Background: Perennial Stems Are a Resource for Pollinators
A perennial garden is a great way to support wildlife, from birds to butterflies to bees. The flowers feed beneficial insects, the seeds feed birds, and the plants provide shelter where even more species can find a safe haven. A garden’s value to wildlife depends not only on what you plant, but also on how you maintain it—hence “leave the leaves” and “save the stems.” But exactly what it means to “save the stems” can be a source of confusion.
Stems of perennial plants can provide nesting and overwintering habitat for several species of native solitary bees (and solitary wasps) in North Carolina. Unlike social insects, such as honey bees and yellowjacket wasps, solitary bees nest individually and do not aggressively defend their nests. Some solitary bee species nest in dead stems that are naturally hollow, and others actively tunnel into previously cut or damaged stems by excavating the pith, that is, the soft, spongy tissue found in the center of most herbaceous stems. (Bees do not generally damage healthy, living stems.) They divide the space inside the stem into a series of chambers and fill each cell with pollen and one egg (Figure 1). The bee larvae develop into adults inside the stem, then fly off to make their own new nest. Depending on the species, larvae or young adult bees may spend the winter inside the stem.
Decisions about when and how to tidy up garden stems can make the difference between habitat creation and habitat destruction for stem-nesting bees. When can you cut back perennial stems without destroying pollinators nesting inside? Should you leave a stubble or cut all the way to the ground? Although many resources on gardening for pollinators suggest waiting until late spring to trim or cut down dead stems, there wasn’t much hard evidence available to support this advice. To find the answers, NC State Extension conducted a study to see when perennial garden stems are occupied by bees and to determine the time of year when plants can be safely trimmed.
We worked with Extension agents and NC State Extension Master GardenerSM volunteers, who sampled stems from 20 gardens in 10 counties across North Carolina. They sent us nearly 3,000 stems in all, collected at four times of year (winter, spring, summer, and fall). Although only about 1.5% of stems contained nests, there were clear seasonal differences. The results show that winter—not spring—is an ideal time to trim stems of perennial plants to provide pollinator habitat.
Figure 1. Bee nests inside perennial plant stems. A. Cocoons of leafcutting bee (Hoplitis) in lollipop verbena. B. Nest of small carpenter bee (Ceratina mikmaqi) in coneflower. C. Nest of leafcutting bee (Megachile) in Anemonella. D. Nest of small carpenter bee (Ceratina calcarata) in Hydrangea.
Photos by Youngsteadt Urban Ecology Lab, CC BY-NC-SA 4.0.
Stems Through the Seasons: Results of the Study
To think about how bees use stems in a garden of perennial plants, we’ll follow a focal group of stems through the seasons—as we did in our study. Think of a perennial plant that sends up new shoots in the spring, flowers in the summer, then dies back in the fall. If you do no garden cleanup at all, these dead stems will remain on the plant throughout the following year. How might bees use them in each season?
Fall–Winter: Bees have to find a place to overwinter no later than the first fall frost. At this time, stems that grew and flowered the previous summer are dead, but they are generally still whole and intact. This means that bees cannot access their hollow or pithy centers to spend the winter. Indeed, in our study, we found no nests or overwintering bees in first-winter stems.
Spring: Once the focal group of dead stems has made it through the winter, they are more likely to have broken ends where bees can access the hollow or pithy centers. In our study, gardeners enhanced this effect by trimming their stems during the winter to leave a stubble of 12 to 24 inches, creating even more cut ends for bees to enter. We found the most nests in stems collected in spring. At this time, about 70% of gardens had at least one occupied stem.
Summer: Even as a perennial plant opens new blooms, the previous year’s stems continue to age on the plant and are still used by bees. In our study, we found a number of active nests during summer, and about 55% of gardens had at least one occupied stem.
Second Fall: The focal group of stems has been dead for nearly a year. Many stems are starting to fall over and disintegrate, but the ones that remain standing can still provide shelter, especially for bees seeking a place to spend the winter. Although the newest stems on the same plant aren’t yet dry or damaged enough to host bees, the year-old stems are perfect. In our study, we found a few occupied stems in the second fall, from about 11% of gardens.
When to Cut Back Dead Stems
The results of this study showed that stems are not occupied in their first winter, so this is a good time to trim them (Figure 2). You can leave the seed heads in place long enough to feed the birds, but if you trim between the first fall frost and the last spring frost, you won’t be discarding bees. If a stem is cut or damaged during the growing season, however, bees could move in at that time. Thus, if you trimmed or deadheaded any stems earlier in the season, do not cut them again in the winter.
Figure 2. Seasonal cycle of stem growth in a perennial plant. In year one, flowering stems grow and die. If they are trimmed in their first winter, they are available as nesting and overwintering habitat throughout the following year—shown here with small carpenter bees in the genus Ceratina.
Illustration by Christina Spence Morgan CC BY-NC-ND 4.0
How Much to Trim
When you maintain your garden, how much should you cut back? Consider three options:
- To create maximum habitat for stem-nesting bees, trim stems back to leave a stubble 12 to 24 inches tall (Figure 3). This amount of trimming can still give the garden a tidy, uniform look. If you cut the stems in their first winter, this stubble will be ready and waiting as soon as early spring bees begin to search for nest locations.
- If you prefer to cut stems all the way to the ground, you can do so in their first winter without destroying bees. Although this approach does not damage stem-nesting bees directly, it also does not benefit them, because it eliminates potential nesting habitat.
- Finally, there is no harm in cultivating an untrimmed, natural look in your garden! From a bee’s perspective, an untrimmed garden will have fewer immediate nesting opportunities than a garden trimmed to stubble. However, untrimmed stems will gradually become available for nesting once natural wear and tear create access to the stem interior.
Figure 3. Ideal stem trimming cycle, seen in summer (A); winter, before trimming (B); and winter, after trimming (C). Solid ovals mark the current year’s stems, which do not contain bees. Dashed ovals mark the previous year’s stems, which may contain bees and are not disturbed.
Photos by Elsa Youngsteadt (summer) and Heather Frantz (winter) CC BY-NC-SA 4.0
What to Plant
Although our study wasn’t intended to identify the best perennials for stem nesting, Table 1 provides a list of all the plants that contained at least one nest. Also listed are plants that other researchers have found to contain nests constructed by bee species that occur in North Carolina. Any plant from this list could potentially be used by stem-nesting bees in your landscape.
Note: This list is not exhaustive, so other plants could also provide nesting resources. The "Source" column indicates whether nests were found in our study of North Carolina gardens or in other published studies. "Mixed" in the "Native to North Carolina" column means that some species in the listed plant genus are native, while others are not.
Who Lives Inside
We found 11 species of bees and solitary wasps nesting inside garden stems. Most nests—and 5 of the 11 species—belonged to small carpenter bees in the genus Ceratina (Figure 4). Not to be confused with the large carpenter bees in the genus Xylocopa, which drill holes in porches and fenceposts, Ceratina bees cannot tunnel into wood. Instead, they excavate soft pith from stems that are already cut or damaged, then use the pith to construct chamber walls within the nest. Each nest can contain about a dozen chambers.
Adult female small carpenter bees start new nests in the spring and raise their offspring over the spring and summer. They are unusual among solitary bees in that mothers reopen the nest chambers to groom the developing larvae to protect them from fungal diseases. Once the larvae develop into young adult bees, the males and females often overwinter together in the nest before dispersing to start new nests the following spring.
Because they are small and not very hairy, Ceratina bees don’t carry a lot of pollen on their bodies (compared to larger, fuzzier species such as bumble bees). As a result, they have received little attention as pollinators in North America. In other parts of the world, however, Ceratina bees are recognized as important pollinators of native plants and crops, and they may be making similar contributions in your garden.
Other bees we found nesting in garden stems included small mason bees (Hoplitis spp.), leafcutting bees (Megachile spp.), and yellow-faced bees (Hylaeus spp.). We also found a few solitary wasps, including grass-carrying wasps (Isodontia spp.) and mason wasps (Euodynerus and Leptochilus spp.).
Figure 4. Small carpenter bee (genus Ceratina). Ceratina bees are about 1/4 inch long.
Photo by Judy Gallagher via Wikimedia Commons CC BY 2.0
Frequently Asked Questions
I don’t want stubble in my garden. Can I cut the stems off at ground level and leave them somewhere else for bees to use? Maybe. We didn’t test this, but logic suggests that if you place cut stems in a partly sunny place, they could still provide useful nesting habitat. Do not leave them directly on the ground, where moisture and mold would destroy any nests. You could try placing them in a large pot with drainage holes or weaving them into a decorative wattle fence.
What about the third year of a perennial stem? In our project, volunteers found that stems started naturally breaking down in their second year—that is, when they had been dead for about a year. If perennial stems are sturdy enough, they could last even longer and have the potential to continue providing nesting habitat for as long as they are standing. Each winter, you would cut the previous season’s growth and leave any remaining older stems alone (Figure 3).
If stems aren’t occupied in the winter, how are bees overwintering in stems? If stems are trimmed according to our recommendations, during their first winter (between the first fall frost and last spring frost), they will not be occupied because the stem was not available for nesting when stem-nesting bees were active during the previous growing season. Bees need the stem to be damaged, broken, or cut so they can access the stem interior and start building their nest. So, when bees are overwintering in stems, they are using stems that were damaged, broken, or cut previously. If you cut your stems during the winter this year, bees can overwinter in them next year.
How do I know if a stem is occupied? You can’t always tell, but a close look at the cut end can give you some clues. If the cut end of a pithy stem has intact pith, there is probably no bee inside. But if there is a hole dug into the pith, it is possible there is, or was, an active nest inside (Figure 5).
Will stem-nesting bees sting me when I’m working in my garden? The species of bees nesting inside stems are small, solitary species. While they may be capable of stinging, they will not attempt it unless they are directly pinched. Unlike honey bees, bumble bees, or yellowjacket wasps, they do not have a workforce to defend the nest. Instead, solitary bees are focused on building their nests and collecting food and are not bothered by people working in the garden.
Figure 5. The stem on the left is not occupied; the pith is undisturbed. The stem on the right is occupied by bees, which have hollowed out the pith and even left a dusting of yellow pollen at the nest entrance.
Photo by Elsa Youngsteadt CC BY-NC-SA 4.0
Related Resources
Roos, Debbie. 2024. "Fall Chores in the Pollinator Garden: Leave the Leaves and Save the Stems!" NC State Extension.
Roos, Debbie. 2025. "Provide Nesting Habitat for Native Bees Through Plant Stems in Your Garden". NC State Extension.
Youngsteadt, Elsa and Meredith Favre. 2022. How to Manage a Successful Bee Hotel. AG-915. NC State Extension.
Levenson, Hannah and Elsa Youngsteadt. 2019 The Bees of North Carolina: An Identification Guide. AG-858. NC State Extension.
Moorman, Christopher and Lucy Bradley. 2025. "How to Create Wildlife-Friendly Landscapes." NC State Extension.
Publication date: May 22, 2025
AG-984
N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.