This article was originally published by the Tufts Pollinator Initiative and has been edited for membership clarity.
Written by: Nick Dorian, Tufts Pollinator Initiative (TPI)
The life cycles of New England’s native bees can provide an instructive lesson on supporting native bees. About 30% of New England’s native bees build nests above ground. Besides bee hotels (many of which have their own issues), a great way to support these above-ground nesting bees is to leave dead plant stems standing in gardens. Bees will lay and provision offspring in these hollow or pithy stems. The Tufts Pollinator Initiative (TPI) members are often asked by gardeners, “when is the best time to cut down stems?” The answer is at least every two years (ideally never), which is longer than you might think. Let’s review bee and plant biology to understand why.
Year 1: Plant stems are growing. Native plants like joe-pye weed, elderberry, wild bergamot, mountain mint, and swamp milkweed produce hollow or pithy (e.g. soft, spongy tissue) stems suitable for nesting bees. Bees won’t nest in these actively growing stems. At the end of the growing season (in the northern hemisphere, December through March), we suggest the stems be cut back to between 6-18” tall. Use sharp tools to ensure a clean cut. By cutting back the stems, you create homes for next year’s bees.
In this example, the Joe-pye weed (Eutrochium purpureum) stems were cut back in December 2020. The stems are hollow and will provide homes for twig-nesting bees during the 2021 growing season. TPI will leave the stems standing until the end of the 2022 growing season to ensure all bee offspring have emerged.
Year 2: Bees active during this year will nest in stems left standing. They will lay eggs in the stem and provision each egg with a nutritious ball of pollen and nectar. Inside the stem, bees will develop from eggs into larvae and adults that hibernate through winter. Bees won’t emerge from stems until next growing season. Remember to cut back the new, green stems produced this year for next year’s bees.
Leafcutter bees (Megachile sp.) will live in your garden if you provide undisturbed stems for them to nest in.
Year 3: In spring of year 3, stems produced in year 1 still contain bees; stems produced in year 2 do not contain bees. Leave both generations of stems standing throughout the year. Spring-active bees will emerge from year 1 stems by June, whereas fall-active species might not emerge from year 1 stems until August or early September. During this time, new bees will nest in year 2 stems, so leave them standing!
While this may seem like an awfully long time to leave stubble in a garden, it is the only way to ensure that native bees find safe, undisturbed places to nest. Posting signage in your garden to inform visitors about how gardens can be managed to balance aesthetic and ecological goals can be helpful.