OF THE MANY PERENNIALS in my garden, few are more versatile and graceful than the thalictrums, or meadow rues. (The common name comes from the resemblance of their ferny foliage to that of the common rue, Ruta graveolens, although in fact they belong to the buttercup family, or Ranunculaceae.) With over 130 species, the genus Thalictrum represents a remarkably diverse group. All, however, share certain traits that make them prized by gardeners: multitudes of small flowers, usually in soft pastel tints, that cluster in panicles, and lacy foliage that is an asset to the border both before and after flowering.
Of the 26 species and cultivars that at one time or another I have included in my garden in Kingston, Washington (USDA Zone 8), I have found that most are best used as secondary components in the border, rock garden, or woodland. In addition to their clouds of color, they contribute polish and continuity to any grouping of plants without overwhelming the intended effect. In most species, the color of the individual flowers is provided by a feathery bundle of dangling stamens. Unlike the stamens of the bold and nectar-rich flowers of other members of the buttercup family, these stamens depend on the wind rather than insects to deliver pollen to nearby plants.
For the most part, thalictrums are quite hardy. Nearly all species will thrive in Zones 5 through 8, and several species will tolerate even lower temperatures. As a group, they are among the most undemanding plants in my garden. Most of the species will grow in partial shade; indeed, in their native haunts, many are found growing as woodland herbs. The taller border species, however, respond best to full sun. Don’t be surprised, though, if their rate of growth isn’t as rapid as that other perennials individual plants will gain in size slowly but surely each year.
The flowering stems of the more diminutive species do not require staking and remain upright and sturdy long after the flowers fade. The taller species, however, especially, if they are grown in rich, moist soil, will easily reach six feet in height and will begin to topple when the flowers start to expand. While some gardeners may find this trait objectionable and wish to provide some kind of support (see “Staking Thalictrums,”), I find that the leaning stems can provide a charming pastel complement to the blossoms and foliage of sturdier neighbors, whether shrubs or other perennials.
Thalictrums for the rock garden
Equally useful as a low groundcover and in the rock garden, T. kiusianum (Zone 6) is a carpeting species that produces finely textured foliage on four- to six-inch stems that increase slowly by stolons. The lilac flowers are held just above the leaves from May through August. This species is found growing throughout Japan, but because it has never been known to produce fertile seed, it is thought to be a naturally occurring but sterile hybrid of unknown origin. Content in full sun as well as in the semishade of the woodland garden, it can be increased by division in early spring, just as growth resumes.
Thalictrum coreanum (also known as T ichangense; Zone 5) has distinctively broad, oval, shining green leaflets that are deceptively similar to the foliage of epimediums. In May, this species produces pale pink flowers on six-inch stems, provided the plants are in full sun; the stems on plants in shady sites stretch to eight inches. If both sexes are, present, seed will be freely produced, though as with other species, the clumps are also easily divided.
For many years I have grown a thalictrum that was first collected in Afghanistan and that subsequently made the horticultural rounds throughout the United States and Europe as “Thalictrum sp. Afghanistan.” Recently identified as T isopyroides (Zone 6), it is easily one of the most beautiful meadow rues in my collection. Tiny, delicate leaves of intense steely blue cloak stems that rise to 15 inches; the small, greenish yellow flowers are produced in early summer. This plant should be grown in a sunny rock garden or in the front of a sharply drained border. I have used it to advantage in a blue and yellow border with a low carpet of Stachys byzantina `Primrose Heron’ in front and the slightly taller Veronica peduncularis `Georgia Blue’ on one side. Though the thalictrum’s flowers appear long after the starry, dark blue blossoms of the veronica have faded, the blue-and-yellow theme is maintained by Lathyrus sativus, which snakes through the meadow rue’s foliage and produces its electric blue pea flowers at the same time as the thalictrum puts forth its yellow ones.
Thalictrums for the border
The common meadow rue, T. aquilegiifolium Zone 5), is perhaps the best known of the cultivated species, and with good reason. In early summer, three- to four-foot stems topped with ethereal lilac-pink flowers emerge from ferny clumps of foliage. For a striking contrast, I have planted this species at the base of a dark-red-leaved Acer palmatum. In another pairing, the nearly black flowers of Iris chrysographes mingle with the thalictrum’s feathery flowers to create a composition of singular beauty. The flower color of T. aquilegiifolium is quite variable, ranging from pure white in the variety album through deep rosepink in the cultivar `Purpureum’. I have found the white-flowered form useful for brightening a planting of dark red double columbines backed by red-leaved barberries and the blue-flushed-pink foliage of Rosa glauca.
In early spring, the young purple-mauve stems and foliage of T. flavum subsp. glaucum (Zone 5) begin their thrust upward from the soil, soon to intensify in color to a striking silvery blue. It is not until early summer, though, that the flowers of soft yellow open above the lovely six-foot columns of foliage. Initially, the stems are upright and strong, but soon after the flowers expand they begin to lean, gracefully at first. In my garden I find these pastel feather dusters a perfect companion for the black-purple foliage of Cotinus coggygria ‘Purpureus‘, which helps to hold the thalictrum’s stems upright, even after rains.
Midsummer sees the flowering of two species that are among the most beautiful of the genus. Thalictrum delavayi (also known as T. dipterocarpum; Zone 4) sports handsome foliage that is more finely textured than that of many other meadow rues. Rather than clustering in dense, terminal, fluffy heads, the flowers are displayed in long airy panicles on stems that rise to five feet. Each flower is like a clematis of minutest proportions, with lavender sepals surrounding creamy yellow stamens. Though I sited my original plant in the middle of the border, self-sown seedlings have crept forward to frontal positions. The plant might seem too tall for this location, but the nearly transparent, misty-lavender veil of flowers enhances the beauty of the perennials behind it. This species offers two superb variants. The well-known white garden at Sissinghurst Castle in Kent, England, relies on a sizable stand of T. delavayi `Album’ to produce a striking, late-summer haze. In contrast, T. d. `Hewitt’s Double’ is the usual lavender, but sacrifices the yellow stamens in each flower for a double dose of sepals. The flowers of this selection are extremely long-lived and will easily provide eight weeks of color, weather permitting, making it ideal for cutting. This clone harmonizes beautifully with the deep-blue-to-pink mophead hydrangea `Kluis Superba’ and offers textural contrast’ to the shrub’s bold leaves and flower heads.
Thalictrum rochebrunianum is the most commonly cultivated of a trio of relatively large-flowered meadow rues that also includes T. chelidonii and T. diffusiflorum. Descriptions of all three species in the gardening literature are so contradictory that it may take years to sort out the taxonomic muddle. Fortunately, this need not interfere with our appreciation of these lovely plants. The large pink and white flowers of T. rochebrunianum (Zone 5) appear in sprays similar to those of T. delavayi, but the stems are dark purple rather than green. Unlike other species I have grown, it is slow to establish, but it seems happy in a partially shaded site on the edge of our woodland garden. A few summers ago I flowered T diffusiflorum (Zone 6) for the first time. Quarter-size blossoms of blue and white appeared, albeit sparingly, atop two-foot stems and generated considerable excitement among our nursery staff and visitors. Thalictrum chelidonii (Zone 6) has also bloomed for us here. Its blue and yellow flowers are astonishing, but the plant’s diminutive six-inch size makes it difficult to enjoy them. The relatively large flowers of these three species make it easy to see their close relationship to the columbines and clematis.
One of the most pleasant surprises of late has bipen T. actaeifolium, A Korean native with broad leaflets that superficially resemble those of the baneberries (Actaea spp.), it has been a reliable performer here in both the woodland garden and the perennial borders. Its pink and yellow flowers are produced on sturdy, two-foot stems for many weeks from mid-to late summer, making it a valuable companion for agapanthus, schizostylis, and Aster xfrikartii.
Given their variety, their sturdy constitution, and their undeniable charm, it is surprising that the meadow rues are not more popular with the gardening public. Even a slight acquaintance with these plants, however, is likely to lead to full-blown infatuation. Whether your taste runs to classic border perennials, rockgarden treasures, or intriguing new introductions, there is a thalictrum for every garden and every gardener.