Gardening with Native Plants: Spring Plants and Pollinators

Two-spotted bumble bee foraging on pendulous flowers of gooseberry.

Two-spotted bumble bee (Bombus bimaculatus) foraging on pendulous flowers of gooseberry (Ribes spp.)

Even if winter lingers in March, April leads to true spring: longer and warmer days, adequate rainfall, and changing colors as tree buds swell and burst. By the end of the month, monochromatic landscapes are brushed with many subtle shades of green. Garden tasks include trimming or raking last year’s plant material, checking that sprouting plants are not buried under thick leaves, looking for early emerging pollinators, and enjoying the earliest growth and spring flowers. In the Native Plant Garden, if conditions permit, our prescribed fire crew will burn off dried plant material in some prairie, savanna, and rain gardens. North of the Visitor Center, I am preparing a new prairie garden area, for planting in May.

If you left plants standing in your garden last fall to protect overwintering insects and provide seeds for birds and small animals, you can trim them back now that we have reached consistent daytime temperatures of 50° F. Some of the insects may have emerged already. Insect- and pollinator-friendly garden “clean-up” practices include: 1) trimming to leave a 12- to 18-inch stubble of dried stems (for those species where stems are still standing tall) which can become stem nesting habitat this coming season, and 2) loosely piling or windrowing the trimmed or raked material so that any insects can emerge from the piles.

Leaf cluster where earthworms pulled the leaves into burrow.
Leaf cluster where earthworms pulled the leaves into burrow.

Late in March and early in April, you may notice earthworm activity in the garden and lawn. Deep-dwelling earthworms move up through the soil, leaving castings at the surface and pulling leaves down partially into their burrow. Look for “tufts” of leaves on the soil as well as castings. We usually do not find surface-dwelling jumping worms and deep-dwelling earthworms at the same place, so finding these signs may indicate that jumping worms are not present. Later in the season, I will sample by doing a mustard pour to see which earthworm species are present.

This month we usually see the first native bees of the season visiting early flowering species like willow (Salix spp.), cherry and plum (Prunus spp.), serviceberry (Amelanchier spp.) and even creeping Charlie (Glechoma hederacea). Photography is an excellent way to monitor bumble bees, freezing action so you can study features, and providing a record of time, place, and plants visited. (Sign up for Bumble Bee Brigade to learn more and contribute to what we know about Wisconsin bumble bees.) The first bumble bee to emerge is usually the two-spotted bumble bee (Bombus bimaculatus). Look for two small yellow spots on the second segment of the bee’s abdomen.

Bumble bee queens are large. Besides foraging for pollen and nectar on plants, she searches for and explores nest sites. She must establish a colony quickly, despite cold or rainy weather. Bumble bees are social bees with annual colonies. The nest may be in compost, an abandoned burrow, in a building foundation. If you see a queen traveling in and out of an opening carrying a pollen load, she has established her nest. Protect the location if you can.

Bird's-foot violet in bloom
Bird’s-foot violet (Viola pedata) needs full sun and well drained sandy or rocky soil. It is the larval host plant for several fritillary butterfly species. It is visited by bumble bees and butterflies, especially skippers.

Only about 15 percent of Wisconsin’s 400–500 species of native bees are social, like bumble bees, or semi-social. The majority of species are solitary bees, meaning that they do not live in colonies. Most of the solitary bees are ground nesting. They emerge as adults from underground chambers and begin foraging. They mate and the female bees begin to lay eggs and provision them with pollen in small cells in the underground chambers. For example, the unequal cellophane bee (Colletes inaequalis) nests in areas of Longenecker Horticultural Gardens where soils are suitable for digging the chambers. The bees are usually active for two weeks or even less, foraging on willow flowers and other plants. They are generalist feeders, not limited to one or only a few plant species.

Some solitary bee species are cavity nesters. An example active in early spring at the Arboretum is the Mason bee (Osmia spp.). These bees use hollow stems, logs, or crevices to lay and provision eggs. We’ve observed these generalists foraging on willow or early blooming cherry trees (Prunus spp.).

How many early season bees are you noticing in April, and which plants are they visiting? Enjoy long-anticipated spring changes in your garden, and the chance to see and learn something new every day.

—Susan Carpenter, native plant gardener