Think about nesting as you prepare for winter – November 2015

Hillary Sardinas

This is the time of year when many of us winterize our gardens. Often this process involves mulching over bare soil between plants in order to conserve soil moisture, reduce runoff, and minimize the number of weedy plants that may spring up. But there is a consequence of mulching that many of us fail to think of: we are likely inhibiting the ability of ground-nesting bees to initiate their nests because mulch covers the patches of soil that are potential nest sites.

Most of the native bee species in the U.S. are solitary ground-nesters. They excavate nests in the ground and provision them with pollen and nectar for their young. The nest entrances are usually nondescript, appearing much like an ant nest without a dirt mound. Sometimes bees create turrets, or rings of mud around their entrances, which can get quite tall and fall over. Species of ground-nesting bees include the green sweat bees (Agapostemon spp.), long-horned bees (Melissodes spp.), sunflower bees (Diadasia spp.), and the aptly named digger bees (Anthophora spp.) and mining bees (Andrena spp.).

Heavy wood or bark chips are the most common types of mulch materials. These persist over time and can make it difficult for ground nesting bees to access the soil. In the wildflower meadows that the Xerces Society helps to create in agricultural areas, we try to use lighter weight mulches such as almond or walnut hulls, whenever possible. In California, these materials usually get blown away by strong northern winds in the spring when the first pulse of native bees emerge. Thus, these light weight mulches help suppress weeds but then allow bees to get to digging once the weather improves.

If you don’t have access to light-weight mulches, consider hand-weeding between your plants. Weeding when the ground is still moist and the plants are small makes the work easier. Another option is to seed between your permanent plants with locally-sourced native wildflower seeds. The seeds germinate and compete with weeds, but leave room at their bases for bee nests. If mulching is a must, then try creating a dirt walkway through your garden and maintain it as bare, compacted soil. Such paths can be attractive nest sites for bees. A fourth option is to mulch in shady areas where bees are less likely to nest, and leave areas of bare soil in sunnier locations. This strategy could help minimize the amount of yard work necessary to maintain bare ground.

If you have already added mulches to help your plants establish, don’t worry — even heavy mulches will break down over time. Once your garden is filled in, then the existing plants can outcompete weeds and leave areas mulch-free for bees to nest in.

This winter as you plant pollinator-friendly species that can be watered in with the rain, don’t forget to create and protect the other key habitat that pollinators need to survive: their nest sites!

To read previous months’ tips, please click here.

Hedgerow with almond shell mulch

Hedgerow with almond shell mulch

Diadasia enavata nest entrance with toppled over turret

Diadasia enavata nest entrance with toppled over turret

All manner of critters will live in a leaf pile, not to mention overwintering bumble bees.

All manner of critters will live in a leaf pile as well, not to mention overwintering bumble bees.

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