Interseeding, or sowing cover crop into a standing cash crop, is a way get a jump on the traditional winter cover crop season. This can lead to an increase in cover crop biomass production, and presumably, better erosion control and soil organic matter enhancement. Earlier establishment also increases your choices of cover crops compared to waiting to sow a cover crop until after a full-season vegetable crop.
The most popular choice for a winter cover crop after vegetable crops is winter rye because it can put on vegetative growth as long as the temperature is above 38 degrees F. Oats or other small grains can also be sown late, although they can't match rye's ability to put on growth in cold weather. In the case of a very late vegetable harvest, or just not getting around to the job, it may even be too late for winter rye to put on enough growth to protect the soil over winter. That's where interseeding can be an advantage by getting the cover in place early.
To successfully interseed cover crops, the timing must be right. Sowing must be delayed enough to minimize competition with the vegetable crop, but early enough so the cover crop can survive competition with the vegetable and then withstand the harvest traffic. The best timing depends on the vegetable - cover crop combination, and of course your location. Farm trials in southern Vermont have been successful with several different combinations, and these will be described below. On you own farm, it's important to start small as you experiment with a new cropping system.
Wimpy vegetable crops are poor candidates for interseeding. Carrots, onions and the like will suffer from the competition. Vigorous vegetables, like winter squash and sweet corn, call for relatively early interseeding so that the cover crop has a chance. Right after last cultivation is a good time to interseed a cover crop - there's a nice seed bed for the cover crop, and the vegetable canopy is not yet fully closed, so there's still some light available to help get the cover crop going.
In trials with winter squash, cover crops were interseeded during the first week of July, just as the squash vines were starting to run. No herbicides had been used on any of the three farms, and field cultivators of various types had left a clean swath of soil between the crop rows. Arlington red clover, Linn perennial ryegrass, and annual ryegrass were then broadcast or drilled. Seeding rates ranged from 12 to 25 pounds for the clover and 35 to 60 pounds for the ryegrasses.
High seeding rates were used to help get a decent stand, and to get an edge on the weeds. All the cover crops were well-established by mid-October. Brillion-seeding left a bare area where the squash had been, whereas broadcasting lead to more complete ground cover. On one farm, none of the crops established, possibly because the grower cultivated the cover crop strips too deeply after sowing.
A different system was used to in trials with bell peppers and kale. In this case, last cultivation occurred in mid-August, and hairy vetch plus rye, rye alone, or oats were broadcast into the crop. Standard seeding rates were used: 30 pounds vetch plus 1 bushel rye, 2 bushels of rye, or 110 pounds of oats. Again, the soil was well prepared to receive seed. A timely rainfall after sowing, or irrigation help with establishment. These crops all established well, the only complaints being from the pickers, whose sneakers got wet in the cover crop treatments but stayed dry in bare ground plots.
A more serious concern with these systems is the possibility of disease enhancement, since crop residues are not plowed under at the end of the growing season. Hence, interseedings are not advisable if there are serious disease problems in the crop. It would then be better to disk the field and try an get winter rye on. Another concern is yield reductions due to competition. In dry years, this could really be a problem. Interseedings are pretty risky unless irrigation is available.
Some growers have tried interseeding with crops like cabbage and found that the broadcast seeds get caught in the leaves, sprout, and make a mess. One grower has found an innovative way to overcome this. He set up his crew with 5 gallon buckets of cover crop seed, and they race down the rows hand-broadcasting seed under the leaves of the crucifers. In smaller fields, it doesn't take that long.
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