Cover crops to plant in early spring
Medium red clover and yellow mustard
After late-harvested vegetables, fields may be left open over the winter with only the decomposing crop residue protecting the ground. There is an opportunity in the early spring to sow a cover crop. There are two early-spring options.
Medium red clover
Spring is the most common time to sow clover. This cover crop is often overseeded into small grain. If mowed at early flowering, it can provide nitrogen and cover through the summer, and even until the following spring.
A new option for a short turnaround, white or yellow mustard sown in March or April puts on biomass in May for incorporation at flowering in late May or early June. For frost seeding, the Tilney variety has the best take. Spring mustard suppresses weeds and can reduce soil disease pressure. Don't follow mustard with vine crops, though, because seedling emergence is often unacceptably low.
When to killcover crops in spring
One of the frightening things about using a rye cover crop is when it rains all spring, and the rye is over your cap by the time you can get to it. Tall rye is really challenging to manage, and even when you get the ground worked, it takes a long time for the soil condition to be good for planting vegetables.
Fortunately, there is no need to cut it close on killing that winter cover crop. Most overwintering cover crops give you the most value if you kill them quite early.
April is the best time to kill many overwintering cover crops. They can be killed with an herbicide that works at lower temperatures, and smaller plants can often be killed with shallow disking. April weather doesn't offer lots of chances to get on the ground, but it is worth taking those chances when they happen.
For getting nitrogen value out of grains like rye, the best time to kill them is when they have recently greened up and just started to grow. Perhaps six to eight inches tall. When rye is larger than that, the nitrogen concentration drops, leading to tie-up when your crop needs it. An early kill can give a 30 to 50 lb N credit (yes, from those little plants!), while killing at boot can cause a significant debit. Killing at boot also makes the rye is also slower to break down, leave less time for it to break down before planting, and the larger crowns make a seedbed more difficult to prepare. Delaying control of rye also raises the risk of missing the opportunity, and having it grow too big to handle.
Annual ryegrass only becomes sufficiently sensitive to glyphosate when it's warm enough for the leaves to grow. Once it is sensitive, don't delay application because the young growth is the source of nitrogen.
Fall-sown crucifers usually die in the fall (radish, mustard) or early spring (turnip). The latter is better for recovering N to use in the next crop. In either case, there is little regrowth in the spring. The reason to control crucifers early in the spring is to avoid volunteers from stray survivors. If you see yellow (or pink radish) flowers in the field, it may be a signal to get your attention.
Despite the disadvantages of killing rye at the boot stage there is one practice where it is good. If the rye (usually a rye-vetch mix) is to be killed by mowing or rolling, the stems become lignified enough at the boot stage that rolling breaks them. The vetch is also at its maximum nitrogen content, creating a favorable C:N ratio. I consider that a special case where the late kill is appropriate.
In my research program, we tested whether the crop inhibition caused by rye is reduced if one uses triticale or wheat, becasue they are less allelopathic. We killed all of them with herbicide at early to mid-boot, incorporated and let them break down. We transplanted tomatoes, peppers and cabbage, and direct seeded corn, beans and cucumber. All these crops showed about a 25% reduction in growth in the first month. The kind of cover crop made no difference. That result shows how deleterious late control of small grains can be, and that the cause is not allelopathy.
It may seem premature to kill cover crops before they put on much biomass in the spring. You do forego some addition of active carbon. However, the cost of adding the extra organic matter just before planting is too high. It is better to get the nitrogen value and the soil improvement from the extensive root growth, and to work on organic matter production at the end of the growing season.
The information on killing in early spring appeard in the March 2011 VegEdge, from the Cornell Vegetable Program.