Much can be said in favor of using mulch in the garden. A layer of mulch covering the ground helps to keep plant roots cool and moist, prevents weed seeds from sprouting, helps slow erosion and can prettify the otherwise bare soil of newly planted areas. The dry splinters of bark mulch will sometimes halt the traffic of terrestrial mollusks towards vulnerable vegetation. (Too thick a layer, however, simply provides cover where slugs and their kin can lurk in ambush.) There are plant- and animal-derived mulches such as manure, grass clippings, straw, leaves, newspaper, cocoa hulls, shredded or chipped bark, etc., which decompose readily and can then be dug into the soil, adding valuable nutrients. There are mineral and manmade mulches like gravel, stones, plastic tarps and shower curtains. And then there are whimsical mulches—bits of tumbled colored glass, scraps of old flying carpets, seaweed gathered by the light of the moon, etc. Alas, Berkeley Horticultural Nursery is not a good source for such specialty mulches, but we do sell a nice selection of chipped and shredded bark, fragrant cocoa bean hulls, weed-cloth and burlap. The bark and hulls are packaged in sacks, the weedcloth and burlap in rolls.

With mulch, usually the thicker the layer, the better, due to its capacity to reduce irrigation, suppress weeds, keep roots cool and prevent rains from rinsing away the soil. However, sometimes it’s possible to get too much of a good thing, and in the case of ground-nesting native bees, even the thinnest layer of mulch will prevent these charming and essential pollinators from digging the small tunnels in the soil that they need for their underground bee nurseries. Given how important these creatures are to the survival of all earth-dwelling species, including homo not-so-sapiens, we really should resist the urge to smother earth with too much mulch. To give the native buzzers a break, UC Berkeley entomologist Gordon Frankie recommends that we learn to love the look of bare earth around at least half of our gardens!

– Margaret

For more information about this topic, please see Dr. Frankie's website on bee-friendly gardening: http://nature.berkeley.edu/urbanbeegardens