IN A RECENT conversation with Doug Tallamy about ecologically minded fall cleanup, he raised the name of Heather Holm, and how some of the pollinator research she’s been part of lately is informing how he shifts his approach to garden maintenance this time of year, and again in spring. I wanted to hear more, so I called Heather to talk about how we can each support native bees, especially, in our gardens beyond the season of bloom, in the offseason, too.
Heather Holm is the award-winning author of the 2017 book “Bees,” and before that of “Pollinators of Native Plants” (Amazon affiliate links). Her expertise includes the interactions between native bees and native flora, and the bees’ natural history and biology (that’s a small carpenter bee, genus Ceratina, above). She joined me on the podcast to tune us into their needs.
Read along as you listen to the October 26, 2020 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).
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ecological landscaping symposium features heather holm
HEATHER HOLM is one of the speakers at Berkshire Botanical Garden’s annual ecological landscaping symposium, taking place online Nov. 15 and Nov. 22, 2020. Learn more about her talk another other expert presenters.
bee-friendly garden cleanup, with heather holm
Margaret Roach: As I said in the introduction, in that conversation about fall cleanup with Doug Tallamy, it dawned on me that as much as I know, and as many experts as I’ve spoken to over the years about pollinator plants and pollinator gardens, I don’t know as much at all about how a bee’s life history really works. What’s a bee’s-eye view of my place other than the flowers? I wondered if you could introduce us a little bit. I know there are many different species of native bees, but generally speaking, clue us into how a bee sees a place like our gardens.
Heather Holm: Sure. As you said, the big take-home message is just bee diversity—3,700 species approximately in the U.S., and basically each is overwintering in a different life stage than another.
It really makes it challenging in our gardens if we’re trying to supply supplemental nesting sites, for example. I always get the question, well, when do the bees leave? [Laughter.] And it always goes back to, well, it depends on the kind of bee. And the thing for people to remember is of those 3,700 species, about 90 percent have a solitary lifestyle. They really have this narrow window of time during the growing season that they’re active as adults.
We have bees that are the first bees to come out in the spring and are active for four weeks maybe in April or May, depending on where you live.
But then on the other hand, we have social bees, such as our native bumblebees, and they will be active throughout the growing season and have very different strategies for overwintering. I can continue, but… [laughter].
Margaret: Yeah. That’s a good one. Bumblebees, I think, is a good group; let’s start with them and give us some examples of what are they looking for or what do they see as a good place, etc.? And again, beyond the flowers, right?
Heather: Right, long beyond the flowers. The flowers, particularly for bumblebees, the fall-blooming plants are critical because what’s happening in the fall is the new queens produced in a bumblebee colony are coming out, and they’re practicing forging. And while they’re visiting flowers to consume nectar, they’re building up fat stores, which will help them for their winter hibernation.
Most people are unaware that bumblebees are annual colonies. And unlike the European honeybee, which can survive the winter and the colony can be perennial, at the end of the growing season, no matter where you live, all the bumblebees will perish, except for those new queens that are preparing to hibernate. They’re hibernating as adults, and they go off and disperse from where they grew up in the nest, where their mothers raised them, and they’re looking for a really nice, insulated place to spend the winter.
That can be a number of different situations in a garden. It could be that they tuck themselves into an abandoned rodent hole. They may find a mouse nest; often they’re attracted to mouse nests. I live in the upper Midwest, so we have very cold winters, so they’re really looking for an insulated place. But if you’re further South, they may tuck themselves just under a heavy pile of leaf litter. Some hibernating queens, people will find in their compost piles. The key is a place that’s providing some insulation.
Margaret: When you said that the lifespan of this queen, who’s going to make the next generation, she came out… These native bees have a full metamorphosis and there was an egg and a larva, so can you go back in her life a little bit, or forward to her next generation?
Heather: Sure. Those new queens are called gynes, and they are produced at the end of the growing season. They overwinter as adults and as you just said, they are the new queens of next year’s colonies. Bumblebee queens lived the longest of any of our native bees, because they are alive for 12 months, for 12 or 13 months.
They’re doing that initial winter hibernation as an adult, and then the following spring, depending on light and other phenological cues, they emerge from their hibernation site and then establish their own colony. But they will perish at the end of that growing season, but produce new queens. It’s a pretty precarious lifestyle, even though it’s social. They have to survive the winter; they have to build up enough energy and fat stores to do so.
They may be impacted by certain practices that we’re doing in our garden. We may be raking up a queen bumblebee tucked away for the winter and putting it in leaf bags. That’s sort of the precarious part. And the other piece is having adequate food supplies, especially in early spring when they emerge, because their energies are depleted. They need calories in order to start that nest initiation process.
Margaret: And in the early spring, I’ll see them at the earliest-blooming flowers, and I always in my head, my very overly simplistic… Almost not understanding at all, but all I knew was she’s hungry, so to speak, but also she’s going to provision a nest. Is that right? She’s eating for herself, and also collecting material to provision a nest?
Heather: Right, right. Correct. She’s got to find an adequate supply of pollen-producing plants because she’s going to create a large pollen ball, where she’ll lay multiple eggs. Plants such as willow and really early spring blooming plants are critical for bumblebees, because they need that pollen supply. Nectar is their carbohydrate source, their fuel. You combine a little bit of nectar in with the pollen stores, but pollen’s the critical piece for raising offspring.
Margaret: And so if we were to pick another type of native bee to contrast against the bumblebee—which you said the queen is the longest-lived of our native bees, the queen bumblebee—there are some that their adult life is what, a few weeks or something?
Heather: The 90 percent that have this solitary lifestyle, it’s just a single female that’s emerging from her nest, where she was raised, and living maybe four to six weeks. And she’s doing all the nest construction and nest provisioning tasks herself, and so it’s a very narrow window of time.
And native bees, the males have a much shorter lifespan than females. Basically their sole purpose in the bee life cycle is to mate with a female, but the females live a little bit longer because they’re doing that nest provisioning.
If we take that back to our gardens and maintenance, the ones that typically can be impacted by our maintenance practices are the 30 percent of native bees that nest above-ground. And they would be building nests in cavities such as holes in wood, plant stems, supplemental nests that some folks will put out. If we’re doing any clearing, or cutting down, or removal of materials that could be cavity-nesting sites, then they would be directly impacted.
Margaret: If I want to do the best job I can—understanding that there’s many kinds of bees with different needs, some are cavity nesters, some under some insulation on the ground or in the ground, like you mentioned, I think an old animal hole or something like that. Some nest in stems above the ground, too, right? Aren’t there stem-nesting bees as well? [Above, a small carpenter bee in the genus Ceratina, a stem nester.]
Heather: Yes. Stems can mean a number of different things. Some of our smaller cavity-nesting bees like pith-filled flower stalks. Plants in the Aster family, for example, are ideal nesting sites for those smaller stem-nesting bees. Similarly, some of our woody plants that are more softer wood, elderberry, sumac, they will have pith-filled centers or hollow centers, and the bees will mine out that styrofoam-like pith in the center and to excavate a nesting cavity.
It depends on the bee and where they preferentially nest, because you can imagine it nesting in an old flower stalk, and the orientation of those stalks is near-vertical the way the flower bloomed on top of those stalks.
Whereas if they are nesting in a hole in a standing tree, those bees would preferentially like more horizontal nesting orientation.
And then the other key thing is the size of the opening. Small bees will seek out very small-diameter cavities, depending if it’s in a hole in wood or a plant stem. And then the larger bees will be looking for a larger cavities.
Margaret: Here we are, it’s fall cleanup, and this is why Doug mentioned because we were talking about that, he mentioned your work. And you’ve been participating in some recent research and you had some aha’s that they seemed to even inform a different way of cleanup in the fall with something like the stem-nesting bees and some of these others in mind. Can you tell us a little bit about that research, and what’s the new news from it?
Heather: Sure. For those stem nesters, in particular, the ones that are in flower stalks, for many years, I’ve been telling people not to do any really fall maintenance or cleanup. And then in the spring to cut down the flower stalks, and leave the flower-stalk stubble, and that is what the bees would occupy for that nesting season.
Going back to bee life cycles, we’re talking about these solitary bees that nest in stems or cavities. They’re egg-to-adult timeframe is 12 months, if they’re producing one generation per year, which is typical. So the confusion with gardening and maintenance and cleanup is if you leave the stem stubble as nesting opportunities, you have to leave it indefinitely. Often people will ask me, “Well then when can I remove it?” [Laughter.]
And so part of the messaging is we need to get beyond this cleanup mentality. I find in my garden, I let the leaf litter fall. I leave everything up for the winter and in early spring, I cut back old flower stalks to provide new nesting opportunities, but I let all that plant debris just fall to the ground. And the leaf litter combined with plant debris is really my weed barrier or my mulch alternative.
It has a couple of advantages for bees, it’s looser material, the bees that are ground-nesting can easily crawl underneath it to excavate their nest below ground, and you don’t have to purchase mulch. And mulch now is becoming a vector for things such as jumping worms, so I think people are thinking a little bit differently about using a lot of mulch in their gardens.
Margaret: Importing stuff, so to speak.
Margaret: A lot of gardeners are listening and they’re thinking, “O.K., now, wait a minute. What can I do that’s still going to feel garden-y, but is looser and so forth?” And people who’ve listened regularly to the show know this: I started out a million years ago as I sort of collector garden kind of person, with all these, many of them non-native plants and more sort of design-y kind of gardens and so forth of unusual things.
And then I got tuned in maybe 20-something years ago to native things and birds and insects and so forth, and more all the time.
And so I still kept a couple of beds around the house that are more that old design-y stuff [laughter] and I’ve added looser areas farther out, so to speak, big areas where I practice more what you’re talking about now and other ways when you talk to gardeners, how do you help them compromise or learn? Do you know what I mean? It’s tricky.
Heather: It is tricky. I think you’ve got the perfect balance. If you have too much leaf litter for your more formalized or structured gardens, you can always transfer that to your more naturalized gardens. And it really depends on the contrast between square footage of gardens and lawns and so on for each particular gardener. My yard is two-thirds of an acre and my lawn is 30 feet by 15 feet, so it’s very, very small. For me to try to move the leaves off the lawn and into the surrounding natural habitat, isn’t a big deal.
But for some folks that have the opposite dynamic, mostly lawn and a little bit of garden, then yes, you would be inundated with the quantity of leaf litter and wondering “What do I do with all this?” and so on.
Margaret: Really, my “solution,” and that’s definitely in quotes, has been more to identify areas—which are getting larger all the time, by the way—that I am managing more loosely and to tip the balance, reduce the lawn, obviously. Areas around trees that were once mulch are no longer mulch, they’re living mulch. Things like that, to loosen up in general the way that things are maintained.
You were talking about solitary bees and social and so forth. The solitary bees, are those the ones that when people buy or make those bee houses, is that who they’re trying to attract? What are those about—with the little pieces of like bamboo or whatever, the hollow stem kind of things.
Heather: Those would be some supplying supplemental nesting sites for the cavity-nesting bees. The bee houses have become rather commercialized, so I tend to not recommend people use them partly because some of them are really poorly designed. They’re very shallow.
You can imagine what’s happening inside a cavity nest for a solitary bee is she’s building multiple little rooms or cells, and each cell is partitioned with some kind of natural material. They’re lined up, cell after cell after cell, with a wall or partition in between. And so if you have a very shallow cavity—and by shallow I mean by 2 to 3 inches is shallow—then she’s only able to really have maybe three to six larval cells developing inside that cavity.
Generally, bees have predators, like most other natural organisms. The first three or four larvae developing inside of that cavity could be predated upon by woodpeckers or parasitoids or you name it. So that that’s not a great situation if the design is poor. You want to have longer cavities, so that they can ensure that the ones at least at the back of the cavity will survive to adulthood.
The other piece in why I don’t encourage people to put these up, is they do require maintenance and cleaning and replacement of stems. Many people have great intentions. It’s much like putting up a nest box for birds and probably only 10 percent or 15 percent of folks would actually do the regular cleaning out of the nest box and maintaining it and so on. That’s what I worry about with the supplemental bee nests. And that’s one reason why I’m trying to encourage people to think, well, what are the natural ways that these above ground bees nest?
Putting logs in your garden, if you’re able to, if you live in suburbia or somewhere where that’s appropriate, that’s a great way to provide nesting habitat. Leaving a standing dead tree, if you have a larger property, or having the tree-removal company leave 10 feet of the trunk as a snag. That’s great habitat for a number of things besides bees—birds, for example.
You just have to think a little bit about the natural ways, and the easy one that I mentioned earlier is the stem stubble. That’s something any gardener can do and that provides a nice array of nesting opportunities.
And the difference between that and the supplemental bee hotels is the bee hotels are tightly aggregated nests, one next to the other. And that’s not how these solitary bees nest, they nest in a cavity here, a cavity there. They’re not nesting right next to another species. What happens with the bee hotels is there’s a higher propensity for disease transmission, too. Those are some reasons why I shy away from recommending them.
Now, there are folks that do a really great job of maintenance and cleaning and stem replacement and so on, and that’s great. They’re also a very useful tool for teaching people about native bees at nature centers or something like that. But for the general homeowner that’s busy and doing a ton of different things, I would shy away from putting them up.
Margaret: It reminds me so much of what you said, like the bird houses and so forth, where people are like, “Well, I put up bluebird houses” or whatever, but then it turns out it was like a pretty cute house, fancy, design-y, but it wasn’t really designed with the species in mind and it wasn’t maintained right and so on and so forth.
Heather: That’s exactly it.
Margaret: In the last few minutes, I wanted to hear one thing, especially. The stem stubble you’ve mentioned a couple of times. O.K., I’m not going to cut back in the fall. Let’s say I have like a meadow-ish planting or just even a bed of perennials. I’m not going to cut it back in the fall, and then come spring and I’m going to do what? Like how high, and just describe it a little more specifically to me.
Heather: You’re out there in spring, you want to be careful not to tromp around in any of your gardens too early in the spring, for all those other insects overwintering under the leaf litter. But what I do if I have a place to stand and cut the plant stems from a sidewalk or a lawn, you can start with those fairly early. Cut down the old flower stalks, and the length that you want to leave is anywhere between 8 and 20 inches.
The research has found that bees will nest in a variety of cavity lengths. I’m using garden scissors, so I chop and try and eyeball at least more than 12, usually 15, inches of stubble [photo above]. And you don’t have to do it for all plants, but this is a great opportunity for a gardener to take a look at the stem. Is it hollow? Is it sturdy?
I mentioned earlier plants in the Aster family, they tend to have very…—goldenrods, asters, black-eyed Susans, coneflowers, and so on—they tend to have very fibrous, sturdy stems. It will last for another 12 months as a nesting cavity. Something like a daylily would be too soft and wouldn’t be appropriate. Concentrate on what you think are more fibrous and sturdy stalks; take a look if the stalks are hollow or have a pith-filled center, both are fine.
Native bees aren’t that big, even those larger ones I mentioned earlier that may nest in larger diameters. You don’t want to leave any hollow stems any bigger than a half-inch diameter. The mason bees and leafcutter bees like three-eighths of an inch diameter. If you go to those really big, tall plants, such as Joe pye weed, they have hollow stems. You don’t want to leave it if the stems edging on to three-quarters of an inch in diameter, that’s not going to help native bees. Discerning by looking while you’re cutting—what would be an experiment.
Margaret: Well, Heather, there are a million more lessons that I need to learn, but your books, both of them, which we’re going to have, as I said, in the giveaway, are so helpful, such great guides. And I appreciate your taking the time today to talk and I hope we’ll talk again soon. Thank you.
(Photos except Monarda from Heather Holm.)
enter to win ‘pollinators of native plants’ and ‘bees’
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the October 26, 2020 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).