The terms “cover crop” and “green manure” can easily be confused. But you can also simplify the terms by stating the purpose behind each. Technically a cover crop is a crop whose purpose is to block out and smother out weeds. A cover crop also protects the soil from bad weather extremes.
A green manure is grown more to add fertility and organic matter to the soil.
Other times we grow plants because of chemical properties that can harm pests in the soil, such as specific mustards green against nematodes. Or we may grow them because they trap bad insects or attract good insects.
The fact is, plants have many more ecosystem services than just providing food. For this article cover crop is catch-all term for beneficial plants with specific services above and beyond edibility.
A Variety of Options
When it comes to cover cropping, everyone has their preferences of varieties, seasons and rotational timings within their home or market garden. Indeed, the cover crops you choose have a lot to do with the climate you grow in and local selection of varieties.
In colder climates, many people grow rye and vetch. Buckwheat works well for mid-season cover cropping because it is frost-sensitive. You can seed rye and vetch, on the other hand, in the summer and into fall.
In more Mediterranean climates, you will also find folks using fava beans and other cold-sensitive plants both early and later in their season. Indeed, when it comes down to it, a cover crop is really any plant that works into your rotation and helps to improve the production.
It is important to start to think of many less-understood plants as being cover crops.
For example, consider the following plants that seldom make it into the traditional lists of recommended cover crops.
The Vegetable Itself
You can plant the actual vegetable crop you already grow. I call these “crop cover crops” or 3C cropping.
With 3C cropping you maximize the capacity of the vegetables you already grow. They become a cover crop after you have harvested the edible portion. This can be particularly powerful when you have a short season and have limited time to remove the previous crop and seed a cover crop into your rotation.
As an example, consider that after two to three cuts of salad mix or arugula you can let these greens bolt (go to seed). This will create towering foliage full of nitrogen and organic matter. The growth will continue to open soil with already established roots.
An extra two to four weeks of growth from an established crop can yield significantly more benefits to your garden than a newly seeded cover crop in the same period of time. You can then follow this cover cropping phase with an additional cover crop. Or you can return to a later season crop.
Sometimes it is best to let the annual weeds grow. Really—they often balance certain conditions in the soil. This is why they dominate the rhizosphere’s germination profile.
Perhaps they grow well in response to an excess of nitrogen. Weeds can help fix and store this in the form of immobile organic matter. Later, it will release this nitrogen through flail mowing and decomposition.
Or perhaps weeds thrive due to compacted conditions that they can dominate over less tough crops. In this case, they will loosen and open the soil for other plants with aggressive roots.
Cover cropping with weeds helps burn up excess seeds in your soil’s seed bank. It can provide many of the typical benefits of traditional cover crops. You can easily manage stands of germinating weeds even in diversity (say primarily a single species, like lamb’s quarter) with a timely flail mowing before they go to seed again.
But either way, when it comes to a lower tillage style management, you can allow weed seeds to germinate as part of a pre-weeding cycle. Then you can fry them with silage tarps. You can also burn them with a flame weeder or simply eliminate with shallow cultivation.
In essence, don’t fear annual weeds. But don’t use aggressive perennial weeds—thistle, grasses and such—for “weed cover cropping” in an annual vegetable garden.
Perennial Cover Crops
Many varieties of perennial plants can grow adjacent to and within gardens and market gardens and used as cover crops. Leaves from broadleaf perennials like rhubarb make a wonderful sheet mulch in garden paths. And, in the fall, you can rake leaves from oaks, maples, pears and plums as mulch over garlic or broccoli.
Other herbs such as thyme, lemon balm and sage work as well to integrate into the mulching and composting processes of your garden.
I mention mulching here rather than cover cropping. But it is a similar process. We cover the soil and then work these materials in with a shallow tillage. Or we can allow them to rot on the soil surface by planting more crop through them or over-sowing new cover crop seeds.
Ultimately cover crops provide many ecosystem services for the farm or homestead. Looking outside the box, we find many more “cover crops” that can benefit the vegetable production rotations and systems.
When time is short, cover crops make good use of pre-established plants. When weeds are abundant, use them to help balance soil conditions. And when perennial agriculture is integrated with annuals, many new mulching materials are available in-situ for use in the garden.