The Perennial Matchmaker

For the most part, you’ll plant and maintain your perennial partnerships just as you would if the plants were growing by themselves. But there are a few special considerations that are worth keeping in mind to get the best out of your combinations.

Some perennials need 2 or 3 years in one spot before they reach their full size and flowering potential; others, such as baptisias (Baptisias) and bugbanes (Cimicifuga), may take 4 years or more. The speed at which they fill in depends a great deal on the growing conditions as well as the sizes of the plants you are starting with. If you’re beginning a bed or border from scratch, setting out the perennials based on the spacings suggested on their pot tags can leave you with a very bare-looking garden for a few years. (It will leave lots of room for weeds, as well, unless you use annuals or lots of mulch to keep the soil covered.) It’s often more satisfying to set the plants closer together the first year, so they look good at planting time, and then be prepared to move some of them in the following years.

Once you’ve been gardening for a while, dividing or transplanting your older perennials in spring or fall gives you opportunities to try out new combinations and to make room for new additions. When you’re creating a combination by pairing an established perennial with a new one, consider dividing the established plant at the same time that you add the new partner, so the clumps will be more closely matched in age; this way, they are more likely to stay in proportion as they mature. Or if you don’t want to disturb the existing plant, set out three or more of the new ones close together to create a clump that’s in good balance, size-wise.

Sometimes the difference between a good combination and a photo-worthy one comes down to a few minutes of attention with your garden shears. To keep perennial pairings in prime condition for as long as possible, regularly trim off dead or discolored leaves (a process sometimes called “deadleafing”). If flowers are the focus of a combination, snip off older blooms as they start to fade or discolor, or cut off entire flowering stems close to the base of the plant when all the flowers are done, unless you also want the seed heads for later-season interest. This “deadheading” not only tidies the plants, but it may also extend their main bloom period or encourage them to flower again later in the growing season, and it may encourage them to produce fresh leafy growth. Perennials that we prize for their foliage, such as heucheras (Heuchera) and hostas, also produce flowers that may spoil the effect of a foliage-based grouping; in that case, you may want to clip off the flowering stems as soon as they appear.

Medium-size to tall perennials may need some help to hold their upright form and produce their flowers at the height you expect. If you find that certain plants are sprawling and spoiling your carefully planned combinations, gently propping them up with half-hoop supports, linking stakes, or Y-stakes may salvage their pairings for the current season. Next year, consider staking them soon after new growth begins; using pruning techniques, as explained below; or giving them shrubs or sturdy perennials as companions for natural support. Or accept that the flowers are going to sprawl, then plan accordingly or just enjoy the often surprising and delightful combinations they can create on their own that way (as long as they’re not smothering their bedmates, of course).

Some well-timed snips with your pruning shears can work wonders with many perennials, changing their height and shape, influencing their bloom time, and affecting the size and distribution of their flowers. Here are just a few ways you can use pruning as part of your combination-care routine; for more specific tips, see the individual plant entries. (Note: Don’t try these techniques with daylilies [Hemerocallis], torch lilies [Kniphofia], true lilies [Lilium], or other perennials with nonbranching flowering stems, or you’ll remove the flowers for that year, too!)

To make plants shorter and bushier. When you remove the stem tips of perennials that have growth points along their stems, you encourage those points to produce side shoots, making for lower, bushier plants. That’s not something you need to do with spring-blooming perennials, which naturally tend to be short, but it’s a big help with summer and fall perennials that might otherwise tend to sprawl once they start flowering, such as chrysanthemums, New England aster (Aster novae-angliae), and upright sedums (Sedum).

Perennial pruning can take the form of light trimming—removing just 1 to 2 inches from the tips—starting in mid-spring. Do this once or twice on perennials that flower in early to midsummer; repeat the process once or twice, a few weeks apart, in early summer on later bloomers. Or simplify matters for the later bloomers with a single hard pruning in early summer, cutting them back to about half their height at that time.

To affect flowering. When you prune perennials to control their size, you’re also changing the way they flower, to some extent. One possible result is a delay in flowering time, which can be very useful if you’d prefer your mid- to late-summer perennials to flower a few weeks later for an impressive late-summer to fall display. Some good candidates for this sort of pruning include chrysanthemums, heleniums (Helenium), ironweeds (Vernonia), Joe-Pye weeds (Eupatorium), and perennial sunflowers (Helianthus). Shear the whole clump by about half once in early summer to delay the bloom time by a week or two, or in midsummer to delay it by a month or so. Or trim off just the shoot tips every few weeks from mid- to late spring into midsummer; most perennials will start flowering 3 to 4 weeks after the last trim.

Perennials that tend to have a few, mostly straight stems in each clump—cardinal flower (Lobelia cardinalis), Joe-Pye weeds, and summer phlox (Phlox paniculata), for example—produce fewer but larger flowers or flower clusters than bushier plants with many-branched stems, and those flowers appear at the tops of the plants. That’s fine when you’re working only with tall plants in a combination or when you’re looking for a real spectacle: In that case, stake the plants instead of pruning them, or give them sturdy companions to lean on. If you’re looking to pair them with shorter plants, though, the same approach you’d use for delaying bloom times can also produce smaller flower clusters that are more evenly distributed over the plant and in better proportion to shorter companions.

Holding Beds

One simple garden project—creating a holding bed—can be an invaluable help in creating outstanding combinations. A holding bed, also known as a nursery bed, is a small plot where you can plant young perennials and let them grow for a while before giving them a permanent place. It’s a great place for raising seedlings and for pampering small mail-order purchases until they’re large enough to go into your garden, as well as for holding excess divisions and impulse plant purchases until you can figure out where you want to use them. It’s also an excellent source of instant replacement plants, if you discover that you’ve lost some perennials after a tough winter or an extended rainy spell.

Though it can be pretty, a holding bed isn’t meant to be ornamental: it’s a practical space for bulking up small plants and observing new acquisitions to see when they flower and exactly what colors they offer. Instead of depending on photos and secondhand information, you’ll see for yourself just how a particular perennial will grow and look in your yard before giving it valuable garden space. And when you’re ready to use it in a combination, you can simply dig it out of the holding bed, plant it next to its new companion, and poof—instant combination!

A holding area can be as simple as a few square feet in one corner of your vegetable garden or a separate space with one or more ground-level or raised beds meant specifically to be a short-term home for perennials. If possible, site it close to an outdoor faucet so you can easily provide supplemental water to the young plants during dry spells. Prepare the soil just as you would for any perennial planting, or enrich it with extra compost to get speedier growth on the young plants. When you dig out clumps for use in the garden, fill the holes with a soil-compost mix, then add new plants as you acquire them.

TIME FOR A TRIM. Many spring and early-summer perennials respond well to a bit of pruning when they finish flowering. Even if they don’t bloom again later in the year, they’ll look much tidier and may be less prone to flopping. Arkansas bluestar (Amsonia hubrichtii), for instance, stays bushier through the rest of the growing season, and lamb’s ears (Stachys byzantina) produces fresh new leaves that are less likely to “melt” in humid summer weather.

PERENNIAL PRUNING EFFECTS. It’s worth experimenting with pruning techniques on your later-flowering perennials to vary their heights, shapes, and bloom times. In the border above, for instance, the clump of Culver’s root (Veronicastrum virginicum) on the right was left untrimmed and began flowering in early July. The clump on the left, cut back by half its height in early June, was shorter and bushier and began flowering a few weeks after its neighbor.

EXTENDING THE SHOW. In larger borders, shearing half of the plants in a particular grouping can stretch the bloom display by several weeks, at least. In the photo at left, the purple coneflower (Echinacea purpurea) plants in front were cut back by about two-thirds in late June, just before they began to bloom. By mid-September, those plants were contributing fresh flowers to the border while those behind them were filled with seedheads. The photo at right shows a large drift of ‘Lemon Queen’ perennial sunflower (Helianthus). The front half was cut back by about half in late June, producing bushier growth that supported the taller, unpruned stems at back. The pruned stems began blooming in late August, about a month after those at the back.