Adrian Higgins - Almanac Unlocks the Timing of Perennials' Blooming Periods

By Adrian Higgins
Thursday, January 15, 2009

What do the demure foamflower of April, the outlandish peony of May and the wild goldenrod of September have in common? They are hardy perennials, plants that bloom in their season and then retreat slowly into the ground, to grow afresh the next year.

A generation or two ago, perennials were viewed as occasional showstoppers. The peony was one, the bearded iris another, the lilies of summer a third. Today, with hundreds of varieties easily obtained, they are the life and soul of the garden. They ebb and flow between the constancy of shrubs and trees and the breathless performance of flowering annuals. Annuals don't know in October that frost is about to wipe the smiles off their faces. Perennials are wiser plants, growing with the warmth of the sun and the soil and shrinking when their flowering is done. They bring a natural rhythm to our gardens and, by extension, our lives.

Tomasz Anisko has written a book that is destined to become a perennial itself, for it unlocks in a practical way one of the hardest aspects of garden design: figuring out which perennials bloom when, exactly.

As curator of plants at Longwood Gardens, he has synthesized plant blooming periods, recorded over seven years at the public garden in Kennett Square, Pa., and the result is "When Perennials Bloom" (Timber Press, 2008, $59.95). The book includes the bloom periods and sequences of 462 varieties. The spotted cranesbill Espresso begins to flower in late April, reaches its peak in mid-May and trails off through June. The flamboyant native rose mallow Lord Baltimore begins to unfurl its huge scarlet flowers in mid-July and peaks into September before waning in early fall. When that hibiscus is well into its peak, the lance leaf hosta joins the party with lavender flower spikes that decline alongside the hibiscus bloom.

Kennett Square is about 100 miles north of Washington, a little less hot and humid than the D.C. climate, but not by much. The bloom sequences might start a week or 10 days earlier here than at Longwood, but the plant choices are essentially the same.

Once you can predict these bloom periods, you can plant perennial combinations with confidence and design for a sequence of blooms at points of the year that interest you. Suddenly, the old idea that the gardening season begins in March and ends by June is quaintly absurd.

"I was trying to put together something that would be of very practical use to both home gardeners and professional landscape designers," Anisko said. "What I found frustrating reading a lot of books on perennials was that the reference to the flowering period is always very vague. If you're a person who enjoys flowers no matter what, that's not so critical. But being around people who are very particular about composing plant compositions and working at a particular effect, this is critical."

There is a dead time during the hottest weeks, between the beginning of July and the end of August. As an armchair exercise in winter, it's fun to look at this almanac and figure out how to correct it. Here are perennials that peak in that eight-week period: purple coneflowers, coreopsis, helianthus Lemon Queen, agastaches, black-eyed Susans, Inula ensifolia, perovskia, swamp milkweed and yarrow Fire King, to name a few.

I have planted areas of my garden for September and October interest, but I wonder what else would work at that time. Lo, the scarlet rose mallow Davis Creek, catmints, knautia, calamintha, caryopteris, toad lilies and the tall tatarian asters.

I am also intrigued by the idea of perennials that bloom for weeks on end, as annuals do, but without the recurring work and constant feeding and watering.

The Fire King yarrow flowers in late May and persists until November. There is a cornflower or centaurea named John Coutts that blooms in May but is still in flower at Halloween. The gaura variety Corrie's Gold cranks up in June and winds down in November.

A lot of this longevity is produced, however, by cutting back the plants as their first flush of bloom begins to fade, spurring them to regrow and rebloom. Many of these rebloomers, Anisko said, are found in habitats where they would be grazed on by animals, and so they developed to resprout and bloom quickly. Flowering, after all, is about reproducing, a primal motivation of all living things.

Interestingly, timing is key to getting a good reflowering. After two varieties of catmint, Blue Wonder and Dropmore, would flower in June and July at Longwood, the gardeners cut them back right away. "Plants rebloomed within 3 to 6 weeks," writes Anisko, "but when trimming back was done after July, rebloom did not occur or was delayed until October."

As he says, "it's not just whether to cut them back, but when." However, some perennials just don't know when to stop, including agastache Pink Panther, astrantia Margery Fish, scabiosa and the Japanese aster Kalimeris pinnatifida. Sometimes, buying the precise variety is key to getting the most out of a plant. Snowcap is a white flowering tradescantia that averaged eight weeks in bloom at Longwood, which makes it a valuable garden plant, while a closely related variety, Concord Grape, was in bloom from April to November, peaking in late May and again in late September.

In an interview, Anisko said it would be a mistake to plant long-flowering perennials alone. "I was careful not to suggest a perennial that blooms for 25 weeks is better than one that blooms for only three weeks," he said. "There's a place for both kinds." Usually, the briefer the show, the more intense and eye-catching it is. Many of the long-flowering perennials work best as filler plants, he said.

It's worth noting that many perennials also are grown for their foliage effect: such varieties as hostas, Solomon's seal and heucheras. The feathery, shrublike Amsonia hubrichtii flowers for a relatively brief time in spring but is a valuable, fine-textured background plant that turns golden yellow in the fall.

The underlying message is that perennials are the class of plant that has drawn us closer to nature in the garden. A garden, by definition, is not untamed nature, but the expanse of perennials at hand today allows us to emulate the meadow, wood and wetlands from March to November and beyond.

"We used to expect that only plants with huge double flowers deserved a place in the garden," Anisko said, "and now plants that used to be relegated to some roadside have a treasured place in the garden as well."

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