By Kathleen Cue, Nebraska Extension Horticulture Educator in Dodge County (Week of October 25, 2021)
If you are a creature of tidiness, here is a thought to ponder. Cutting back the foliage of flowering perennials, ornamental grasses, and small shrubs is damaging to native bees that overwinter in the hollow stems of plants. Moisture seeps down through the open ends, killing the eggs, larvae, and pupae of native bees that are set to emerge as adult bees come spring.
According to Dr. Jody Green, Entomology Educator with Nebraska Extension, roughly 30% of solitary bees are cavity nesters, utilizing the hollow spaces in plant stems, and twigs of trees and shrubs to lay their eggs. These eggs must stay dry over the winter months so eggs can hatch, larvae will develop into pupae, and pupae into adults. Hollow stems serve as incubators for this process, underscoring the necessity for protecting the integrity of plant stems from freezing rain and snow.
Where possible, holding off on stem removal until spring is the best thing for native solitary bees. Granted, there are instances when cutting back the diseased foliage of peonies and other perennials is necessary to decrease the amount of disease incidence next year. Where plant foliage has been disease-free, however, these materials can be left in place to help the native solitary bees.
In spring, freezing rain no longer poses a threat to bees, so the stems of flowering perennials, small shrubs, and ornamental grasses may be cut back then. “But wait! There is more to this,” says Dr. Green. “Solitary bees emerge over an extended period, so removing all the spent foliage in spring risks killing bees that haven’t emerged yet.” To remedy this, Green suggests cutting back stems, leaving lengths from 8 to 24 inches intact. This allows native bees a fighting chance at survival. The new foliage, as it develops, will cover up last year’s stems, and old stems will eventually decompose.
Bee hotels are an artificial way to add hollow “stems” to a landscape, but be aware these nesting sites concentrate bee numbers, making it easier for diseases and parasites to run rampant through populations. From a bee health perspective, nothing beats intact hollow stems of plants for overwintering success.
A coneflower set in spring. Photo credit to J. Green.