Saturday, March 26, 2016

Saving more bees: How and why I'm rethinking mulch in my garden

For years I have been a member of the chorus of natural gardening advocates who tout the benefits of using mulch liberally to suppress weeds, maintain soil moisture, control disease by keeping dirt from splashing onto plant leaves, and keep soil-dwelling pests at bay.

Even better, I have said, is to put down layers and layers of newspapers to form a solid—but natural! decomposable!—barrier to really suppress the weeds for a full season or longer. 

Between the boxwoods: lots of mulch, and emerging crocuses

Then I learned about ground-nesting solitary bees. These gentle hardworking pollinators include several genera of wild bees, sometimes called digger bees or mining bees, and they need access to bare soil to excavate their nest tunnels. 

This 5-minute video from the University of Minnesota condenses an hour of effort by one such bee (genus Colletes), diligently working to move a couple of small wood chips out of her way and then digging a tunnel in a sheltered spot next to a rock. Imagine that bee laboring to move several wood chips out of the way and then encountering a thick barrier of newspapers—or worse, black plastic. Game over, little hapless bee.

The widespread use of mulch is a matter of real concern to bee advocates, like those at the UC Berkeley Urban Bee Lab, who call it "mulch madness"—and the use of black plastic weed barriers they dub BPI  for "black plastic insanity." (If you click on the link, you'll see a photo of a beautiful little green sweat bee,  genus Agapostemon, emerging from her underground nest.)

There's no reason to fear these gentle bees; they don't have a hive to protect, and it is very unlikely they will ever sting you. While the Cornell Department of Entomology says these bees will not sting unless handled, many species will even tolerate gentle handling, as the children of Sabin Elementary School in Portland, Oregon, will testify. Their playground is home to thousands of Andrena bees—the kids call them "tickle bees" and have adopted them as the school mascot.

A little patch of bare soil to the left—the beginning of a more bee-friendly garden

So what's a conscientious gardener to do?

Mulch less, mulch sparingly, mulch lightly—and don't replenish those bare spots that emerge as our old mulch breaks down or gets moved around. Plant groundcovers for living mulches that leave space for the bees. Get into the zen of pulling weeds once in a while. When we don't have the time or inclination to weed, we can focus on edging to give the perception of a neatly contained garden.  (Trust me on this—I have gotten compliments on a very weedy garden with tidy edges.)

And no more laying down of layers of newspapers—unless we want to shred them first, as researchers at the University of Ohio did to encourage ground-nesting squash bees, which they found to be more effective than honeybees at pollinating squash, pumpkins, gourds, and other related crops. They determined that a mulch of shredded newspapers and grass clippings was best at both keeping weeds down and supporting these valuable pollinators.

Other loose, lightweight mulches, such as leaves and the hulls of various types of nuts, are also good alternatives to wood chips and barriers. These are not only easy for the bees to work around, but they also get blown about by the wind, exposing a bit of soil here and there in the process, as found by the Xerces Society in their test fields in California.

Ground covers make attractive natural airy mulches. In one of my gardens, I planted periwinkle (Vinca minor) amongst the bulbs and perennials and a chokeberry (Aronia) shrub.  The periwinkle has spread and interwoven itself throughout the garden so that I really couldn't spread woodchips without burying it, so I stopped trying to mulch it, and have found that I only need to pull a few weeds from time to time.

Tulips emerging amid a very loose mulch of leaves, plant debris, and perwinkle

This garden is lightly mulched by happenstance with the leaves that fall on it, the debris from previous years' ferns, astilbe and other plants, and the periwinkle sprawling everywhere. I have come to appreciate the aesthetics of the variety of textures and shapes in this mix of light mulches and plants. And by mid June, the perennials hide most everything else anyway. 

Most importantly—for the bees especially—here and there, patches of bare soil have become exposed. And that's where I first noticed little bees hovering over the ground last spring, shortly after learning about them.

Another part of the same garden, with coral bells, emerging star of Bethlehem (Ornithogalum), more periwinkle, and a little bare soil, which you can see in the lower right 

I didn't plan it that way, but now I'll be looking to replicate that happy mix elsewhere in my yard.

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