Green manure is not fresh out of the horse. In fact, it’s not even manure. It’s another name given to cover crops, or crops that are planted for one reason only: to benefit garden soil. Think of it as a grow-your-own organic fertilizer, literally. Cover crops also stabilize soil and attract beneficial bugs, providing them with food and shelter.
Choose which crops to grow based on what your garden will need. For instance, do you need lots of nitrogen replaced, or is erosion control more of a concern? Some cover crops replace nitrogen, others use it; some crops are better than others at erosion control. Do you live in a mild or harsh climate? Some cover crops do not survive frost. Of course, your choice of winter cover crops should also fit in with your plant family crop rotation plan.
Here is a list of several green manure crops you can grow during the winter to fertilize and/or stabilize your garden soil.
Alfalfa – Plant alfalfa from Spring to Fall; it adds nitrogen and has a deep root system that is ideal for loosening up heavy, compacted soil. This isn’t used as often as other cover crops, as it needs one full growing season to ripen before tilling into the soil.
Barley – Start interplanting in the Fall. It helps to prevent erosion while growing and adds organic matter once it is tilled into the soil. Winter hardiness is hit-or-miss, depending on how cold it actually gets; does very well in milder climates.
Buckwheat – This works well as a companion cover crop during the warm months, attracting beneficial bees and bugs. Because it is easily frost-killed, this should be used as a winter cover crop only in very mild climates.
Clover – Interplant from Spring to Fall; this crop adapts well to most conditions and thrives in the shade. During the active growing season it makes a great groundcover between garden beds. During the winter it helps prevent erosion and adds nitrogen to your soil. Crimson clover is the easiest to till into the soil, but can attract harmful nematodes.
Oats – Start interplanting in late Summer/early Fall. Oats help prevent erosion and use nitrogen while growing, but are killed by frost and will then release nitrogen into your soil during decomposition.
Vetch – Start interplanting in early Fall; this crop thrives in winter temperatures, particularly the hairy variety. It adds nitrogen to your soil while growing and plenty of organic matter once it is tilled into the soil. You may need to mow it down a few days before tilling it in to make the task easier, as it gets very thick.
Winter Rye – Start interplanting in early Fall; this crop thrives in winter temperatures. Root system draws potassium up from deep underground and provides good erosion control. Uses nitrogen rather than adding it to your soil. Harvest for straw mulch before tilling under.
Winter Wheat – Start interplanting in early Fall; this crop thrives in winter temperatures. Uses nitrogen rather than adding it to your soil. Provides erosion control and adds organic matter when tilled into the soil. Harvest straw mulch before tilling under.