Note from the editor
Like death and taxes, pests and diseases are about the only things in the garden that are absolutely certain. The result is that they often bulk rather too largely in our imaginations, preventing us from enjoying our gardens to the full. The RHS, for example, receives thousands of enquiries every year about them so we can assume there are a great many more discontented gardeners out there. But at least the Society can provide its members with accurate information to ensure that no more time than is necessary is expended on identifying the problem, and doing something about it–if possible. If not, we will be told just to accept it, which is probably better than nothing.
Pests come in all shapes and sizes it seems: wasps (perhaps), rabbits, roe deer, even exotic parakeets, to name just a few. They are not even confined to lower mammals–humans play their part as well, as Vanessa Berridge has discovered about her neighbours. Many diseases are always with us. Dutch elm disease, for example, was devastating in the 1970s and is still a threat, while rose replant disease and grey mould can be controlled or circumvented, but never eradicated.
Of course, the list and virulence of pests and diseases change over the years: some are solved or neutralised, while others continue to arrive (often from warmer climes) to plague us. Nowadays, we should think ourselves really lucky if we were plagued by house sparrows or skylarks in our gardens, as earlier generations have apparently been, but at least those before 1940 were saved from the cursed lily beetle which makes lily cultivation so difficult these days, the citrus longhorn beetle that threatens trees, not to mention pathogenic horrors like Cylindrocladium buxicola (box blight), Phytophthora ramorum (sudden oak death) and Chalara fraxinea (ash dieback). Sometimes it seems a miracle that we can make thriving gardens at all.
URSULA BUCHAN, editor
Birds versus Gardening
Some gardeners speak as though horticulture would benefit by the extermination of all birds, whilst others would strive to whitewash the sparrow and endeavour to prove that the sooty suburban raider is a very slightly disguised angel. Being myself fond of both birds and garden, I have tried to speak without prejudice and judge each of our feathered visitors to the garden on its merits.
“…the majority of the sparrow’s iniquities in the garden are ascribable to sheer love of mischief rather than stealing to satisfy his hunger”
Taking our foes first, the sparrow heads the list. The most annoying part of it is that the majority of the sparrow’s iniquities in the garden are ascribable to sheer love of mischief rather than stealing to satisfy his hunger. In the early spring, when even a few Crocuses are a joy to the amateur gardener as an earnest of summer glories to come, the sparrow makes that joy extremely brief by rending the flowers in pieces and strewing them about the border. Curiously enough, he seems to have a strongly developed colour sense, and attacks the yellows more persistently than any other shades. A ‘Primrose by a river’s brim’ is a subject of indifference to a sparrow, but he seems to take a fiendish pleasure in pulling out all the flowers from those which have been the objects of the gardener’s solicitude. The cultivation of the Gooseberry is almost a hopeless labour where these pests abound, as they pull out all the buds from the twigs as soon as swelling commences. Something may be done by dusting the trees with lime, soot, etc., and by threading the branches with cotton. It is also wise under these circumstances not to go in for hard pruning, as birds seem to find more difficulty in abstracting all the buds from slender whippy shoots than from shortened stumps, and by leaving a sufficient number of whiplike shoots on lightly pruned trees you ensure at least a partial crop. I have seen beds of Carnations absolutely ruined by being pecked to pieces by this destructive nuisance.
I was reminded this afternoon not to forget the lark in my list of feathered enemies, and I regret to say that there is some reason for its inclusion [because] it also visits the garden and completely skeletonises all the Spring Cabbage. No one would be Philistine enough to wish to lose the ‘fine careless rapture’ of the skylark’s song, but at the same time one is led to regard the slight annual thinning of its numbers for table purposes as so far beneficial as not to call for discouragement.
Turning now to the brighter side of the picture, one is glad to be able to chronicle a small army of friends whose manner of life causes the gardener to regard them with unmixed benevolence. These are the purely insectivorous birds, including, among residents, the modest hedge sparrow, the graceful wagtail, the lively wren, and the robin. This last, though sanctified by common sentiment in this country, is a pugnacious little rascal, fighting to the death intruders on the small domain he has marked out as his own; he has also one blot on his character, his principles failing to keep him in the path of honesty when ripe Cherries are about.
Among the migratory hosts all the warblers, the chiff-chaff, willow-wren, white-throat, fly-catchers, etc., are without reproach and ought to be carefully protected. The cuckoo, too, though needing a kindly veil over its domestic affairs, is a gardener’s friend, being a destroyer of caterpillars, and is the only bird I know of which will tackle the long-haired section of them.
Parakeet Protection Reduced
Non-native parakeets colonising parts of southern England and causing problems in many private and public gardens can now be controlled more easily. From the beginning of the year, rules obliging landowners to apply for a special licence to cull the species have been lifted (they must apply for a general licence to control them legally).
“Their exotic, bright green plumage is a common sight at RHS Garden Wisley…”
Populations of naturalised ringnecked parakeet (Psittacula krameri) have exploded in the last decade and now number tens of thousands. Their exotic, bright green plumage is a common sight at RHS Garden Wisley, where they cause considerable damage to apples. ‘Parakeets hang on the branches and peck at the more brightly coloured fruits,’ says Fruit Supervisor Alessandra Valsecchi.
The Wisley vineyard, which began producing wine in 2006, has yet to be affected but at nearby Painshill Park, grapes that could make thousands of bottles of wine are eaten by flocks of parakeets each year.
Why Welcome Wasps?
Wasps have entered the rich pantheon of English simile. The phrase ‘as welcome as a wasp at a picnic’ has joined other ironic adages such as ‘as passive as Japanese knotweed’ or ‘as organic as DDT’. Certainly wasps are slightly scary, given their propensity to sting humans without any apparent provocation. That only females inflict this injury makes wasps a tricky subject for a politically correct male writer. Worse still, the male wasps (most evident around August Bank Holiday), being completely unable to multi-task, spend their short lives feeding on plums and trying to mate with newly emergent queens–albeit not stinging anybody.
“All a freshly fertilised queen wasp wants to do is to hide in the dining-room curtains until next spring”
Believe it or not, for much of their lives common wasps (Vespula vulgaris) are beneficial to gardeners. All a freshly fertilised queen wasp wants to do is to hide in the dining-room curtains until next spring. When she emerges from the Laura Ashley she starts a new colony. She finds a suitable nesting site such as a disused mouse burrow, your loft or the rafters of a shed and creates a small structure using a type of papier-mâché made from chewed-up wood. You may already have had the pleasure of watching wasps gnaw away at your favourite garden furniture or sheds.
The young queen builds several cells in her new nest and lays an egg in each. These fertilised eggs hatch into grubs which the queen feeds with captured insects. (Wasp larvae are carnivorous, unlike bees, which feed only on pollen and nectar-points, if they remember their card.) Wasp larvae secrete sweet saliva, which serves as a food for the queen.
Infertile female workers hatch out from the first few eggs and fly off to munch up more garden sheds in an effort to enlarge the nest. As the colony increases in size, worker wasps will use their stings to paralyse caterpillars and other garden pests before carrying them off as fodder for their grubs. The queen concentrates on laying fertilised eggs until mid-August when she lays a few unfertilised eggs in some slightly larger cells. These hatch into males that will mate with females from other nests.
At this point the colony’s work is finished and it begins to disintegrate. Unfortunately for us gardeners, the unemployed worker wasps then change from being largely beneficial caterpillar and aphid killers to predators of our plums, pears and apples.
Undoubtedly there will be times when a persistent wasp needs to be shown a rolled-up copy of The Garden, but these insects do have a useful place within the fauna of our gardens. So if any wasps are reading this as it descends towards them at high speed, take note: my plums are off limits.
Asian Beetle Import Alert
UK import regulations have been stepped up in a bid to prevent citrus longhorn beetle (Anoplophora chinensis), recently found in parks and gardens in the Netherlands, from entering and establishing in this country.
The Dutch Plant Protection Service has now destroyed all deciduous trees within 100m (330ft) of a find in Boskoop. All plant nurseries within a 1.2-mile radius (said to be more than 550) had to cease trading while inspections were carried out. Boskoop is a major horticultural production area for Europe.
The beetle could be potentially devastating in gardens. Adults eat foliage and young bark, but the larvae do more damage, feeding inside trunks on the pith and vascular system of a wide range of deciduous trees and shrubs. Each female beetle can lay 200 eggs.
The discovery has had (and may still cause) serious repercussions for British gardeners, including increased prices and restrictions on the availability of some plants. Andrew Halstead, RHS Principal Entomologist, says there is no treatment. ‘The only thing you can do is destroy infested plants, which is serious if they are important specimens or have historic significance.’
The Food and Environment Research Agency (FERA) estimates it would cost around £300m to deal with a substantial outbreak of citrus longhorn beetle in the UK–yet this would not guarantee eradication. It is asking gardeners and those in the horticultural industry to be extra vigilant for signs of the pest, particularly on its favourite host, Japanese maple (Acer palmatum).
“…be extra vigilant for signs of the pest, particularly on its favourite host, Japanese maple…”
The agency speculates the pest could have arrived in the Netherlands as early as 2001, given the numbers of plants affected in the Boskoop area. Larvae are difficult to spot as they can live within plant stems for up to three years. Breeding populations have been found in Lombardy, Italy and France as well as the Netherlands since 2000.
In the UK, the Plant Health and Seeds Inspectorate is targeting inspections of large shipments from Boskoop and the Far East (the main source of the beetle), and some British nurseries and garden centres. The only way to detect larvae is by ‘destructive testing’ of a sample of plants in a shipment. Seventeen hosts including Cotoneaster and Betula have been identified, but acers imported to the UK from China have been the plants most commonly infested.
Adult beetles are 21–37mm (¾–1½in) long, with antennae up to twice the length of the body. The most obvious visual sign of the beetle’s presence are 6–11mm (¼–½in) diameter exit holes at the base of trunks. More difficult to detect is bleeding sap (where eggs have been laid) and bulges in the trunk (indicating a pupal chamber).
Anyone who suspects seeing symptoms should contact their local Plant Health and Seeds Inspector.
Book-Bugs
How do insects learn botany? It has been another sad season of elm attacks by Scolytus scolytus and his little cousin Scolytus multistriatus, with their fungus-infected feet, leaving young trees burnt or bare in the hedge. I am hardened to that, 30 years after their brutal arrival destroyed all but one of our elms in, or close to, the immemorial class. But this year, to my horror, they have been at the botany books and discovered that the graceful zelkovas, cousins to the elm in the niceties of botany but in appearance quite different, are fair game too.
“Our Zelkova carpinifolia turned limp and jaundice-yellow almost overnight in early July”
We have three: two of the comparatively common Zelkova carpinifolia from the Caucasus and one of the rarer and even more elegant Z. serrata from Japan. Our Z. carpinifolia turned limp and jaundice-yellow almost overnight in early July. Z. serrata is seemingly following, with brown sprays in the canopy and, worse, brown stains appearing on the bark of the lower trunk. One of the last of the East Anglian breed once known as Ulmus nitens, the shining elm, for its gleaming dark leaves (most elm leaves are rough and dull), is under branch-by-branch attack, to which we respond laboriously with a saw. How soon will the scholarly scolytus cotton on to Celtis australis, the so-called hackberry, another elm relation and one of the best Mediterranean street trees? Ours looks fine now, but how do I tell it not to breathe?
Autumn Precautions
In fully-fenced hill pasture voles swarm beneath tussock or snow, gnawing young trees’ stems (hazel and birch excepted). We baffle them by tube guards of old 2-pint milk cartons or double storeys of cat-food tins (800g size–years of them, all Natural Rust Brown). These are easily renewable and if not attractive, neither are 200 girdled oaklings. (How many cat-food tins? Well, after all, don’t we keep Felis catus to protect our plants?)
“…roe deer–unlike the execrable goat–don’t seem to push down trees…”
Trees that I plant outside the fence proper require individual protection against rabbits and roe deer as well. Small oaks and such like begin there in tall (420cm) plastic tree-shelters (expensive and non-durable) but are weaned into cans and rabbit-netting rolls (420 × 10cm diam.) that ease apart as the boles expand; taller saplings start off in the rolls. A temporary post prevents weak stems bending invitingly under snow-load. These rolls prevent the lethal fraying of young stems by bucks’ horns, and as roe–unlike the execrable goat–don’t seem to push down trees and posts, they prevent nibbling of branches as well. They also keep off rabbit teeth in hard two to three-foot deep snow, when rabbits are really hungry and low guards useless.
Secateur-Happy Neighbours
The old Victorian adage ‘Growth follows the knife’ may be true–but within limits. It’s those limits, sadly, that my neighbours just do not recognise. Good friends all, they will unhesitatingly run you and your child to A&E, lend a cup of sugar, or feed the cat. But let them loose with a pair of secateurs–and problems begin.
“…rambling roses, a foaming Acacia and scented jasmine have all fallen to that deadly snip, snip”
‘She’s in the garden,’ I shriek. My husband sighs wearily, knowing I’ll shoot out like Dickens’ Betsey Trotwood shooing away donkeys. Experience has shown that my neighbour will be busily clipping away at stems that have unwisely poked their way through to her side and I’ll find my Clematis ‘Perle d’Azur’, previously a mass of vibrant blue blooms, hanging lifeless from the fence. The initial warning was an assault on her cooking apple tree, which once provided us with welcome borrowed landscape and its creamy-pink blossom in spring. ‘Why do you think it’s died?’ she wondered, apparently unaware that July is not a key month for savage orchard pruning. Her lofty Cupressus, which removes light from our garden by 2pm for much of the year, would have been a better victim.
On the other side, a Victorian pear once soared to 18m (60ft), its blossom an avalanche of snow in April. It died some years back, but its cadaver still haunts our skyline–partly because our other secateur-happy neighbour is too busy trimming back my carefully interwoven Clematis armandii and white Solanum to reveal an uninterrupted view of his drain pipe and a Stalag 17-like shed in the garden beyond.
London gardens have enough challenges, such as slugs, snails and the squirrels that rip through my tulip bulbs unless I cover the pots and beds in winter with unsightly chicken wire. Then there’s my soil–cold London clay. To put that into context: a 1930s lease on a house a few streets away had a restrictive covenant forbidding brick-making in the back garden. So I’ve added sand, humus and helpings of our own compost to the soil, and planted damp shade-lovers. I’ve divided the 9m × 18m (30 × 60ft) space into four, using clipped box and arches covered with roses and evergreen Muehlenbeckia to draw in the eye and hint at further delights. But I can’t rely on my boundaries, as rambling roses, a foaming Acacia and scented jasmine have all fallen to that deadly snip, snip. Would it seem unfriendly to build walls?
Hopes for Ash Come Back
To date there have been more than 600 reported cases of ash dieback (Chalara fraxinea) in the UK. However, good news has emerged from the National Trust’s Holnicote Estate, Somerset, home to a 2001 plantation of 6,000 ash trees. The disease was identified there in September [2013], but only 10 percent of the population is infected and some trees are thought to display signs of resistance. Because of their location, the Trust’s Natural Environment Director Simon Pryor believes that the trees were not infected by wind-borne spores but already had the disease when they were planted more than 12 years ago. ‘Even the trees affected have not suffered as much as we’d have expected, and few have died, despite apparently having had the disease for nearly a decade,’ he said.
“…good news has emerged from the National Trust’s Holnicote Estate, Somerset…”
Phytophthora
In recent months there have been many reports about the spread of Phytophthora ramorum, its threat to oaks and Japanese larch as well as its ability to infect Rhododendron and other shrubs. But what is this disease, and what does it mean to gardeners?
Phytophthora ramorum has been responsible for the widespread death of oak (Quercus) species and tanbark oak (Lithocarpus densiflorus) in parts of the USA, where the disease was dubbed ‘sudden oak death’.
“Plants that are susceptible should be pruned only in dry weather and the tools used cleaned and disinfected”
It was soon detected in Europe, and there have now been hundreds of findings in the UK. However, our native oak species (Q. robur and Q. petraea) are much more resistant to infection. In the UK, the majority of cases have been in the nursery trade, affecting container-grown ornamentals such as Camellia, Rhododendron and Viburnum. There have, however, been significant outbreaks in gardens, amenity areas and woodland. In the UK and Europe the disease tends to be referred to as ‘ramorum dieback’.
Phytophthora ramorum (and a closely related organism, P. kernoviae) are known as ‘notifiable pathogens’, which means that suspected cases must be reported to the appropriate plant health authority. Until recently, common beech (Fagus sylvatica) appeared to be the most susceptible tree under UK conditions. There have also been significant findings on a common heathland plant, bilberry (Vaccinium myrtillus). A major epidemic could thus have far-reaching consequences for woodland and heathland habitats, as well as for the horticultural industry, gardens and amenity plantings.
Infected Japanese larch trees (Larix kaempferi) were first found in August 2009 in southwest England. There since have been outbreaks on larch in Wales, Northern Ireland, the Republic of Ireland and, most recently, in Scotland. Larch has added a new dimension to the problem: this is the first time that the disease has been found affecting a commercially important conifer species; it is also the first time that large numbers of any tree species have been infected in the UK; numerous spores are produced on infected larch needles, which have the potential to spread the disease widely.
Thousands of hectares of affected Japanese larch plantations are currently subject to statutory control measures. Affected trees are being felled, although the timber can still be used.
Gardeners can help prevent the spread of the disease mainly by following good garden practice. Plants that are susceptible should be pruned only in dry weather and the tools used cleaned and disinfected. It is better to water these plants in the morning than at night and, to help prevent the disease spreading, allow good air movement by keeping plants well spaced. Damaged leaves can be more liable to infection, especially in wetter weather.
All gardeners should familiarise themselves with the symptoms of P. ramorum and P. kernoviae and keep an eye on plants and trees known to be hosts. The diseases do not kill shrubs such as Rhododendron and Camellia but will weaken the plants, and their infected leaves can generate spores that can pass to susceptible trees.
By being able to recognise the symptoms, by reporting suspected cases, and by following sensible cultivation techniques, gardeners can help prevent the spread of these diseases.
False Alarm
There was an unnerving article in last September’s journal about a new box fungus of such potency that it threatens all the box hedges, the parterres, the dinky little pompons, the cloud hedges–all the pretty horticultural contrivances from nature’s most malleable plant.
“There was a pause while the scientific staff went to work. The spores of Cylindrocladium take a while to incubate”
This spring a yelp of fright went up from Saling. Our most-photographed feature is the bank of what are fashionably called clouds clipped over the past 25 years by Eric Kirby with ever-growing ingenuity.
In March they started to die. In part, that is, but enough to scare us silly. I noticed that the dead leaves and shoots were principally on hollows and crevices where fallen leaves had lain, sodden, winter-long. Hoping that this, rather than an imported fungus, was the proximate cause, I sent samples of dead wood to Wisley. As I cut it, though, my spirits rose. Down at the base the old bushes were sprouting new growth. These were not exactly at death’s door.
There was a pause while the scientific staff went to work. The spores of Cylindrocladium take a while to incubate. Volutella, a relatively benign fungus, was found. Three weeks later came the all-clear: no Cylindrocladium. Meanwhile with fingers crossed we had cut the infected bushes back to good new shoots, sprayed them with copper fungicide, fed and mulched them heavily and given them a foliar feed for good measure.
I see now how the bushes were weakened. A quarter-century of clipping, on a gravelly bank, with feeding neglected, leaves a plant with few reserves. Accretions of wet leaves gave fungi an open door. It won’t happen again..
Rose Replant Disease
Replant ‘disease’, or soil sickness, is a problem that can occur when re-establishing plants on sites where the same species has been grown previously. It affects many different plant species, but may be most common with roses.
“Some gardeners line the hole with a cardboard box with its bottom removed; by the time the cardboard rots, the plant should have established”
Symptoms are variable, but above ground are characterised by stunting of growth and a general lack of vigour, often with yellowing of the foliage and poor flowering. If the plant is lifted, on inspection the roots are usually small, dark and compact. Some of the fine roots may be rotten. A healthy plant put into affected soil will deteriorate and may die. More often it survives the decline and slowly improves, but is unlikely to catch up with unaffected plants, even those in close proximity. Moving a stunted plant to fresh soil often aids in its recovery.
The exact causes of replant diseases are not completely understood. In most cases they are thought to be due to microbial agents in the soil, as soil sterilisation prevents the disease. Causes probably vary with different plant species, and in some instances it is possible that other factors–such as exhaustion of certain nutrients or impoverished soil structure–are more important than microbes.
For small areas, changing the soil may be effective (it either removes the microbes or overcomes soil impoverishment). A planting hole should be excavated wider than the full spread of the roots–at least 60cm (2ft) in diameter and 30cm (12in) deep. Fill the hole with soil from another part of the garden or buy in topsoil. Some gardeners line the hole with a cardboard box with its bottom removed; by the time the cardboard rots, the plant should have established. There is some evidence that boosting plant growth by applying high nitrogen fertiliser can help to counteract replant disease (follow the manufacturer’s application rate: too much can do more harm than good).
Choosing plants with resistant rootstocks may help avoid the problem, if the grower indicates which they use–some Rosa canina rootstocks are prone to replant disease. Rosa x dumetorum ‘Laxa’ shows some resistance. Some rose nurseries recommend mycorrhizal fungi–there is some evidence that these can be effective in preventing replant diseases, but the results are not conclusive. They can be applied as granules or dips for bare-root plants. No chemical soil sterilants are available to home gardeners to control replant disease.
Bees Help Counter Strawberry Grey Mould
Spraying strawberries against fungal infections could become a thing of the past, as researchers have found that bumblebees can do the job just as well. Commercial growers often use bumblebee hives for good pollination.
“As they forage through strawberry plants, the bees transfer some powder into each bloom…”
Scientists at East Malling Research in Kent, with agricultural consultant ADAS, have designed a dispenser to fit into the hives that bees must move through on their way out to forage for nectar. As they do so, a tiny quantity of a biofungicide, Gliocladium catenulatum (a fungus that suppresses the growth of grey mould), sticks to their bodies and legs. As they forage through strawberry plants, the bees transfer some powder into each bloom, preventing grey mould being carried onto developing fruit. The bees’ control was just as good as on plants that were sprayed, and fungicide residue on fruit decreased.
