Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
There has been an increase in interest in the use of certain Brassica species as cover crops in vegetable rotations for biofumigation when planted in late summer.
Rapeseed has been used as a winter cover and has shown some promise in reducing the levels of certain nematodes and disease organisms in the soil. To take advantage of the biofumigation properties of rapeseed you plant the crop in late summer, allow the plant to develop until early next spring and then till it under before it goes to seed. It is the leaves that break down to release the fumigant-like chemical. Mow rapeseed using a flail mower and plow down the residue immediately. Never mow down more area than can be plowed under within two hours. Mowing injures the plants and initiates a process releasing biofumigant chemicals into the soil. Failure to incorporate mowed plant material into the soil quickly, allows much of these available toxicants to escape by volatilization.
Several mustard species have biofumigation potential and a succession rotation of an August or early September planting of biofumigant mustards that are tilled under in October followed by small grain can significantly reduce diseases for spring planted vegetables that follow.
Fall growth of biofumigant mustards. When tilled under in October and then followed by small grain such as rye, diseases in spring planted vegetables can be significantly reduced.
We are planning a project for later this year in which we will measure the biomass production of biofumigant mustard planted on different dates. This will allow us to give better recommendations on the planting window for fall biofumigation.