Apr 7, 2007

Brassicas Have Multiple Values As Cover Crops

For many vegetable crops, three- to five-year rotations are ideal.

Rotations provide a “time out,” during which disease spores, insects and nematodes decline because they are deprived of a host. The idle time can be used to undo compaction, elevate organic matter, conquer problem weeds and adjust pH and nutrient levels.

But given the value and scarcity of good vegetable land, is a long rotation feasible? Is there any way to get the value of a long rotation in a shorter time?

The answer to that is cover crops, said Mathieu Ngouajio, a Michigan State University horticulturist. He showed off several that he likes during a tour of the MSU Muck Research Farm in early August.

A cover crop is a fill-in between two crops, something to keep the ground covered, and not a cash crop in and of itself. It uses idle time and not time that could otherwise be growing a cash crop.

Finding a good cover crop for a niche can be difficult, Ngouajio said.

In onions, for example, the problem is the long growing season. He’s not yet found a good cover crop to plant after onions have taken 120 days out of the season.

Sorghum-sudangrass produces a lot of biomass and has disease-suppressing properties, but it likes hot weather. It can be used to follow early-harvested crops like peas or beans, but not later crops like onions and potatoes.

Rye and wheat can be planted later, in early fall, but the real biomass accumulation starts late the next spring. Then, they can be hard to kill and incorporate in time for spring planting.

For these reasons, Ngouajio said brassicas are good choices as cover crops that can be planted late, grow fast and heartily in cool weather, scavenge nutrients, die over the winter and leave a manageable residue for spring planting. In addition, they release glucosinolate compounds similar to those in the fumigant Vapam and thus contribute to control of insects, nematodes, diseases and weeds.

The members of the mustard family he recommends include oilseed radish and oriental, yellow, brown, black and white mustard. Oilseed radish will cost $20 to $40 for the 10 to 20 pounds of seed it takes to seed an acre ¬– somewhat more expensive than the others.

“Whether planted in spring, late summer or early fall, oilseed radish grows quickly and produces a large amount of biomass (about four tons per acre) in a relatively short time,” he said.

Oilseed radishes produce large taproots, like the Daikon radishes you see in market produce sections. Upon decomposition, they leave large holes in the ground that improve water infiltration.

It emerges quickly and establishes fast, even under moderate drought conditions, providing good soil protection from wind and water. It is a good choice for muck soils, he said.

When tested against other crops like hairy vetch, crimson clover and soybeans, it suppressed fall weeds far better.

“An oilseed radish cover crop can fit into diverse cropping systems,” Ngouajio said. “It can be seeded after harvest of a short-season crop such as pickling cucumbers, snap beans, wheat, rye, early potatoes or celery. In the fall, it will cover the soil, smother weeds and recycle nitrogen for the next crop. It can also be planted in early spring to provide green manure for cash crops planted in late May or early June.

“Oilseed radish leaves low-surface residue in the spring, so it is very appropriate for crops that require a well-prepared seedbed. This is particularly important for small-seeded crops and for efficient operation of transplanters.”

Oilseed radish doesn’t fit everywhere. It is not a good choice in rotation with cabbage, broccoli or other brassicas. While it is killed by winter, if it is planted early and goes to seed, it is potentially a weed.

Seed can be hard to find, Ngouajio said. Several varieties are available, but the common cultivar produces as much biomass – and more root – than the named varieties.