Black walnut trees are a beautiful and and useful addition to the landscape. Their only fault is a chemical called juglone that is produced by the roots. Juglone has a deleterious effect on plant growth.
Susceptible trees, shrubs and perennials planted too close to a black walnut tree will begin to decline and die, usually within a few months. Luckily, a few ornamental plants tolerate juglone successfully.
Columbines are low-growing perennials that bloom in a wide range of colors, depending on the variety. They are tolerant of shade and will grow in full sun if given regular water at least once a week during hot weather. Prune back foliage and spent flower stalks after flowering to tidy the plant and encourage more flowers the following spring.
Columbines tend to have a rather short lifespan of three to four years, but they self-seed readily if the flower spikes are left on the plant. Most columbines are hardy in USDA zones 3 through 9, but a light coating of mulch in colder zones will prevent the crown of the plant from freeze damage.
Brunnera has large, heart-shaped leaves widely veined in silver. Some cultivars such as 'Looking Glass' have such extensive silver coloration that the foliage appears chrome colored. This is a great perennial for brightening up the darkest shady areas beneath large trees.
Brunnera blooms in early summer, bearing clouds of tiny blue flowers high above the foliage. Although drought tolerant once established, young plants should be watered weekly. Brunnera is hardy in zones 3 through 8.
Heucheras are primarily grown for their foliage, which can be shades of purple, gold, chartreuse, variegated and cream. They look their best planted in large groups. Flowers are small but striking, growing on large spikes in colors of pink and white. Heucheras will grow in sun or shade, with paler cultivars requiring more shade than those with darker leaves.
Heavy rain can wash away soil from the base of the plant, leaving the root and crown exposed. A light dressing of compost in the spring and fall will cover the root and increase winter hardiness. Heucheras grow in zones 3 through 9.
Hostas have a staggering number of varietal differences. Foliage color can range from pale cream to blue, with numerous variegated cultivars as well. Hostas are very sturdy plants that will grow in sun or shade, but new plantings in full sun will need regular water. Flowers are small but lovely, with colors such as white, pink, and lavender.
Hostas are the most reliable perennials for growing beneath a black walnut tree, as they are tough, drought tolerant, and seemingly immune to juglone. Hostas are hardy in zones 4 through 9.
Tricyrtis hirta (Toad Lily) is an beautiful plant with an unfortunate name. The toad lily produces flowers that resemble miniature orchids, accompanied by dark green foliage on jointed stems. Tricyrtis has the added advantage of blooming in late summer, when the rest of the garden is winding down for the season. Toad lilies make excellent cut flowers, although fragrance is minimal.
Most species will grow in partial to full shade. Variegated varieties need shelter from hot afternoon sun. Tricyrtis is hardy in zones 5 through 9.