Background: Perennial Stems are a Resource for Pollinators
A perennial garden is a great way to support wildlife, from birds to butterflies and bees. The flowers feed beneficial insects, the seeds feed birds, and the plants themselves provide shelter and texture where even more species can find a safe haven. It’s no secret that a garden’s value for 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.
Perennial stems can provide nesting and overwintering habitat for several species of solitary native bees in North Carolina. Some species nest in dead stems that are naturally hollow, and others actively tunnel into the pith of previously cut or damaged stems. (Bees do not generally damage healthy, living stems.) They divide the space inside the stem into a series of chambers, each one filled 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 throwing away pollinators nesting inside? Should you leave a stubble or cut all the way to the ground? Since there wasn’t much hard evidence available, NC State Extension conducted a study to see when perennial garden stems are actually occupied by bees, and to find a time of year when they could be safely trimmed.
We worked with Extension agents and Extension Master Gardener℠ 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.
Figure 1. Examples of bee nests inside perennial plant stems. A. Cocoons of the leafcutting bee Hoplitis in lollipop verbena. B. Nest of the small carpenter bee Ceratina mikmaqi in coneflower. C. Nest of leafcutting bee (Megachile) in Anemonella. D. Nest of the small carpenter bee Ceratina calcarata in Hydrangea.
Youngsteadt Urban Ecology Lab CC BY-NC-SA 4.0
Stems Through the Seasons: Results of the Study
Winter: At this point, stems that grew and flowered the previous summer have died. Stems are mostly whole and undamaged—meaning bees have had no opportunity to enter the hollow or pithy centers. As expected, we found zero nests or overwintering bees in first-winter stems.
Spring: Now last year’s stems have made it through the winter, and are more likely to have broken ends where bees can access the hollow or pithy centers. In our study, participants trimmed their stems to leave a stubble of 12-24” in the first winter, making even more cut ends where bees could enter. Spring was the season when we found the most nests in stems. At this point, about 70% of gardens had at least one occupied stem in their stem samples.
Summer: The stems have aged a few more months. We still found a number of nests during summer, and about 55% of gardens had at least one occupied stem.
Fall: The stems have been dead for nearly a year. Many are starting to fall over and disintegrate, but the ones that remain standing are still providing habitat. We found a few occupied stems in the fall, too, from about 11% of gardens. Bees that are inside the stems in the fall will probably stay there the rest of the winter.
When to Cut Back Dead Stems
The results of this study showed that stems are not occupied in their first winter, and 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 in the winter (between the first frost and the last frost), you won’t be throwing away bees.
If a stem is cut or damaged during the growing season, however, bees could move in at that time. So if you do trim or deadhead any stems earlier in the season, you would not cut those stems 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 (genus Ceratina).
Illustration by Christina Spence Morgan CC BY-NC-ND 4.0
How Much to Trim
When you do clean up your garden, how much should you cut back? Consider three options:
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To create maximum habitat for stem-nesting bees, trim stems back to leave a 12-24” tall stubble (Figure 3). This amount of trimming can still give the garden a tidier, more 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.
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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 by removing the stems.
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Finally, there is no harm in leaving an untrimmed, natural look in your garden! From a bee’s perspective, an untrimmed garden will have fewer nesting opportunities than a garden trimmed to stubble. Even so, untrimmed stems will gradually become available for nesting as natural wear and tear creates access to the stem interior.
Figure 3. A plant undergoing the recommended stem trimming cycle, as seen in summer (A), winter before trimming (B), and winter after trimming (C). Solid ovals indicate the current year’s stems, which do not yet contain bees. Dashed ovals indicate the previous year’s stems, which could contain bees and are left undisturbed.
Elsa Youngsteadt (summer), Heather Frantz (winter) CC BY-NC-SA 4.0
What to Plant
Although our study wasn’t designed to identify the best stem species for 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.
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 1B, Figure 1D, Figure 4). These small, solitary bees build nests by excavating soft pith from cut or damaged stems, then use the pith to construct chamber walls within the nest. Each nest could contain about a dozen chambers.
Adult 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, protecting them from fungal diseases. Once the larvae develop into young adult bees, the brothers and sisters often overwinter together in the nest before dispersing to start new nests the following spring.
Because they are small and not very hairy, Ceratina 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 regions, however, Ceratina bees are 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; Figure 1A), leafcutting bees (Megachile; Figure 1C), and yellow-faced bees (Hylaeus). We also found a few solitary wasps, including grass-carrying wasps (Isodontia) and mason wasps (Euodynerus and Leptochilus).
Figure 4. Small carpenter bee (genus Ceratina). Ceratina bees are about ¼” long.
Judy Gallagher via Wikimedia Commons CC BY 2.0
FAQs
I don’t want stubble in my garden. Can I cut the stems all the way off, 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 be 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, well-drained pot, or weaving them into a decorative wattle fence. Vertical stem orientation may be preferred, but you could try either way.
What about the third year of a perennial stem? In our project, volunteers found that stems started naturally breaking down in the second year. If perennial stems are sturdy enough, they have the potential to continue providing nesting habitat for as long as they are standing. Each winter, you would cut the previous summer’s growth, and leave any remaining older stems alone (Figure 2, 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 frost and last 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 one 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, these solitary bees are busy building their nests and collecting food, they are not concerned with 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, who have hollowed out the pith and even left a dusting of yellow pollen at the nest entrance.
Elsa Youngsteadt CC BY-NC-SA 4.0
Related Resources
Fall Chores in the Pollinator Garden: Leave the Leaves and Save the Stems!
Provide Nesting Habitat for Native Bees Through Plant Stems in Your Garden
How to Manage a Successful Bee Hotel
Publication date: Dec. 18, 2024
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