Another doubly beneficial group of winter annuals are the forage brassicas, including kale and daikon, sometimes called forage radish. When seeded more densely than for harvest as a vegetable crop, these deep-rooted crops offer plenty of nutritious leaves for composting, says Sarrantonio. After the tops of the brassicas are harvested for compost, the large taproots die back, leaving wonderful nutrient-rich channels in the soil. These open spaces allow microbial populations to flourish, enhance good drainage, and create a nice structure in the soil. Seeds of cold-tolerant forage kales are often sold in bulk quantities by farm-supply stores, but home gardeners can get the same results from familiar culinary varieties such as ‘Dwarf Siberian’ and ‘Blue Curled Scotch’.
Some compost crops feed both the gardener and the compost pile. Most legumes that are cultivated for their edible beans don’t retain much nitrogen in their leaves and stems after the seeds have fully matured—they have concentrated most of the nitrogen in their seeds. But cowpeas, a subtropical legume species that includes black-eyed peas and red cowpeas, are mostly indeterminate; the plants will flower continuously and leaves will stay green until killed by frost in northern climates. Rather than tilling the plants into the soil in place, the tops can be removed and added to the compost pile, Sarrantonio says. The seeds of another legume, the soybean, can be harvested while green for edamame, while its foliage is still green and full of nitrogen. Throw the soybean plants on the compost pile after harvest.
Comfrey and borage are both dynamic accumulators of nutrients—they have a reputation for accumulating phosphorus and zinc, in particular, says Sarrantonio—and both produce a tremendous amount of aboveground biomass. They grow so fast and so large that you can use them for many purposes, and certainly as a source of compost-pile ingredients.
When planning your garden this spring, make room for plants with a double ben-efit—a hardworking root system or edible seeds in addition to plentiful, carbon- or nitrogen-rich foliage to feed your compost pile.