Actually, it’s always time to plant cover crops. Here’s why.
What are cover crops?
Cover crops are crops grown with the intention of plowing them back into the field, or roller flattening them, or otherwise sending the crop straight from growing up to composting down, all without harvesting any valuables from it.
Why grow a crop just to wreck it?
There are two reasons for growing cover crops: (1) what happens, and (2) what doesn’t happen. What happens is that leguminous crops (peas, beans, etc) and other “nitrogen fixers” will convert atmospheric nitrogen (of which there is an abundance) to plant soluble nitrogen (of which there is not so much). Little nodules of nitrogen will collect along the roots. When the plant is killed and the roots rot, these nodules will act as slow release fertilizer tablets for the next crop in the field.
Non-nitrogen fixing plants are also valuable as cover crops. Not only will their roots bind up any excess nitrogen in the soil, but they’ll also act as wedge (loosening the dirt), net (collecting the soil and preventing erosion), and sponge (organic matter absorbs and releases rainfall in a very helpful way). The tops of non-fixing cover crops create thick mulch mats of organic matter, pure gold to an organic vegetable farm.
What does not happen when a cover crop is in place is also important. Plants shade the soil, protecting it from direct sunlight, which would cause soil nutrients to oxidize. Picture the soil steaming nutrition right into the air. Not good.
What are good cover crops?
I need to learn more on this, but here’s what I know so far. (Feel free to chime in in the comments section.) If you plan to return to planting that plot early the next spring, it’s helpful to use an annual that is killed by the winter, such as annual clovers and Sudan grass. This winter kill is so effective where I am (zone 5) that tilling the next spring is a moot point. Why muck up the nice deep mulch that’s been created?
If you are waiting to plant summer vegetables in this plot, then winter hardier combinations, such as a peas and oats mix, balance each other out well. Legumes (peas) often enhance the availability of phosphorus (P) while grasses (oats) often increase the availability of potassium (N). While this combination winter kills in most northern areas, comparable combinations of hairy vetch and winter rye are likely to survive; luckily, both roller kill well when in flower / pollen-shed season (~May). Just in time for transplanting season!