February is usually snowy and cold but always the shortest wintry month. Thawing and increasing daylength hint at the end of winter and the beginning of mud season. Garden tasks include pruning and checking for storm damage. Indoor activities include presentations, planning the annual native gardening conference, research, and finalizing plant orders.
As the Arboretum Native Plant Garden developed over two decades, small trees planted in the early 2000s grew quickly and now shade several garden beds. Savanna gardens were planted with appropriate understory forbs and grasses, but the tree and shrub bed began as a mulched bed with scattered specimens. Over time, we have planted forbs and sedges in shade and have seen many other species seed into the beds. This conversion from mulch beds to plant-filled beds increased plant diversity. Using less mulch reduced habitat for jumping worms, which were found in the garden twelve to thirteen years ago. The dense perennial ground layer and increased shading from trees reduce weed success, as many garden weeds require disturbance and more sun. Exceptions include bird-dispersed fruit-bearing tree and shrub seedlings such as buckthorn (Rhamnus cathartica), invasive honeysuckle (Lonicera sp.) and burning bush (Euonymous alatus) or shade-tolerant weeds like garlic mustard (rare in our garden).
A frequent question from gardeners is how to manage space under mature trees where turf grass will no longer grow. Instead of trying to reseed turf grass or applying a thick mulch, try “soft landings” using diverse native plants, an idea that is well illustrated on Heather Holm’s website. In this approach, in the area from the tree trunk to the dripline (the area under the outer circumference of the branches), carefully plant native plants (avoid disturbing tree roots) and leave fallen leaves in a loose layer. Do not add soil, fertilizer, or compost as those won’t benefit native plants, but may harm the tree’s root system. If the tree you are planting under is a native species, it may be host to many species of native insect larvae. Some of the larvae are eaten by adult birds, nestlings, or fledglings. After feeding on foliage, some of the insect species spend a pupal stage in the ground layer under trees and then develop into adults. The “soft landing” refers to the zone you create by following the practices noted above and not mowing.
The most appropriate species for these plantings are woodland or savanna species, depending on light conditions at each site. Consider the path of the sun, edges where light changes in the garden, exposure (which direction does it face?), filtered light through the tree or heavy shade, effect of buildings nearby, etc. Suitable species for dappled shade and edges include American bellflower (Campanulastrum americanum), downy wood mint (Blephilia ciliata), Short’s aster (Symphyotrichum shortii), and blue giant hyssop (Agastache foeniculum), all with blue or purple flowers held on plants 3 to 4 feet tall. These self-seed readily.
The following white-flowered species are easy to grow: Culver’s root (Veronicastrum virginicum), hairy wood mint (Blephilia hirsuta), and wild strawberry (Fragaria virginiana). The first two have small flowers held on spikes on plants 2.5 to 4 feet tall, and wild strawberry is a ground cover with tiny edible fruits that spreads vegetatively by runners.
Wild geranium (Geranium maculatum) is about 2 feet tall when blooming and has pale purple flowers, and Sweet Joe-pye weed (Eutrochium purpureum) can reach 6 feet tall when blooming, with pink flowers. Wild columbine (Aquilegia canadensis) and zigzag goldenrod (Solidago flexicaulis) bloom early and late in the season, respectively. Columbine, with red-and-yellow flowers, will self-seed in edges on bare or disturbed soil, and the goldenrod will spread by rhizome (underground stems). In light shade, grasses like bottlebrush grass (Elymus hystrix) or prairie dropseed (Sporobolus heterolepis) and sedges like Sprengel’s sedge (Carex sprengelii) and Pennsylvania sedge (Carex pensylvanica) may thrive.
Many of these native plants are available from the Friends of the Arboretum online sale. Order plants now through March 15 and pick up your order in May – just in time to plant a “soft landing” area this spring.
—Susan Carpenter, native plant garden curator