Spring arrives this month, with meteorological spring on March 1 and astronomical spring (the equinox) on March 20. The meteorological season marks the beginning of the three months between the coldest and warmest quarters of the year. Throughout March, the average high temperature in our region rises about 10 degrees Farenheit, the average low temperature rises about 11 degrees Farenheit, and day length increases by one-and-a-half hours. By any measure, spring is welcome.
Garden tasks include plant orders, pruning, trimming last year’s dried material, and interviewing summer student gardeners. We begin scheduling garden volunteers for sessions held when spring arrives in earnest. Depending on weather late in the month, our professional prescribed fire crew may burn sections of the native plant garden to stimulate growth and health of fire-adapted vegetation.
In sites with full sun, shrubs bearing spring or early summer flowers will attract pollinators and later yield summer fruits that birds and wildlife use. Buttonbush (Cephalanthus occidentalis) is a shrub in the Madder family that grows best in moist soils. Its flowers, held in one-inch-diameter globe-shaped clusters, begin blooming in June. Insect visitors include bees of all sizes and many butterfly species. This shrub is highly branched with a rounded form and can grow 10 to 20 feet tall.
Three shrubs in the Rose family also bloom early in the season: American plum (Prunus americana), ninebark (Physocarpus opulifolius), and black chokeberry (Aronia melanocarpa). American plum’s white flowers bloom March to May for about 7 to 10 days, before the leaves expand. Floral visitors include flies and a variety of bees, especially spring solitary Andrena species. It reaches 8 to 10 feet in height and may produce suckers, resulting in a cluster of shrubs over time. The fruits in summer and leaves in fall turn yellow, orange, and red. Ninebark blooms as clustered white flowers with yellow centers from May to June, after the leaves have expanded. This shrub is up to 10 feet tall and has multiple stems, with shredding bark on the mature trunks. It can be grown under a range of moisture and light conditions. Insects visiting its flowers include bees, wasps, flies, and butterflies. It has dry fruits and is effective at self-seeding. Black chokeberry is a smaller shrub (up to 8 feet tall) and can grow in wet or dry soil conditions. Its white flowers, held in small clusters, bloom over several weeks from May to June. They are visited by bumblebees, early season solitary bees, and flies.
Many herbaceous plants for full sun bloom in the spring and early summer seasons. In dry to mesic sites, prairie-smoke (Geum triflorum) and butterfly-weed (Asclepias tuberosa) bloom in spring and early summer, respectively. Because of its small size, prairie-smoke should be grown in open sites without competition from larger plants. Well suited to rocky or gravel soils, it is visited by bumble bees seeking nectar and pollen. It spreads by wind-dispersed seed and can also form small plants from rhizomes of a larger plant. Butterfly-weed also spreads by wind-dispersed seed but is not rhizomatous. Its flowers can vary from coral-red, orange, or, rarely, golden yellow, and are visited by butterflies, bees, flies, wasps, and hummingbirds that feed on nectar. It has no milky sap, but monarch larvae, milkweed beetles, and milkweed bugs feed on the plants.
In dry-mesic to wet-mesic soils, the bright yellow flowers of golden Alexanders (Zizia species) are among the first to bloom. They are easy to grow and self-seed readily from year to year. In full sun, on mesic to moist soils, blue flag iris (Iris virginica) spreads clonally with thick rhizomes and blooms in June. When the patch becomes large, rhizomes bearing buds can be cut and transplanted. If you leave the flower on the plant after blooming, it will develop into a pod packed with brown seeds when mature.
The native plants highlighted here (and more) are available through the Friends of the Arboretum’s online plant sale (orders due March 15 for pickup in May) and tent sale (held at the Arboretum on May 20). Consider adding spring and early summer blooms to your sunny native plant garden this season.
—Susan Carpenter, native plant garden curator