The Sullivan Arch Garden in Chicago.
Inspired by nature, planting designers are creating intermingled combinations using either plants or seed. Here we look at some of the most exciting and clearly articulated of these approaches currently being applied by colleagues across three continents.
We have to suppose that there is a zeitgeist at work here. It certainly feels as if many people have had the same good idea at the same time. Inspired by plant ecology science, or just a desire to try to replicate the way plants grow in nature, there are now a number of different but related approaches to creating perennial-based mixes which aim to blend and intermingle varieties. Whereas design in the past focussed on precise plant placement and juxtaposition, these techniques all aim to create the apparent spontaneity of natural vegetation. They do this not by setting out a plan but by planting a mixture – or, to put it another way, they are about creating a vegetation.
A planting in Noel Kingsbury’s garden in Herefordshire on the England/Wales border, in August, illustrating a mix derived from a tall-herb flora model developed for use in public spaces. Very robust perennials are chosen to minimize weed competition on fertile soils in regions with a long growing season. The yellow is Telekia speciosa, the mauve-blue Campanula lactiflora, the red Persicaria amplexicualis, and the pale pink in the foreground Geranium endressii.
A version of Prairie Morning, a mix developed by Cassian Schmidt at Hermannshof, Weinheim, Germany, for dry to medium soil. The pink daisy is Echinacea tennesseensis ‘Rocky Top’, the dull purple Amorpha canescens, the grass Nassella tenuissima, and the pink cup Callirhoe bushii.
The idea of randomized planting at first hearing sounds strange – almost the antithesis of what design in all about. The immediate analogy is with a wild plant community: a wildflower meadow or prairie looks random. In fact, it is not, because there are what ecologists call assembly rules for how and why plants associate. To our eyes, though, these wild plant communities are effectively random. Creating a deliberately randomized mix is essentially a way of creating a mix of plants which is designed off-site: species are chosen for a given environment, to be compatible at equal spacing, and for particular design criteria, which typically include performance for a given season, color and height. A good mix of different structures is also included. The mix can then be rolled out for any space deemed suitable. It is therefore modular, potentially usable for tens or hundreds or thousands of square meters.
Some in the design community are almost offended by the idea. This may be interpreted as the professional jealousy of those (sometimes not designers but vegetation scientists, entrepreneurs or nursery owners) who have developed the mixes. More crucially, perhaps, there is a sense in which any modular approach is seen as mass-produced, and insensitive to what has become almost a dogma: that of design being site-specific. However, what randomized planting offers is a kind of democratization. Just as industrially mass-produced furniture can bring quality design to people who before could not afford it, sometimes to the chagrin of those who can, so randomized planting brings the possibility of large-scale sophisticated plantings to clients who could not afford to employ someone to undertake planting design on a large scale: local government, not-for-profit organizations, community groups, private gardeners with large or difficult to maintain areas, schools and other institutions.
Many commercial projects tend to squeeze the expenditure for landscape; the building comes first, and cost overruns eat into the landscape budget, and then because the planting usually happens last, resources for this are squeezed further. Bringing down the cost of quality plantings inevitably involves compromises, but randomized mixes offer hope of more visually interesting, seasonally changing and biodiverse planting combinations. In particular, they offer the opportunity of visually rich plantings as an alternative to the green cement which in the past has sometimes given the landscape profession a bad name.
The wildflower meadow mixes developed initially by British and German practitioners in the 1970s and the prairie mixes developed during the same period in the American Midwest involve randomization because they are seeded. Seeding is also part of the process used by James Hitchmough and collaborators at the University of Sheffield for creating mixes, each one based on a natural reference model. The idea of applying random mixes to planted combinations was first developed in Germany by Walter Kolb and Wolfram Kircher in the 1990s, with the first public plantings of the mix, Silbersommer (Silver Summer), being made in 2001. Since then more than twenty ‘Mixed Plantings’ have been developed at a number of educational and research institutions in Germany and Switzerland. In addition, other mixes have been developed by garden and landscape designers and individual nurseries.
Here we look at a number of approaches to randomized planting, and then in more detail at the method actually called ‘Mixed Planting’ in German, which is by far the most deeply researched and influential.
DAN PEARSON – AN EXPERIMENT WITH MODULAR PLANTING
Dan Pearson enjoys a reputation as one of Britain’s leading garden designers. The key to understanding him is that he was practically born with a trowel in his hand. Plants and gardening have always been a part of his life, and his design skills have grown out of this personal experience and intimate knowledge. Wild plant communities have always been an inspiration for him; he discovered the limestone meadows of the Picos de Europa mountains in northern Spain as a teenager, and an early experience involved working at the Jerusalem Botanic Garden, which introduced him to the spectacular spring flowers of the Middle East; he has gone on to encounter many other rich wild plant communities. Like many practitioners he has to tread a delicate balance between designing and implementing tried and tested planting combinations and being experimental. One particular project has allowed him to make what he describes as being ‘a risky experiment, not yet repeated’, but which so far has seemed successful.
The Tokachi Millennium Forest in Hokkaido, Japan, is a 240 hectare ecological park created by a media entrepreneur, Mitsushige Hayashi, and aims to offset the carbon footprint of his newspaper business. Working with landscape architect Fumiaki Takano, Pearson has helped to create an environment which greets visitors (mostly very urban people) with a gardenesque environment near the visitor center that ‘will draw them in, gradually familiarizing them with plants and nature’. The idea is that they will begin to feel more relaxed about exploring the main part of the project, a forest landscape with an immensely rich ground flora recovering from a period of deforestation. This area, the Meadow Garden, uses a modular planting style.
Hokkaido lies on the same latitude as New England and has, if anything, an even more Continental climate, with a short growing season (April to September), cold winters (down to minus 25°C) and a short, hot and humid summer. The 1.2 hectare Meadow Garden is designed to be an attractive experience for visitors, but as the planting uses mostly non-native species, there is a concern that some might escape into the wild. To help prevent this, the garden area is surrounded by hedge walls of pine, willow and other species, some 10 to 20 meters deep, with a thick, weed-free mulch so as to minimize seeding. Within this boundary are a series of 14 panels, each one a mix of 5 or 6 perennial species. Each panel is color-themed, and bands of grass Calamagrostis ‘Karl Foerster’ are used to create screens to avoid color clashes. ‘The paths’, says Pearson, ‘are designed to have the same panel mix on each side, to get a sense of walking through. . . . Some mixes have an additional component added to get a sense of one panel segueing into another.’ A key element in the macro level of design is the use of emergents: shrubs such as Rosa glauca or large perennials like Aconogonon ‘Johanniswolke’ ‘to bridge the panels and provide a unifying element, and be rocks around which everything flows. . . . The idea is a river of incidental happenings.’
Each panel mix uses a repeating combination module which Pearson describes as being ‘like a strand of DNA. . . . We have designed a system in which the combination within the mix is never repeated the same way twice so that it is random. . . . The idea is that the mixes will develop their own balance and rhythms.’ A computer program is supplied to the client which is used to generate a pattern based on repeating the modules. The resulting plan can be divided up by a grid, and then the plants set out on to a corresponding grid on the ground.
Different combinations have had different outcomes. In one of the mixes a particular species has become too invasive and has had to be replaced, but the great majority of combinations have been satisfactory and indeed have begun to ‘develop their own dynamic’.
As with the Mixed Planting systems developed in Germany, each mix combines plants from several structural categories: what Pearson describes as ‘emergents, procumbent [that is, sprawling] plants, space fillers and treats’ (treats being occasional unexpected splashes of color or form).
An excerpt from the plan for perennial mix areas at the Tokachi Millennium Forest (2006). Each block, or panel, indicates one of the plant mixes A to N. Shrubs and large perennials also form another layer to this area, indicated on another plan.
One of the mixes used for the Meadow Garden at the Tokachi Forest. The strip at the bottom is one of the repeating units or modules, analogous to a tile. The cimicifugas (20 percent of the plants), rodgersias (3 percent) and sanguisorbas (10 percent) have flower-heads that function as emergents. The asters (43 percent) and the euphorbias (20 percent) are space fillers, while the two Paeonia species (5 percent) are the ‘treats’. Spacing is 30 centimeters between plant centers, except for the paeonias and the rodgersias, where it is 50 centimeters. It is instructive to read the accompanying notes on the plan:
ROY DIBLIK – PLANTING GRIDS
Roy Diblik is a nursery owner in southern Wisconsin. Always something of a pioneer, he was one of the first to grow Midwest natives as containerized plants. Working in a region where there is little history of growing perennials as garden plants, and where many misconceptions exist about the kind and level of maintenance required, Diblik has developed a system called the ‘Know Maintenance’ planting system. Aimed at homeowners in his Zone 5b area (minimum of minus 26°C), plantings are based on a ‘segment’ measuring 2.4 by 3.7 meters and divided into a grid of 30 centimeter squares, which can be repeated as a modular system using connecting plants he calls ‘Integrated Action Plants’ between each module. Outlined in a book (see Further Reading) intended as a manual for domestic gardeners, he sets out some 40 combinations. He does stress, however, that these should be seen as a starting point, and that by using his combinations gardeners can gain the confidence to start making their their own selections.
Among his public works, Diblik has designed a 1,400 square meter planting scheme for the Art Institute of Chicago. A new area at the side of the building, it is within sight of Piet Oudolf’s Lurie Garden and consciously makes a connection, but it uses a grid system so it is clearly distinct. Diblik describes how ‘I was inspired by the colors of a Pierre Bonnard painting in the collection, “Earthly Paradise”. I picked out the color tones in the painting and tried to repeat them in the garden.’
The Meadow Garden at the Tokachi Forest. Flowering are deep purple Salvia nemorosa ‘Caradonna’, white Gillenia trifoliata and blue Baptisia australis.
An example of one of Roy Diblik’s border planting modules, including both perennials and bulbs. This is ‘Elegant #4’, for an open site and average soil where a height of 45 to 60 centimeters is required.
June in the Sullivan Arch Garden of the Art Institute of Chicago, Illinois, where Roy Diblik has used a range of about 60 perennials to create an intermingled style planting. The pale yellow is Achillea ‘Inca Gold’, a relatively reliable and persistent cultivar, the pink Stachys officinalis ‘Hummelo’, and the blue Kalimeris incisa ‘Blue Star’.
MIXED PLANTING
The Mixed Planting system developed in Germany (and the very similar Integrated Planting System in Switzerland) is an example of public investment (through universities and other institutions of higher education and research) going into research and development for plantings which ornament and improve the public realm. Those in other countries where this does not happen can only look on in wonder and envy. One of the advantages of this work being in the public sector is the fact that trialling can take advantage of a nationally or culturally defined unit and be carried out openly and publicly. Silver Summer, for example, was trialled in 13 different locations in Germany and Austria.
Mixes are developed for different habitats, but are also based on visual themes, particularly color. These themes are especially important for the marketing and public perception of the plantings. The majority of mixes have been created at the Hochschule Sachsen-Anhalt (University of Applied Sciences) at Bernburg in the east of Germany, an area with a marked Continental climate and low rainfall. Other locations in Germany and Switzerland where mixes have been developed have less extreme climates. The support of the German Perennial Nursery Association (Bund deutscher Staudengärtner – BdS) has been vital; clients can buy the mixes from members of the association.
A successful mixed planting is one which can function with relatively little maintenance, almost as an artificial ecosystem. Component plants have to be able to coexist with little input for at least ten years together, while the survival of individual plants is less important than the survival of the whole. The species chosen are overwhelmingly long-lived and resilient, but a minority of shorter lived species may also be included in order to create interest in the early years while longer lived but slower species establish. These shorter lived plants may also self-seed, but over the long term their seedlings will find less and less habitat for their replacement as the more permanent components increasingly dominate the space. The same will also be true of species which spread rapidly through vegetative means such as runners but whose short stature renders them vulnerable to overshadowing by taller plants.
There has to be a structural balance, so the categories of structural, companion and ground-cover plants (discussed in chapter three) are key to building a successful mix. Short-lived filler plants may or may not be included. Some bulbs or other geophytes may also be included; indeed, in some mixes they play a key role. As a general rule, the summer-aspect perennial components of the mixtures include 5 to 15 percent structure plants, 30 to 40 percent companion plants and at least 50 percent ground cover. Interest through the year is provided by flushes of flower, with some structural interest from seedheads or evergreen foliage for the winter. Floral interest through the year is, however, dependent on the availability of species to choose from for a given habitat. Some conditions, such as shade or dry soil, tend to have fewer species flowering later in the season.
The mixes developed at Hochschule Wädenswil in Switzerland, by Axel Heinrich and others, include annuals which are sown after the completion of the perennial planting; varieties of species such as Eschscholzia californica, Nigella damascena and Alyssum maritimum germinate rapidly from seed, fill gaps between perennials in the first year and may also self-seed for the second year. Short-lived perennials such as Digitalis lutea and Aquilegia vulgaris are a feature of some mixes – how long they survive in the plantings is dependent on how much they are out-competed by longer lived components. One mix, the most recently developed, Shade Pearl, even includes a shrub, Diervilla sessilifolia, which is cut down to the ground every two to three years.
By October grasses Sporobolus heterolepis and Eragrostis spectabilis are much more prominent, especially when laden with morning dew. The yellow is Rudbeckia fulgida ‘Viette’s Little Suzy’.
Mixed planting Indian Summer is successful for average to dry and sandy soils. Here flowering in mid-summer the orange Asclepias tuberosa is a dry prairie plant well known as being a food source for monarch butterflies. The yellow and brown flower is Echinacea paradoxa, the small yellow in the foreground Coreopsis lanceolata. The grass is Nassella tenuissima.
Silver Summer was the first of the Mixed Planting systems to be developed – intended for dry, alkaline soils. Here it is in the city of Mannheim. The yellow spike is Phlomis russeliana, the blue Veronica teucrium ‘Knallblau’ and the white is Geranium sanguineum ‘Album’.
The table below shows the Mixed Planting ‘Bernburger Staudenmix Native Perennial Flower Steppe’ developed by Wolfram Kircher and his collaborators at Hochschule Anhalt, for dry, open spaces on alkaline soils. Linum perenne is short-lived but self-seeding, while Campanularotundifolia runs extensively underground: good at short-term space filling but eventually displaced by larger plants or those, such as Carex species, which form densely packed clumps. Numbers given are for plants per 10 square meters.
Seasonal Interest of the ‘Bernburger Staudenmix Native Perennial Flower Steppe’ planting mix
A version of Silver Summer developed by Bettina Jaugstetter, for the ABB-Company businesspark. Early to mid-summer sees a strong yellow and blue-violet color scheme with gold Achillea ‘Coronation Gold’ and paler A. ‘Terracotta’, deep blue-violet Salvia nemorosa ‘Caradonna’ and pale blue Nepeta ‘Walker’s Low’. The spherical seedheads are Allium ‘Mount Everest’ and A. ‘Globemaster’.
The table above shows the ‘Bernburger Staudenmix Flower Shade’, also developed at Hochschule Anhalt, for shade, including (up to a point) dry shade with tree root competition. Numbers given are for plants per 10 square meters. Flowering is heavily concentrated in the spring, as relatively few shade-tolerant species flower reliably in mid- to late summer in central Europe, which is usually dry; however, there is good foliage for the rest of the year. In climates such as the eastern USA or the Far East, where summer rainfall is at least reliable or even heavy, a greater range of species could be used.
Trials have experimented with a range of spacings. Generally, more open spacings (4–6 plants per square meter) have been found to be preferable. Close spacing (8–12 plants per square meter) results in early intense competition with resulting high rates of loss, and the over-dominance of the more aggressively spreading species. Gaps in more widely spaced plantings may be filled temporarily with annuals (as in the Swiss-developed Integrated Planting Systems) or with miniature varieties of Sedum, which can be introduced simply by scattering shoots on to the soil surface (at a rate of 30 grams per square meter), as is done for green roofs.
The proponents of Mixed Planting suggest that high numbers of species are a good guarantee of their long-term survival. The mixtures marketed under the name Bernburger Staudenmix contain between 15 and 19 species, while Silver Summer has 30. While plant ecology provides evidence that species diversity improves the resilience of a planting, as losses and gaps are more likely to be filled if there is a wide range able to occupy a variety of ecological niches, as yet no experimental evidence demonstrates that this is so for designed plantings.
How effective mixed plantings are for small areas of just a few square meters is questionable. It is unlikely, and probably undesirable, for plants to be set out at random in such situations, as particular species will be seen as more appropriate for the ‘front of border’ position: bushy plants rather than ‘leggy’ ones, for example. The number of species also needs to be restricted, as there will not be enough repetition in a small space if a large number are used.
Mixed plantings are designed for extensive maintenance: management operations that treat the planting as one, with no individual attention for individual plants. The main task is the annual removal of dead material at the end of the year, which can be done with brushcutters or other heavy-duty machinery. Trials in Germany have also experimented with a mid-season mow instead of an end-of-season cut for some mixes, particularly those which flower primarily in early summer (typical of dry habitat combinations). This mimics the mid-summer hay cut of traditional agriculture in much of Europe. There are many advantages: an attractive fresh regrowth of foliage, some repeat flowering, more light for autumn/winter growing bulbs like Muscari species, reduced height for late flowering species and easy access for removal of rubbish.
Mineral mulches like gravel and chipped stone add considerably to the cost of plantings but save money by reducing weed infiltration and therefore ongoing maintenance costs – particularly important for public situations where maintenance may be irregular or difficult for access reasons. In high visibility situations such as private gardens, a mineral mulch also creates a tidy appearance and makes an open planting look finished even when it is very young.
The array of Mixed Planting systems developed by 2011 are shown in this table, which is adapted from Norbert Kühn, Neue Staudenverwendung (2011).
Brand name of mixture and originating institution* |
Visual character |
Habitat |
Silver Summer, AP (Silbersommer) |
Mid-height, mostly mid-summer yellows and blues |
Dry, calcareous soils, sun |
Indian Summer, HHOF |
Mid-height, prairie species, range of colors, grasses for late season interest |
Dry to average light spaces |
Prairie Morning, HHOF (Präriemorgen) |
As above but blues and purples dominate |
As above |
Prairie Summer, HHOF (Präriemorgen) |
As above but taller pinks to purples dominate |
As above |
Native Flower Transformations, AN (Heimischer Blütenwandel) |
Low, scattered, mid-height, subdued colors in spring |
Sun to half-shade |
Mid-height, yellows and violets |
Half-shade to shade |
|
Flower Border, AN (Blütensaum) |
Low to mid-height, subdued blues and violets, spring and early summer |
Sun to half-shade |
Exotic Flower Transformations, AN (Exotischer Blütenwandel) |
Mid-height, yellows and violets |
Half-shade to shade |
Flower Border, AN (Blütensaum) |
Low to mid-height, subdued blues and violets, spring and early summer |
Sun to half-shade |
Exotic Flower Border, AN (Exotischer Blütensaum) |
Mid-height, wide color range |
Sun to half-shade |
Flower Shade, AN (Blütenschatten) |
Low mix of spring-flowering species with decorative foliage perennials |
Underplanting for woody plants, dry to moist |
Flower Wave (without summer mowing), AN (Blütenwoge) |
Low, scattered mid-height, strong yellow and blue contrast |
Dry, sunny |
Flower Wave (with summer mowing), AN |
As above, but with fresh look in late summer |
Dry, sunny |
Flower Steppe, AN (Heimische Blütensteppe) |
Low, subdued violet-blues and yellows |
Dry, sunny, similar to natural steppe habitat |
Exotic Flower Steppe, AN (Exotische Blütensteppe) |
Low, subdued yellow-greens to blue |
Dry, sunny, similar to natural steppe habitat |
Flower Veil, AN (Blütenschleier) |
Low, gray-leaved, multicolored in spring, later yellows, violets, some pinks |
Dry, sunny |
Grass Dance, ERF (Tanz der Gräser) |
Low perennials and taller grasses, multicolored |
Dry to moist, sunny |
Veitshöchheimer Flowering Mosaic, VT (Blütenmosaik) |
Low, yellows and blues |
Dry, sunny |
Veitshöchheimer Flower Magic, VT (Blütenzauber) |
Mid-height, blue-dominated, later blues, yellows and reds |
Dry to moist, sunny |
Veitshöchheimer Flower Dream, VT (Blütentraum) |
Mid-height, multicolored |
Dry to moist, sunny |
Veitshöchheimer Colorplay, VT (Farbenspiel) |
Low early, later higher, multicolored |
Dry to moist, sunny |
Low to mid-height, yellows, blues, whites |
Sun to half-shade |
|
Summerwind, wäd (Sommerwind) |
Low, pastels – violets, yellows – silver foliage |
Dry, sunny |
Summerfresh, wäd (Sommerfrische) |
Low with some taller, violets, yellows, grasses important |
Dry to moist, sunny |
Indian Summer, wäd |
Warm colors from yellows to oranges, red autumn color |
Dry to moist, sunny |
Pink Paradise, wäd |
A variety of pinks |
Dry to moist, sunny |
Summer Night’s Dream, wäd (Sommernachtstraum) |
Blue-violets with purple foliage, grasses important |
Moist |
Shade Pearl, wäd (Schattenperle) |
Mid-height to taller, yellows and blue-violets, reds, pinks later |
Shade, underplanting trees |
* German brand names are given in italics. Abbreviations are shown below.
AP = Arbeitskreis Pflanzenverwendung BdS (research group of the German Perennial Nursery Association.
HHOF = Sichtungsgarten Hermannshof, Weinheim (Display Garden).
AN = Hochschule Sachsen-Anhalt, Bernburg (University of Applied Sciences). All mixes produced have the brand name: Bernburger Staudenmix.
ERF = Landesversuchsanstalt für Gartenbau, Erfurt (State Horticultural Research Institute).
VT = Landesanstalt für Wein-und Gartenbau, Veitshöchheim (State Institute for Viticulture and Horticulture).
WÄD = Zürcher Hochschule für Angewandte Wissenschaft, Wädenswil, Switzerland. (Zürich Canton University of Applied Sciences).
The Mixed Planting system is undoubtedly impressive, and appears to be commercially successful. Since 2009, the number of nurseries participating in the provision of plants for the schemes has risen dramatically. At the time of writing, some 40 BdS member nurseries are supplying plants for the older mixes such as Silver Summer, with around 25 for the more recently developed ones. Some non-BdS members are also participating. It is based on an empirical and science-based methodology and extensive trialling, and because it is a product of public funding, the plant mixes are in the public domain. Not surprisingly, it is a concept which is having a growing influence in the countries of eastern Europe, newly emerging into a world where there is more available funding for public and private landscapes.
This mixed perennial prairie planting concept was developed for the ABB-Company businesspark at Ladenburg in the Rhine Valley, Germany, by Bettina Jaugstetter, with a yellow and white color theme. The tall yellow is Helenium ‘Rauchtopas’, the white is Aster divaricatus, the silver foliage Artemisia ludoviciana ‘Silver Queen’. Also present are Echinacea purpurea ‘Alba’, E. p. ‘Sunrise’ and E. p. ‘Sundawn’, Kalimeris incisa ‘Blue Star’, Liatris spicata ‘Alba’, Pennisetum orientale ‘Tall Tails’, Coreopsis verticillata ‘Grandiflora’ and Sesleria autumnalis as a ground cover.
HEINER LUZ AND SEASONAL THEME PLANTS
Related to the Mixed Planting approach is the work of Heiner Luz, a landscape architect who is the third generation of his family to be in the profession. As a design professional, it is not surprising to hear Luz outline his thoughts on planting combinations in clear terms; he stresses the well-known mantra ‘less is more’, but finesses it to ‘uniformity on a large scale and variety on a small scale’ as the fundamental principle which underlies all his company’s work. The projects for which he designs mixed plantings are on a large scale, primarily garden show sites where several hectares need planting. Garden shows in Germany are a major aspect of the country’s landscape and horticulture industry. Sites are chosen for summer-long shows which leave behind a permanent legacy of quality infrastructure and planting, usually as part of urban parks. They are thus a tool of regeneration. Much of the innovation in planting design with perennials in Germany is driven by the garden shows as clients for landscape designers’ work.
Luz’s key concept is that of Prinzip der Aspectbildner – difficult to translate but ‘seasonal theme plants’ is probably the most descriptive. ‘Theme plants’ (Aspektbildner) are species which visually dominate a planting for a period of several weeks each, making a dramatic large-scale effect. A mix will typically contain 3–6 theme plant species forming around 70–75 percent of the total, and the remainder companion (Begleiter) species. Theme plants have to be very distinctive in terms of flower, foliage or structure during the period they are on stage, but may be relatively insignificant before or after. The companion plants play a supporting role, often counterposing or complementing the qualities of the theme plants, or looking good at other times. They make less impact on the large scale because there are more species and so their impact is diluted. However, when the viewer is close to the planting, for example when walking on a path alongside it, they add variety and a sense of constant change – a result of the many different possible combinations that result from their juxtaposition with the theme plants.
Flowering tends to be in flushes, so two to three spectacular periods in the growing season last for several weeks, between which the more subdued companion plants maintain interest. All plants are randomly placed, and for rapid effect planting is relatively dense: 10–12 plants per square meter.
An example of the seasonal theme plant principle is a series of plantings at Landscape Park Riem near Munich, developed from 1995 onwards for a garden show held in 2005, and maintained since then as the park for an exhibition and convention center. Some 2.5 hectares are divided into three areas: the Iris–Mint Meadow, an ornamental reedbed area, and a wet meadow soakaway area. The theme plants for the Iris–Mint Meadow are shown in the table below. The companion plants are Alchemilla epipsila, Camassia cusickii, Geranium wlassovianum, Lythrum salicaria, Mentha pulegium, M. spicata, M. longifolia, Molinia caerulea subsp. arundinacea, Sanguisborba officinalis and Valeriana officinalis. This project required some 230,000 individual plants – clearly expensive to produce.
The considerable costs of making plantings involving very large numbers of plants at high density can be reduced by using custom-designed seed mixes to reproduce similar proportions to the planted combinations. Maintenance and the future development of plantings and other installations are seen as a key part of all the Luz practice work from the very beginning of the planning process. Annual cutting is done with a mower on a high setting or a brushcutter.
Seasonal Interest at the Landscape Park Riem in Munich
The plantings at Landscape Park Riem, Munich, by Heiner Luz’s studio. From top: May with Iris sibirica most prominent; August with Boltonia latisquama; September with the some Boltonia still flowering; and December.
OTHER APPROACHES TO PLANTING BLENDS
Interest in randomized planting mixes is growing, with every year seeming to bring another gardening practitioner into the marketplace bearing a nursery crate full of plants – each metaphorical crate contains enough plants for one module of a planting system. Criteria for combining plants may be primarily aesthetic, or based on matching soil conditions with appropriate plants. In some cases mixes are being developed by independent garden experts and sold online solely as plans (aimed at amateurs or garden designers), and in other cases by nurseries (aimed at landscape designers and local government managers of public space). The evaluation and trialling of these mixes are almost certainly not as thorough as in the German and Swiss Mixed Planting schemes, if they happen at all, and they are not in the public domain.
Planting blends and mixes are increasingly site-specific, so a mix is designed for one location and not repeated elsewhere; very often these schemes involve several custom-designed mixes for different habitats or particular visual qualities. The always innovative Oehme, van Sweden & Associates of Washington, D.C., are beginning to apply this approach for both private and public commissions. This has also been my own approach for a number of years. I design a mix for a sample area (often 100 square meters) and then replicate it for the whole area. This saves on planning costs, an important consideration for cash-strapped clients – in my case a local government department. A particular concern in my work has been to design planting mixes which are resilient to minimal and unskilled local authority contract maintenance in the south of England, where a long growing season makes for very strong weed growth. Necessity requires a focus on robust clump-forming species.
Site-specific mixed plantings have long been a feature of German garden festivals, with Rosemarie Weisse’s work at the 1986 Munich International Garden Show (IGA) being the first to create an impact internationally, especially the steppe planting, which is still looking good all these years later. Much work has been done by Urs Walser over the years, and increasingly other designers are working in this style. Prominent among them is Petra Pelz, who for many years used large monocultural blocks of perennials, reminiscent of Oehme van Sweden’s work in the US. She is now creating some exciting mixed planting combinations at garden shows.
Planting by Noel Kingsbury, working with HTA Landscape, on the Promenade at Bexhill-on-Sea, Sussex, an extremely exposed coastal site with frequent salt spray. Five randomized mixes of species known to thrive in coastal environments are used, each one comprising around 15 species, with around half shared with other mixes. Prominent here are gray Ballota pseudodictamnus, Nepeta ×faassenii and yellow Achillea filipendulina and Rudbeckia fulgida ‘Goldsturm’.
At Bexhill, the seafront planting by Noel Kingsbury features pink-flowered Osteospermum jucundum with silver-leaved Stachys byzantina, Artemisia ‘Powis Castle’, Phlomis italica and Eryngium bourgatii amongstothers. The silver-gray is Ballota pseudodictamnus. In the coastal and other exposed situations where these species would naturally grow, plants would be densely intertwined, supporting and protecting each other. The idea here is that with randomized planting a similar meshing together of foliage will occur over time.
A planting for the Koblenz Federal Garden Show (BUGA) in 2011, designed by Petra Pelz. The tall yellow spikes are Eremurus ‘Moneymaker’, and the red is Penstemon barbatus subsp. coccineus; a prominent foliage plant is Helianthus salicifolius, with tall stems covered in narrow leaves.
A planting by Sheila Brady of Oehme, van Sweden & Associates Inc., showing an area of the newly redeveloped Azalea Garden at the New York Botanical Garden. Perennials and bulbs have been incorporated to provide context and to lengthen the season of interest. The grass here is Molinia caerulea subsp. caerulea ‘Strahlenquelle’, and the blue is Gentiana makinoi ‘Marsha’.
THE ‘SHEFFIELD SCHOOL’
Sheffield is known best as an industrial city, and its increasing profile as a center for horticultural and industrial innovation has come as a surprise to many. This reputation is largely down to two members of the University of Sheffield’s Department of Landscape: James Hitchmough and Nigel Dunnett. Both are pioneers of horticultural ecology – the application of plant ecology science to designed plantings. ‘I have spent much of my life understanding how to assemble sustainable, designed plant communities that are attractive to the public,’ says Hitchmough, ‘in particular, how to apply the ecological rules that govern what can be designed. . . . These rules are universal and blind; they don’t distinguish between wild and cultivated plants.’ Nigel Dunnett has worked on creating sustainable plantings for a variety of situations, and it is his work with annual seed mixes which has so far created most public interest – an inexpensive way for local government and other managers of public spaces to create vibrantly colorful and wildlife-friendly summer plantings. James Hitchmough’s work uses perennials grown from designed seed mixes; these are based on naturally occurring, visually interesting plant communities: central European meadow, North American prairie and South African montane grassland are three groups he has worked on. Fundamental to the Sheffield School approach is what is referred to as the wow factor: they must be designed to brighten people’s lives. As Hitchmough describes one notable public encounter, ‘When a massive bloke comes up to you, with no neck and covered in tattoos, with a pit bull terrier on a chain, and says that what you’ve done is the only thing that gets him out of bed in the morning, you know you’re doing something right.’
The use of native plants in Sheffield School plantings is limited to projects where their use is particularly appropriate – a reflection of the very reduced British flora, and the presence in the flora of a number of aggressive species (mostly coarse pasture grasses) which can, paradoxically, if used indiscriminately reduce the potential for supporting biodiversity. In any case, in many urban environments the primary aim is often the creation of relatively stable artificial ecosystems that, in Hitchmough’s words, ‘meet human and wildlife needs rather than the production of facsimiles of native plant communities’.
Planting aims at matching the fertility of the site with appropriate species. For example, perennial plantings on fertile soils are very prone to invasion by weeds, especially since most perennials drop their leaves in winter, and aggressive grasses and others with winter-green foliage are able to take advantage during this period of dormancy. The use of tallgrass prairie plants from an ecosystem with moist, high fertility soils can minimize this, however, as at a high density their leafy shoots and root systems so monopolize light and soil that any unwanted species which establish are often held in check. Conversely, low fertility sites are appropriate for species derived from plant communities which are used to extracting the most out of limited resources, such as European dry meadows.
Central to Hitchmough’s designed perennial communities is establishment from seed. Seed-derived plant communities are much more able to resist weed infiltration than those which are planted out, because of their much greater density – up to 150 plants per square meter – at least as long as all sources of potential weed competition are eliminated from the soil on site before sowing. Seed is sown into a 75 millimeter layer of sand or similar material spread over the soil surface. Seed allows for plants to arrange themselves according to natural processes, to sort out their own relationships and ecological niches, rather than have their arrangement imposed entirely on them by the designer. Genetic diversity also allows for greater resilience to stress, pests and diseases, as there will be a variety of levels of resistance to problems. Plantings can be created by a mix of planting and seeding – useful if the seed of certain component species is not available in the quantities needed, or a particular cultivar is required, or the rate of establishment from seed is so slow that such species may be too easily out-competed. Very importantly, planting is also more predictable, something which appeals to risk-averse clients.
Plant community mixes include several foliage layers, which increase the coverage of the soil (reducing water loss, erosion and weed infiltration), increase the value of the planting for habitat and add to its visual richness, and in particular increase length of flowering. Layers can also be manipulated for aesthetic reasons: low basal foliage with taller emerging naked flowering stems rising out of it is a favorite Hitchmough visual effect. A low canopy layer can include many spring or early summer flowering species (often relatively shade tolerant), with taller emergents for late summer and autumn interest. Grasses are included but only as a minority component, unlike the wild communities from which they are derived, as the high proportion of grasses in wild or semi-natural grassland communities (typically at least 80 percent of biomass) reduces the visual impact. Needless to say, all species used are trialled extensively first to assess their needs in cultivation, and to ensure that they do not show any indication they might become invasive; in fact, the vast majority of species used have either been in cultivation in Britain or have very close relatives which have, and have not shown any inclination to problematic spread.
Maintenance is extensive although the option of additional selective horticultural maintenance is always good to have. Hitchmough says that the planting ‘should be designed so that a management operation is applied to all the plants across the whole site and which disadvantages the plants you don’t want and advantages those that you do. . . . This is an alien concept to most designers.’ Mowing is an obvious management technique. Mulching is another, to suppress weed seedlings. Burning is also highly effective against the annual weed seedlings which emerge very early in spring in maritime climates, against early leafing perennial weeds, and against slug and snail populations, but has no effect on the largely dormant perennials; it can also benefit the later emergence of seedlings of the component plants. As has been found in Germany with Mixed Plantings, cutting in mid- to late summer also limits the growth of weedy species, and reduces the growth rate of the more competitive components; the disadvantage is that late summer to autumn interest is likely to be reduced.
These artificial plant communities have been developed in the context of researching how to apply a knowledge of plant ecology to ornamental horticulture, and the need to publish data on the results achieved. Having proved they are possible, the next stage might be for other practitioners to mix and match between the different communities, to produce truly global planting combinations.
A prairie planting by James Hitchmough at the Royal Horticultural Society garden at Wisley, Surrey, in September, created by sowing a seed mix designed to highlight emergent plants – yellow Silphium laciniatum is one such. Pink Echinecea pallida is a longer lived species than E. purpurea, its flowers just topping the main foliage canopy.
Layers making up a montane South African community designed by James Hitchmough. The uppermost layer shown is the species-rich base layer to which taller species are added, flowering later but in diminishing frequency through the year, to avoid shading out the shade-intolerant basal layer.
The South African area in London’s Olympic Park (2012), designed by James Hitchmough, with white Galtonia candicans, pink-red Gladiolus papilio ‘Ruby’, pink Dierama pulcherrimum, blue Agapanthus varieties and low-growing pink Diascia integerrima. The grass is Themeda triandra.
An area of British natives at the Olympic Park. These are all ‘flowers’ – non-grassy species – including white Leucanthemum vulgare, pink Ononis spinosa, yellow Leontodon autumnalis and pale pink Malva moschata. Apart from Ononis, all were sown.
The table below lists plants used in the Drakensberg South African community designed for one of the gardens in London’s 2012 Olympic Park. The numbers refer to how many plants are added to each square meter. The same principle of taller species being used at much lower densities is illustrated. A planting rate of 0.1 means one of these plants every 10 square meters.
Plants used in the Drakensberg South African planting at London’s 2012 Olympic Park
Hitchmough’s colleague Nigel Dunnett also uses a randomized approach to planting, but with a plant palette that is perhaps more familiar to gardeners as much of it uses species well established in cultivation. His work is particularly geared toward functionality and specific design applications – especially rain gardens and green roofs. At the Olympic Park he has created plantings using plants and seed rather than seed alone, based on Western European hay-meadow and East Asian woodland-edge vegetation. The range of European species available has enabled him to create some color-themed mixes. An advantage of using plants from these areas is that a wide variety are commercially available in Britain and throughout Europe and North America. Rain garden and green roof mixes are designed around very individual environmental conditions; rain gardens need plants with the ability to cope with drought and occasional flooding, whereas green roofs need species which can survive drought, temperature extremes and a root run of limited depth.
The table below shows the Asia garden at the Olympic Park, designed by Nigel Dunnett. Planting densities vary, but the numbers give an idea of the proportions of the plants in the mix. The planting is designed as a series of swathes, each with a specific plant combination.
Plants used in the Asia Garden at London’s 2012 Olympic Park
