Meadow rues offer beauty for the garden

The merits of meadow rues -- Thalictrum spp. -- include delicate flowers, handsome foliage and graceful habits.

These are diverse perennials well-suited to a variety of garden settings including sunny borders, woodlands and rockeries. The wealth of meadow rues available to gardeners ensures a profusion of pretty blossoms from spring into fall.

Thalictrum is a member of the buttercup family (Ranunculaceae), and a close relative of Aquilegia, Delphinium and Helleborus. Many of the 130 species worldwide are indigenous to Northern temperate zones.

These plants have clump-forming or rhizomatous habits ranging from several inches to 10 feet tall. Their fine-textured leaflets give meadow rues a fernlike appearance.

Chicago Botanic Garden, in USDA Hardiness Zone 5, trialed 20 Thalictrum taxa from 1998 to 2003. These plants included 18 cultivated selections obtained from commercial sources and botanical institutions and two collected from the wild in Russia.

The goal was to find species suited for full-sun gardens. Plants were evaluated for ornamental traits, disease and pest resistance, cultural adaptability and winter hardiness.

Plants received at least 10 hours of direct sun during the growing season and were planted in clay-loam soils with a pH of 7.4 Maintenance was kept to a minimum.

Water was provided as needed but no fertilizer was applied. Plants were not routinely cut back after flowering, but occasionally pruned to the ground to rejuvenate foliage and plant health.

The results

In general, the meadow rues exhibited strong habits and good flower production. The lack of any serious pest or disease problems, combined with winter hardiness, contributed to high ratings for many plants.

Eleven taxa received four-star ratings for overall good performance.

T. flavum ssp. glaucum, yellow meadow rue, is a robust, rhizomatous plant. Spreading stems reached up to 6 feet from the original plant by the fourth year of the trial.

Emerging leaves and young stems are purple, changing to dusty blue-green in June. Yellow flowers (composed of stamens only) are held on broad, 6-inch-wide, terminal inflorescences in June and July. Plant health declined after flowering so stems were regularly pruned to the ground in August.

Leaf miners caused slight cosmetic damage in 2001 and 2003.

T. flavum ‘Illuminator’ foliage emerges yellow with a bronze tint. Leaves eventually change to blue-green as flower buds swell in June.

Stems remain upright through the bloom period, but were occasionally cut after the yellow flowers died back to rejuvenate plant health.

A small amount of leaf miner damage occurred in 2002 and 2003, but it was superficial and didn’t affect plant health.

T. kiusianum, Kyushu meadow rue, was the smallest species in the trial, reaching only 2 inches tall without the flowers. Flower production was especially strong each year, smothering the green foliage in fuzzy, pink flowers at peak bloom.

It remained healthy through the trial and performed better than the slightly taller T. ichangense. No winter injury was observed but, like T. ichangense, it did not compete well with weeds early in the season.

T. lucidum has distinctive linear leaflets. These bi- or tri-ternately compound leaves emerge purple, eventually changing to lustrous green.

This species put on an outstanding show in bloom, producing puffs of pale-yellow flowers in pyramidal clusters up to 9 inches wide. The fluffy effect was due to the clusters of prominent stamens subtended by tiny, insignificant sepals.

T. lucidum has a robust habit with stems that remain upright all summer.

T. minus ‘Adiantifolium,’ lesser meadow rue, has flowers with pendulous yellow stamens held in loose clusters above the foliage. The greenish sepals are inconspicuous and abscised early.

The floral effect is more textural than colorful due to the small size of the flowers and the openness of the inflorescences. The fine-textured foliage, resembling the fronds of maidenhair fern (Adiantum), emerges with a bronze cast before turning blue-green.

Stems stayed upright during the flower period, but typically declined in late summer. In some years stems were cut back in August and plants quickly regenerated into healthy mounds of foliage.

Lesser meadow rue has a slow-growing, rhizomatous habit, but plants remained discrete during the trial period.

T. pubescens, tall meadow rue, was grown from wild-collected seeds from Mount Cuba Center in Greenville, Del. Consequently, variation in habit, stem height and flower production were noted with this species.

In general it exhibited heavy flower production, a robust habit and stiff stems. Creamy-white flowers (stamens only) were produced on stem terminals and in lower leaf axils.

The upper portions of stems typically defoliated after flowering creating an open effect in late summer. Stems were cut back twice in the trial, which led to speedy regeneration of basal leaves.

T. rochebrunianum, lavender mist meadow rue, produces prominent flowers with lavender sepals surrounding clusters of yellow stamens. The persistent and colorful sepals set this species apart from many of the others.

Leaves emerged purple, eventually fading to blue-green by late May.

This species typically has a sparse or see-through quality in its habit. This trait is accentuated by the fact that the lush lower leaves present in early season gradually turn yellow and drop in summer.

Stems remained upright all summer with secondary bloom noted in the lower leaf axils into September.

T. rochebrunianum ‘Lavender Mist’ and ‘Purple Mist’ were both good performers. There’s no discernible difference in floral traits with these cultivars.

Both have foliage that emerge purple, and they’re of comparable size to the species.

But while the species was pest free during the trial, both ‘Lavender Mist’ and ‘Purple Mist’ had minor powdery mildew infections in 1999, 2001 and 2003.

Seedlings of T. rochebrunianum and its cultivars were commonly observed in the test plots.

T. x ‘Elin’ is reportedly a hybrid of T. rochebrunianum and T. flavum ssp. glaucum. But it resembles the former species more than the latter.

Flower color and size is comparable to T. rochebrunianum but ‘Elin’ blooms slightly earlier. A secondary flower period was also noted in the lower leaf axils later in summer too.

The new leaves of ‘Elin’ remain purple longer than T. rochebrunianum, beginning to fade only when flower buds swell in mid-June. The vigor of ‘Elin’ surpasses both parents and the stems ultimately reach 8 feet high.

‘Elin’ maintains a dense, bushy habit through summer and does not drop leaves or thin out. However, severe powdery mildew was observed in 2001 and 2003 and slight leaf miner damage was noted most years.

T. aquilegifolium ‘Purpureum,’ as the species name implies, has foliage that resembles columbine leaves. Its pomponlike flowers are made up of pale-lavender stamens held in broad inflorescences up to 6 inches wide.

Flower color was variable within the test group and only slightly darker than the species. ‘Purpureum’ was generally more robust and floriferous than the species.

This cultivar and its species were weakened by the cultural conditions of the site. Inconsistent moisture and full sun resulted in rapid foliar decline after flowering.

On the other hand, these were the first meadow rues to bloom so the premature decline may have been due to natural senescence.

Powdery mildew was a minor to moderate problem following the flower period each year of the trial.

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- Richard G. Hawke