Late summer is the time to get fall cover crops planted. Cover crops are planted not for a harvest that will be taken away from the field, but to provide some benefit to the soil. There are many reasons to plant a cover crop: to increase organic matter in the soil, to prevent erosion over the winter, to soak up excess nutrients from the previous crop, to bring subsoil nutrients up to the surface, to break up compaction, to capture nitrogen from the atmosphere or to smother weeds and reduce weed germination. It's impossible to do all of the above at the same time, so cover cropping is usually a matter of juggling some of those objectives with the available time, weather and cover crop species.
Here I've planted a combination of white clover and tillage radish. The tillage radish is a specialized cover crop that has been selected for extremely deep taproots. It will send the taproot down through hard layers of subsoil, breaking up compaction. The plant dies when the temperature reaches -10 C, leaving a deposit of organic matter deep in the soil. As the plant decays, it will also provide a channel for surface water to better infiltrate the soil, adding to the deep water table and reducing flooding on the surface. The clover, by contrast, has a shallow, fibrous root structure that primarily adds organic matter near the surface and reduces erosion. Its main ability is to fix atmospheric nitrogen (take nitrogen gas out of the air and turn it into nitrate for the soil), which reduces the need for fertilizer inputs next year. It fits well in this spot because it can be established in the shade of the radishes, and then do most of it's growing next spring, when the weather would be too cold and wet to plant something new.
The other combination I use for fallow beds is oats and peas. Oats are in the grass family, and like so many other grasses, do their best growing in cool weather. They will add a large amount of organic matter and reduce soil erosion. They also die around -10 C, so as long as the temperature reaches that point before they set seed, they will leave the organic matter behind with no actively growing plant to incorporate in the spring. I've found that the best point to establish them is around September 1st in this climate. Too much before that and they may set seed (which is a potential weed problem, but also diverts nutrients from the plant tissue to the seeds) and too much after means that they will not be large enough by the time the real cold hits to grow well. Peas fill the nitrogen fixing niche in this combination, but unlike the clover they are killed by cold weather and use their tendrils to climb the oats so they are not growing in shade in the fall.
Finally, where I can I have interseeded clover in beds that will not be finished in time to squeeze in a full bed cover crop. The example above is crimson clover starting to come up in the cabbage bed. The clover establishes under the shade of the main crop and continues to grow after the crop is harvested. I have generally used white clover for this purpose but this year I am also trying crimson clover, which grows a little taller and fixes a bit more nitrogen.
It's tough during a busy harvest season to put aside time for cover cropping. Ultimately, the payoff in terms of building soil health from year to year and reducing the need for fertilizer make it well worth the effort.