Here's a riddle: What will weed and till your soil as well as feed your plants? If you answered "me," you are right. But there is also another answer, and that is "cover crop," which is a plant grown primarily and temporarily for soil improvement.
A cover crop keeps out weeds by smothering them before they gain a foothold. Some cover crops even exude substances that prevent weed growth. Cover crops need to be sown now if they are going to have enough time to do their job before cold weather arrests growth.
Three categories of plants are commonly used as cover crops. The first category consists of grasses such as rye grain, oats, and wheat. Rye and wheat grow late into autumn, go dormant for the winter, then begin growing again in early spring until they ripen their seedheads in summer. Oats also grow late into autumn, but then die where winters are cold, so do not need to be turned under to clear the way for spring plantings of vegetables and flowers.
These grasses also "till" the soil with their extensive roots. A cubic foot of soil beneath a grass will have many miles of roots plowing through it, crumbling it and opening up channels for water and air.
Legumes -- peas, beans, clovers, and vetches -- are the second category of plants commonly used as cover crops. While legumes' roots are not as dense as those of grasses, legumes are valuable for feeding the soil. Bacteria in legume roots take nitrogen gas from the air and convert it into a form usable by subsequent plants.
(Actually, all cover crops, even grasses, indirectly "feed" the soil. They latch onto nutrients that might otherwise wash through the soil and enrich the soil with humus, which makes nutrients more available.)
As with the grasses, there are legumes that live through cold winters and those that do not. Some commonly grown, winter-hardy legumes include hairy vetch and white sweet clover. Legumes that thrive in cool weather but die in cold winters include woolly-pod vetch, crimson clover, and field peas.
The third category of plants that function as cover crops -- often inadvertently -- is weeds. Many offer the same benefits as do legumes and grasses because many weeds are legumes or grasses. Alas, if only weeds were not excessively exuberant in clothing the soil. Rather than allowing a weedy cover crop, a better idea is to deliberately sow a more easily controlled cover crop.