Winter Cover-cropping in the Mixed Vegetable Field
Winter cover cropping in the short season mixed vegetable field
Choosing the right cover-cropping schedule, one that accommodates the time and space required for growing the main crop to maturity, can be tricky for the small farm. We small farmers usually don't have ample extra space for adopting a multiple year crop rotation plan which puts portions of the field out of market production for a year or more for cover-cropping. In the future, this is my goal as we open up more of our field for cultivation. For now though, I am currently dealing with this common challenge of how to best utilize winter and summer cover-cropping opportunities to gain the benefits of cover crops, without sacrificing the growth of the main crops.
Winter Cover Cropping
In many ways, winter is the ideal time for me to cover-crop. Water is plentiful and I'm not growing anything but garlic in my main field at that time. I plant a winter hardy 50/50 mixture of rye and hairy vetch. Rye is a very hardy cereal crop which grows fibrous root systems. These roots will hold onto the soil during winter wind and precipitation, reducing erosion. Rye is fast growing and produces a lot of organic matter that can be incorporated in the spring. Hairy vetch is a hardy, nitrogen-fixing legume. It grows more slowly than rye and doesn't cover or hold onto the soil as well, but it makes a good companion as the nitrogen fixer in the mix.
The fact that these are winter-hardy cover crops means they will need to be incorporated into the soil in the spring. I'll do this by tilling about three weeks prior to planting the crop that follows. This allows the organic matter to break down without giving too much time between crops for nutrients to leach out or weeds to establish. Because I'm dealing with a number of crops planted at different times in the spring, I can till the cover crop progressively. Leaving the cover crop growing until I need the space helps in spring weed control and I'm able to get the maximum benefit of the nutrients and organic matter the cover crop can provide. However, the cover-crop must be incorporated while its still in the succulent vegetative stage. As soon as it begins to flower, it is going to be taking more from the soil than it is giving, and nutrients will move from the stems and leaves into the flower and seeds. I will also incorporate a cover crop early in the event of an exceptionally dry late winter or spring. In these conditions, the cover crop is stealing water stored in the soil that will be needed by main season crops.
Utah State University Extension recommends planting winter cover-crops 30-40 days before the frost. This allows the crop to begin growing and get established before winter's intensity hits. Not much actual growing happens during winter months, so if the intent of the cover crop is to have enough established roots to diminish erosion the fall growing period is important. The NRCS recommends planting cereal crops like rye from August 15 to October 15. Hairy vetch is recommended for planting earlier, from August 1 to September 15. This is accurate to my experience, hairy vetch grows much slower and will not establish well if planted in mid October. Rye seems to do fine planted as late as late October, but vetch will not so I must plant them both earlier.
Many of my vegetable and seed varieties need every day they can get to fully mature. And there's the rub with winter cover-cropping. In the fall I'm watching the forecast every day to determine when I must lift my potatoes, bring my winter squash inside, and harvest the dry beans still drying down on the plants. I don't want to lift the potatoes or other root crops before I need to, because I use cold storage to store them. It's not climate controlled and if it's too warm outside, it's too warm in my cold room. In fact on September 15, the latest date advised for planting hairy vetch, most of the field is still very much occupied. Few publications on winter cover cropping acknowledge this reality faced by vegetable growers!
So far I have utilized a few practical solutions for dealing with this problem. First, if there is any part of the field vacant at the ideal planting time, I plant it. This might include where the peas, cabbage, or spring greens grew, if I have not followed them with another crop. Second, I interplant or undersow cover crops between or under existing crops where I can. I have permanent raised beds with established pathways so I don't plant in the paths, but I can manage to sneak some rows of cover crop between onion rows that are drying down. Both of these solutions require more work, you are seeding more precisely and several times instead of broadcasting it all at once.
The reality is that the bulk of my cover crop goes in late. It does so both because the field is full, and because that's when the rain arrives to water it in. I soak vetch seeds before planting which hastens germination and I am documenting just how late I can get the vetch and rye mixture in and still be successful. As long as the crop has enough time to begin growing in the fall, it will continue growing in the spring.
More on cover-croppoing methods and seeding rates:
http://extension.usu.edu/boxelder/files/uploads/hg521.pdf
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_005818.pdf
