- Malheur Experiment Station
Growers employing organic farming methods can utilize several options before and after planting onions. There are organic herbicides, and these methods could also be useful for ‘conventional’ growers as part of the overall weed IPM program. Several precautionary, pre-planting measures to implement are cover cropping, deep plowing, pre-plant weed germination, and soil solarization. Cover crops are crops planted for a purpose other than food production; for example, certain types of brassicas and legumes can be mulched to fumigate fields before planting onions, therefore reducing weed pressure. Also an effective organic growing method, deep plowing can be “a tillage technique that buries weed seed or propagules of perennial plants below the depth at which they can germinate”. Another option, pre-plant weed germination, involves “the use of irrigation or rain to stimulate weed seed germination before planting onions or garlic…emerged seedlings are then killed by shallow cultivation, flaming, an organic herbicide, or a combination of these treatments.” Soil solarization involves laying down a sheet of plastic on the field to trap heat from the sun in the topsoil, effectively killing perennial weeds in an inhospitable environment (Smith et al., 2007).
After the onions have been planted, growers can use cultivation, mulch, and hand-hoeing to get rid of weeds. Ideally, cultivation tills out weed seedlings as close to the row of onion plants as possible without disturbing the plants. Dark-colored mulch has also been used to block and absorb sunlight, preventing weed growth (although this would be expensive to implement in large onion crops and can reduce storage quality of the bulbs (Reitz, S., 2013)). Hand-hoeing is the least efficient weed control method, but is not as time-consuming as using hand-hoeing alone when used alongside other organic farming practices listed above (Smith et al., 2007).