Growing Aster Praealtus

Hardiness

Willow aster is a herbaceous perennial that survives over winter in a dormant state underground in its root system. Above-ground tissues are killed by progressively colder frosts in autumn, and the remaining debris helps insulate the roots from winter cold. Grow willow aster in U.S. Department of Agriculture plant hardiness zones 4 through 8, where winters are briskly chilly to very cold. Frost-free, mild winters prevent this perennial from becoming fully dormant and can lead to diminished vigor or increased bouts of root rot.

Across its native range, willow aster prospers in full to partial sun exposures, no less than five hours of direct sunlight daily. The soil is at least moderately fertile, as demonstrated by the success of other meadow grasses, weeds and wildflowers nearby. A moist, non-alkaline soil is needed for best growth of the willow aster. It prospers in low, moist soil pockets in fields or ditches, on woodland edges or along ponds and streams. Loam soils are better than fast-draining sands or dense-textured clays.

Depending on soil fertility, abundance of sunshine and soil moisture, willow aster may attain a mature height of 2 to 5 feet. The perennial often is tall and upright, with limited branching. Stake plants in exposed locations to prevent arching stems and floppy growth habit, especially after a plant reaches a height of at least 24 inches. To create a fuller plant, willow aster may be pruned back to one-half its height in early summer to encourage more branching. Do not prune this aster past midsummer, as you risk removing tissues that are beginning to form the year's flower buds. In late winter, cut back the winter-killed stems to the ground to allow more air and light to reach the roots where new growth emerges.

To increase the number of willow asters in the landscape, allow the flowers to remain on the plant across fall into the winter. Each pollinated blossom develops into a seed structure called an achene. The achene ripens to brown with white hairs that catch the wind and help move them across the landscape. Seeds are shed from mid-fall to spring, and germination occurs in spring where the soil is moist and basked with at least a few hours of sunlight. When cutting back the dead tissues in late winter, violently shake the debris to dislodge any seed achenes.