In May spring gardening is at its peak, with consistently warming temperatures, long day lengths, high sun angle, workable soils, and vigorous new growth. In woodlands, early season natives bloom and begin to set seed. For later blooming species, May is an ideal planting time. Plant orders arrive and the Friends of the Arboretum will hold their annual Native Plant Sale on May 13. Volunteers and students join in garden and restoration work. Visitors can choose from eight May tours in our gardens and restorations or simply admire the Arboretum’s spring flowers.
This month in the woodland gardens, sunlight levels decrease on the forest floor throughout the month as the trees leaf out. Spring ephemerals complete this season’s growth and flowering. Some early species, like spring beauty (Claytonia virginica), Virginia bluebells (Mertensia virginica), or toothwort (Cardamine concatenata) have set seed. In these species, the entire plant senesces, turns yellow, and goes dormant until next spring.
Note that some spring ephemerals for sale at the FOA plant sale could be past blooming and dormant. Simply plant the roots (and soil), as you would if they were not dormant, in a hole slightly wider than the soil in the pot but at the same soil surface level. Water the area carefully and apply a light leaf mulch to conserve moisture. You may want to mark or map the locations so you remember the plant’s location.
Larger spring-blooming woodland species like bellwort (Uvularia grandiflora), early meadow-rue (Thalictrum dioicum), blue cohosh (Caulophyllum thalictroides), bloodroot (Sanguinaria canadensis), and miterwort (Mitella diphylla) emerged more than a month ago and will finish blooming and develop fruit this month. These plants reach full height before tree leaf-out. Their leaves are more shade-tolerant and will remain green even after the tree canopy closes and the seeds have been dispersed. Typically, mayapples (Podophyllum peltatum) ripen fruit toward the end of the month. A few late summer blooming woodland species like zig-zag goldenrod (Solidago flexicaulis) and Short’s aster (Symphyotrichum shortii) show in the woodland gardens as short rosettes.
In our savanna and prairie gardens, a few species act as spring ephemerals. For example, shooting-star (Dodecatheon meadia) emerges early and blooms this month before the leaves yellow and senesce later in May or June. Its capsule-bearing stalk is persistent for months; you will know where it is located in your garden after the leaves are gone. Wait until the capsules are dry and brown before you harvest seed from them. This species grows slowly from seed—reportedly reaching flowering size in several years!
Other prairie and savanna plants can be characterized as cool-season or warm-season plants. These terms correspond to the times of year when the plant has its most active growth. Cool-season plants thrive in the early months, reaching full size and blooming during May and early June, then becoming dormant or slowing growth during the summer. As the weather cools in fall, they may begin growing again. Some native examples include: blue-joint grass (Calamagrostis canadensis), the rye grasses (Elymus species), June grass (Koeleria macrantha), golden alexanders (Zizia species),s and angelica (Angelica atropurpurea).
Warm-season species emerge and have their best growth during the summer months. They can begin vigorous growth in May after a spring burn, or later in the absence of fire. The bluestems (Andropogon gerardii) and (Schizachyrium scoparius), switch grass (Panicum virgatum), and many other prairie and savanna grasses and forbs are in this category. They go into dormancy in the cooler fall months.
You will find these plants and many more at the FOA Native Plant sale under the tent on May 13. See you there!
—Susan Carpenter, Arboretum native plant gardener