New Insights on Cutbacks and "Save the Stems" for Overwintering Insects

You may have heard that many solitary bees and wasps depend on standing stems over the winter, hence the garden practices of "save the stems" and "garden stubble" (if you haven't heard of these, check out our blog post).

As native insect enthusiasts, we want to make sure we are providing the highest quality winter homes for our friends, but there are still a lot of questions out there about what the bees like best! Here at Rooted Gardens, we tried sending out an extensive survey to our insects about their housing concerns, but ran into trouble as none of them seem to be able to write and we never got any responses. Well, alas, we will just have to rely on this thing called "science."

Here's what we know: in the fall or spring, solitary bees and wasps seek out hollow stems or excavate pithy stems. They then segment the stem into little chambers that they fill with pollen and one egg each (everyone gets their own room!). The eggs hatch into larvae and depending on the species, either the larvae or young bees will spend the winter in their stem before emerging in the spring.

A carpenter bee nest in a hollow stem (photo courtesy of the Xerces Society)

Generally we've heard that late spring is the best time to cut back stems, but recent research from the North Carolina State Extension in collaboration with NC State Extension Master Gardeners says this might not be the case. They collected nearly 3,000 stems from across North Carolina and found that the best time to cut stems is in the winter (between the last fall frost and the first spring frost). Bees will generally only use stems that have an entry point (a broken or cut stem) made for them already- they won't create a hole in an intact stem. Cutting in the winter means that you won't have to worry about disrupting a nest, since nobody has moved in yet. This also has the benefit of actually creating new habitat for them to use the next season if the stems are left 12-24" tall. In the spring, residents can move into the newly renovated apartments, where some species will spend the entire summer and next winter raising their offspring. Others will find a stem during the fall to overwinter in.

If you prefer to cut your stems back all the way, winter is still the best time to do it as you can be certain you won't be disrupting a nest! However, this doesn't provide the benefit of creating new potential bee houses, so we suggest you "save the stems!" These brown stems of last season quickly get covered up with the green flush of spring growth and will be quietly increasing habitat in your garden all season long.

One sign that someone's living in a plant stem is a pile of wood shavings!

There's still a lot of information we don't know, like what plant species are favorites for bees, but the study provides a non-exhaustive list where bees have been found. Some of our favorites include purple coneflower (Echinacea purpurea), Blazing Star (Liatris spicata), Bee Balm (Monarda fistulosa), and Ironweed (Vernonia spp). Consider adding some of these to your garden to create a little bee city! And if you find a bee nest in your garden, submit it to the Xerces Society so they can figure out what resources our bees need!