A mother's work is never done

I felt I had a personal encounter this week with the biggest bumblebee I’ve ever seen. Maybe I’m wrong, but I seemed to spot the same rotund, furry bee visiting flowers near the house several times in the course of a sunny afternoon. 

    Perusing Heather Holm’s Pollinators of Native Plants, I learned that the bee I was watching was probably a queen looking for pollen and nectar for her offspring. 

Earth bumblebee (Bombus terrestris)-photo Ivar Leidus

Bumblebee queens can be three times as large as worker bumblebees. The orange ball I saw on one of her back legs was probably her corbicula, or pollen pouch. When I watched her moving from flower to flower, she was collecting pollen grains on her head, thorax and abdomen, combining them with nectar, and storing the mixture in the pouch to carry back to her nest.

The orange ball is her pollen pouch-photo Tony Wills

    Another bee behavior I’d happened to notice also made sense now. A few days before, I saw a bumblebee fly into a small hole in the mound of sheet compost I started last month. The mound is topped with chopped straw and hay over a layer of compost; under that there’s a thick pile of wood chips. Now I know that this could be an appealing spot for a bumblebee queen looking for a place to dig a ground nest. 

The top of the sheet compost offers a good bee nesting site

    Bumblebees can sting repeatedly without dying, but none has ever threatened me. The ones you see sleeping in flowers in the early morning are probably males, because once they leave home, they don’t have a nest to go back to.

    I learned that except for rising queens, all bumblebees die at the end of autumn. A queen spends the winter underground, emerging early next spring to start her colony. 

Bumblebee on a crocus-photo Rasbak

She looks for a hole that another animal has made in the past or for some loose, open sandy or loamy soil where she can dig tunnels. Sandy loam is what I’ve got, but I tend to cover open soil with mulch. In future, I’ll make a point of leaving some unmulched areas for bee nesting sites. Meanwhile, the loose, uncompacted surface of the sheet compost mound is providing nesting opportunities.

    I’d thought bumblebees were solitary, but they live in eusocial colonies, where mothers and daughters work together to raise the next generation. With the results of her foraging, my queen will create nectar pots and pollen balls, lay her eggs, and seal each one in wax with some food for the larva when it hatches. When the larvae pupate and emerge as adult bees, those tapped to be next year’s queens will get more food and grow bigger. How does the queen decide which of her daughters will succeed her?

    Bumblebees are generalist pollinators: they visit many kinds of flowers, in contrast to some other native bee species that specialize on a few kinds of flowers. That’s why bumblebees are so useful to farms and gardens and why it makes sense to protect them by providing habitat and eschewing pesticides.

It's not hard for bees to find flowers in the garden in May. August will be more of a challenge.

Good news for trees

Spotting a little green caterpillar on a weed I pulled from the perennial bed reminded me that this is the time when winter moth larvae (Operophtera brumata) make their way up trunks of trees such as the old oak that stands over this area of the garden.

Winter moth caterpillar

    That caterpillar was in for a surprise, though. Thanks to a team led by University of Massachusetts entomologist Joseph Elkinton, a parasitoid tachinid fly, Cyzenis albicans, is laying its eggs on leaves consumed by winter moth caterpillars. The fly’s offspring will kill most of the caterpillars that eat those leaves. What used to be an epidemic of nonnative pests defoliating trees in eastern Massachusetts has turned into no big deal.

    We used to notice clouds of male winter moths when they appeared in late fall, often lighting on windows and doors and sometimes making their way into the house. 

Winter moth caterpillars in November 2016

The wingless females can’t fly, but they can climb tree trunks to lay their eggs in bark crevices. The larvae emerge in March and devour leaf and flower buds, or they produce strands of silk that allow them to “balloon,” floating to other trees to spread the destruction. 

     Winter moth surged in Massachusetts in 2003 and found no native predators. At the height of the moths’ population boom, they affected most of our street trees, favoring the plentiful maples and oaks. It was alarming to walk down the street and see every tree leaf riddled with holes.

Birch leaf chewed by winter moth caterpillars, 2015

    To protect ornamental trees, the usual approach was to apply pesticides, and I did that for several years. I was telling myself that the treatment was relatively benign, because it used spinosad, an agent derived from naturally occurring soil bacteria.

Although it's a natural product, spinosad kills bees as well as winter moths

     Elkinton’s team took a different tack. In Europe, where it originated, winter moth has numerous insect predators. The scientists tested these to see which would be safe for release in New England, seeking an insect that targeted only winter moth and wouldn’t harm native insects. For the past 14 years, they’ve been releasing the parasitoid fly Cyzenis albicans to kill winter moth caterpillars. 

Cyzenis albicans-photo James K. Lindsey

    This year Elkinton has declared victory. After years of painstaking monitoring, his team has determined that winter moth populations are down to manageable levels. The parasitoid fly has become established, maintaining sufficient populations to keep winter moth under control.

    Predators need prey. If the flies eradicated all the winter moths, they’d have no food for their larvae. They would die out too. With both maintaining a stable presence, they can continue their interdependent dance.

    This is one of those scientific and ethical dilemmas where human beings have created a problem—in this case by unintentionally importing nonnative winter moths—and there’s no perfect response. If we did nothing, we’d likely lose many trees. 
  

     If we kept spraying pesticides, even targeted ones, we’d inevitably cause collateral damage to other insect species. That's why I stopped applying spinosad. So far, introducing nonnative Cyzenis albicans seems like the happiest solution for a thorny environmental problem.

Winter moths won't be troubling our oak anymore

Groundcover throwdown

Last fall I wrote about uprooting smooth Solomon’s seal (Polygonatum biflorum) under a crabapple that occupies a very visible spot in our landscape. This native plant has spread widely in my yard. I brought home three as a groundcover for shade, and now 25 years later, their descendants are taking over every shady spot in the yard. They may be beneficiaries of our sprinkler irrigation system, which keeps the soil from drying out in the summer, unintentionally catering to smooth Solomon’s seal’s preference.

Smooth Solomon's seal shows up in every shady spot

    This spring I wasn’t surprised to see new shoots of the plant emerging in the section I’d tried to clear last year. It’s hard to remove every scrap of the white rhizomes. What’s needed, I realized, was something to compete with smooth Solomon’s seal. I should choose a shade-loving plant that would be tough enough to hold its ground, but not so aggressive that it would become a problem. Easier said than done. Smooth Solomon’s seal, along with some other unfortunate choices, has taught me that plants can take years to reach a critical population mass that enables a bid for hegemony.

Will fragrant Solomon's seal also turn into a thug? Time will tell

    To avoid bringing home neonicotinoid-treated plants, I’ve learned to look around the garden for things to transplant before heading out to the garden center. I could certainly move some Canadian wild ginger (Asarum canadense) to the area under the crabapple. Its round matte leaves stay clean and fresh-looking through the season. There’s already a lot of it in the yard, though, and its foliage is quite similar to that of bloodroot (Sanguinaria canadensis), which is already growing along the path that delineates the bed. I was hoping for a new look.

What can succeed in territory temporarily cleared of smooth Solomon's seal?

    European ginger (Asarum europaeum), another in the same genus, might also work. While smooth Solomon’s seal claimed territory in the yard over the years, clumps of European ginger have slowly expanded, and now new volunteers are popping up and forming colonies on their own, a sign that the species is finally feeling at home in the garden. I love its shiny round leaves with their subtle pattern of lighter-colored veins. I can dig some clumps from a bed along the driveway to try their luck under the crabapple.

European ginger making itself at home next to the garage

    At the Massachusetts Horticultural Society’s Garden Fair last weekend, I spotted an interesting option, Carex siderosticha ‘Snow Cap,’ with leaves that are almost all white. 

Easy to see why this broad-leaved sedge was named 'Snow Cap'

The volunteer who sold it to me warned that it needs lots of shade, and that’s exactly what it’ll get under the crabapple. With no competition, this rhizomatous, creeping sedge might make a bid for world domination. Let’s hope the energy it expends holding back smooth Solomon’s seal will keep it in check. With so little green leaf surface, it can’t be a photosynthetic powerhouse.

    I’m planning to start with those two and any wandering bloodroot seedlings that I can find and transplant. It’ll be interesting to see what happens under the crabapple once these plants go in. I suspect a guide and referee may be needed.

European ginger and bloodroot coexisting elbow to elbow

Leaving the leaves

Last fall I departed from past practices and let fallen leaves lie on all my garden beds through the winter. The reason for the change was a new recognition that this would help native insects make it through to spring. As the weather warms and daytime temperatures stay moderate, I’m having the first opportunity to assess how this worked out.

Bloodroot emerging from last fall's leaves

    Years ago, I tried using whole leaves as mulch to improve soil in the front yard. I soon recognized that this was an un-neighborly act, because the leaves blew off our property and ended up on other people’s carefully raked lawns. This blunder led to the purchase of a leaf shredder. Shredded leaves stay put. I used them as mulch for about 20 years. 

Shredded leaves make nice mulch and don't blow around

    Then I learned that by chopping up the leaves, I was probably also chopping up desirable insects. Some dormant adult insects spend the winter hiding among the leaves, and others lay their eggs there. If I let the whole leaves lie through the winter, the eggs could hatch and adult insects could emerge when the weather warms in spring. 

Beneficial lacewings can winter in leaf litter

Those emerging insects would contribute to a healthy balance of insect populations in the garden. That’s why last year I not only let leaves that fell on the backyard beds lie un-shredded. I also dragged in as many leaves as possible from my block and a neighbor’s lawn. All those brown leaves are now lying on my garden.

    I’m still working out this system. My approach this spring is to rake the top layer of fall leaves gently off perennial beds. New shoots from the perennials don’t seem to be able to reach the sunlight through mats of undecomposed whole leaves. I see new growth heaving up a section of leaf mulch, and I can’t resist lifting the leaves off to uncover the emerging yellow-green stalks. 

I peeled away the top layer of leaves to uncover emerging perennials

This may be a remnant of an old way of thinking. Maybe next year I’ll have learned that even this careful raking isn’t necessary.

    Around trees and shrub, though, the whole leaves seem to be doing nothing but good. Like wood chips, they’re keeping the soil steadily cool and moist. They seem to be suppressing weeds too, like any good mulch. 

No need to rake away fall leaves that surround trees and shrubs

Tough, leathery leaves from our neighborhood’s red oaks decompose slowly. Even so, they’ll eventually break down and add organic matter to the soil. Like my sheet mulching project, this is essentially a way to let compost happen with less intervention. Instead of moving leaves to the compost pile and then carting them back to the beds after they decompose, I’m letting soil organisms do the work where the leaves fall. Imitating natural processes is a lot less work!

    I do need to move some of those leaves to the compost piles, though, because the bins are pretty depleted. I’ll need some compost to make homemade potting mix. No worries—there are plenty of leaves to spare.

Leaves will help make compost for peat-free potting mix

 
   

What goes with chartreuse?

After the beautiful elms that lined our streets succumbed to Dutch elm disease in the mid-twentieth century, my town, like many others, thought they’d found great street trees to replace them: Norway maples (Acer platanoides). 

Surviving elms in New York's Central Park

After all, the maples were tough, adaptable growers that could survive the tight root space, thin, rocky topsoil, and plentiful road salt that our narrow curb strips offered.
    
    In one way, choosing these maples was a success. Almost all our streets are now lined with mature specimens. Over their lifetime they’ve rolled with the punches, putting up with limbs torn off by ice storms and passing trucks, finding water by pushing their roots under the sidewalks into lawns and garden beds, growing around holes gouged in their canopy by pruning around power lines.

Norway maple street tree-photo Famartin

    The problem with these nonnative trees is that they’re too successful. Each tree produces thousands of viable seeds every spring, packing pairs of them into the helicopter-like samaras that we see twirling to the ground. 

Winged seed pods of Norway maples

They lie in cracks in the sidewalk, in garden beds, and on any bare soil or pile of organic debris. It seems as if most of those seeds succeed in producing seedlings the following spring. 

    This is the time of year when gardeners like me become obsessed with weeding out those seedlings. We know that by next year, this season’s tender little sprout will have grown a woody stem and sent down a deep root. If we don’t catch them this year, they’ll be harder to pull with every passing year.

One-year-old Norway maple seedling, left, with this year's on right

    When we moved into our house, a dense thicket of Norway maples that soon reached 20 feet high grew behind the back fence, just because our neighbors had left some seedlings alone to grow. The trees make dense shade, their shallow roots crowd out competitors, and they exude allelopathic chemicals that keep other plants from thriving in their vicinity.

Norway maples shading the backyard in the mid-90s

Luckily for my garden, we were able to remove those trees. Their relatives have continued to colonize the city, though. 

    When the Norway maples flower at the end of April, my heart softens toward them, despite my struggle to curb their reproduction. I love the lacy silhouettes the flowers make against the street lights after dark. The rosettes of chartreuse flowers and tiny new leaves are a heartening sight when many trees haven’t yet put out any foliage. 

Norway maple in bloom

A cloud of yellow-green covers the whole city, reminding me of Robert Frost’s line, “Nature’s first green is gold.”

Norway maples paint the landscape a vibrant pale green

    In recent years, our city has gotten wise. Where street trees have died or been cut down, our urban forestry director now chooses to plant a range of resilient tree species—but no more Norway maples. Soon we’ll have a diverse mix of street trees that are less inclined toward world domination. Meanwhile, we should remind ourselves when choosing spring-blooming bulbs and shrubs that they’re going to flower against a chartreuse background. Whether we like it or not, those maples won’t disappear anytime soon.

Nesting time

It’s great to hear the dawn chorus of bird song at this time of year and see birds collecting material from the garden for nests. 

When magnolias bloom, it's time for birds to build nests

    I’ve put out mesh balls filled with feathers again to aid in the nest-building effort. An attractive ball made from bent twigs filled with fluffy raw cotton that I bought a few years ago from a catalog and hung from a branch drew no bird customers at all. I noticed that birds seemed more interested when the ball contained feathers. 

Feathers make more popular nesting material than a wad of raw cotton

    That’s when I started saving onion bags and ordered a big bag of goose feathers from a company that sells them for topping up feather comforters. I fill the bags with feathers, close them with twist ties, and hang them from branches like Christmas ornaments.

 

Onion bag feather ball hanging in a Norway spruce

It’s fun to see a bird hover or cling to the mesh to snag a feather or two and then fly off purposefully to where the building project is underway.

    The bad news is that many of the birds taking me up on this offer are nonnative house sparrows (Passer domesticus). 

European house sparrow

This highly adaptable species originated in Eurasia and North Africa and now lives on six continents, everywhere but polar areas. Flocks of house sparrows evict native birds from their nesting sites and hog the food supply.

    My goal is to foster a generous garden ecosystem where a diverse range of bird species can find the plants, insects, water, shelter, and nesting opportunities that they need. For that reason, I’ve tried to stop inviting these sparrows to my yard. Not filling birdfeeders during the growing season has made a big difference. Once the feather balls come down this summer, I won’t do anything else to encourage house sparrows to return. 

    I’m encouraged to see birds using some garden debris for building nests. That’s what I hoped would happen with the transition to intentionally bird-friendly gardening (Grow Native Massachusetts offers this guide to providing food and habitat for birds in your yard).

    This week I’ve noticed American robins (Turdus migratorius) busily flying back and forth and scolding when I get near certain trees. On Saturday I spotted faded hosta leaves hanging from a nest in a tall Japanese cedar (Cryptomeria japonica). 

Hanging hosta leaves make this robin's nest noticeable

The penny dropped: that’s why last year’s hosta leaves are migrating around the yard and showing up draped over low branches. 

The robins are picking them up to carry to their chosen nest spots, dropping some along the way. Even wilted and dried out, those large leaves must make a heavy load for a 3-ounce robin. 

I noticed hosta leaves woven into another nest located on a downspout under the eaves of the house. Saving the garden clean-up for later this spring has allowed nest-building robins to make use of last year’s leaves, stalks and twigs.

    True, hostas are Asian imports, not native to New England. If they’re useful to the robins, though, I feel their inclusion in the garden is sustainable enough. That’s fortunate, because they’re pretty too. 

Nonnative hostas and flowering astilbes in July

 

Stalking invasives--in moderation

At this time of year, every morning there’s something new happening in the garden. This weekend daffodils and magnolia blossoms opened. 

 

Star magnolia and Magnolia 'Leonard Messel' popped into bloom overnight

Also burgeoning: invasive plants. These super-survivors start early and finish late, sprinting out of the starting blocks while competitors are still dormant.

My arch-nemesis, Norway maple, gets a jump on the season

    Before everything else leafs out, providing camouflage, it’s relatively easy to spot the nonnative problem plants. Now is also I time when I’m not dizzily spinning from task to task, as I will be in May.

    First on the list, I need to check on Japanese knotweed (Fallopia japonica) that’s surfaced along the south fence line. This could be the year I stick to my plan to foil this potentially powerful land grab. I’m sure you’ve seen Japanese knotweed, because it lines highway shoulders and any other areas where soil is disturbed and then left uncovered. 

Japanese knotweed seizes any open ground

Once it starts growing, it’s almost immortal. Like many invasives, it makes lots of seeds. The underground part of the plant is the real menace, though. To eradicate stands of Japanese knotweed, the Massachusetts Department of Transportation has to remove all soil down to 3 feet and burn it. That’s because if even a nubbin of root is left, the plant will grow back. My strategy for Japanese knotweed is to cut it to the ground every two weeks. Eventually if it can’t make energy through photosynthesis, it should dwindle and die. At least that’s the theory.

Last fall I tagged this sprig of Japanese knotweed so I won't miss it

    Garlic mustard (Alliaria petiolata) is another rapid spreader in our area. It’s easy to uproot. I need to catch it all before it flowers and sets seed in its second year, though, or it’ll take over. Garlic mustard emits allelopathic chemicals that suppress other species’ growth. This is one of the few invasive plants in North America that thrives in a shady forest habitat, such as what prevails in much of my yard.

Garlic mustard impersonating an early perennial

    When I pull or cut down these plants, I can’t throw them on the compost pile. If I did, I’d be spreading the unwanted plants whenever I distribute compost. It goes against the grain to bag invasives and throw them in the regular trash, but adding them to the yard waste would pass them on to someone else’s garden.

Volunteer pulling garlic mustard at Cuyahoga Valley National Park-photo NPS/Arrye Rosser

    Of course, officially recognized invasives aren’t the only plants growing aggressively in my garden. Lots of plants I chose myself, including native smooth Solomon’s seal (Polygonatum biflorum), have turned out to be thugs that I have to beat back every year.

Smooth Solomon's seal shooting up from the leaf litter this week

    I don’t like to get too hysterical about invasive plants. They’re mostly responding to human alterations of plant habitat, diving in to pioneer newly disturbed soil. The nonnative invasives in my yard didn’t parachute in from abroad, they just made it under the fence from adjoining properties where people are less interested in policing the landscape. Spending some gardening time keeping them at bay does seem worthwhile, though. The more garden space they claim, the less there’ll be for a diverse landscape that includes native plants.

With help, I hope native inkberry will grow instead of  Japanese knotweed