In highly weedy fields, an 18- to 36-month remedial covered-fallow period that uses integrated weed management strategies can lower the weed seedbank and improve the soil, creating an environment in which crops can thrive. Conventional tillage practices have been suggested for areas with heavy infestations of resistant weeds, but for several reasons, many researchers do not recommend converting lands back to conventional tillage to control resistant weeds. If tillage is used, not all of the weed seed will be buried, and resistant weeds will continue to germinate after tillage. If weed seed is buried, continued tillage will bring resistant weed seeds back to the surface where they will germinate. Even if tillage provides enough seed burial to control heavy infestations, if neighboring lands are not managed for resistance, the resistant weeds can easily reestablish. In these situations, the long-term benefits from conservation tillage practices are lost and the resistant weed is still not controlled .
A recommended remediation plan to prepare fields for conservation tillage vegetable production is outlined in Table 11.3. Using cultural practices and, when appropriate, chemicals, to improve weed management and restore the soil health and productivity of a field, this plan pursues three objectives:
- Reduce the weed seedbank by stimulating consumption (germination, decay and predation) and preventing production of weed seeds .
- Increase soil organic matter by applying lime and nutrients, compost and high-biomass cover crops as needed [10, 15].
- Increase the effective water- and nutrient-holding capacity by increasing vertical soil distribution of organic matter, lime and nutrients .
The plan incorporates proven integrated weed management strategies. Fast-growing perennial sods or a series of annual high-biomass cover crops will smother weed growth, and foster decay and insect predation of weed seeds [3, 15]. Stale seedbed techniques will stimulate weed seed germination so that the subsequent seedlings can be destroyed with shallow cultivation, flamers or herbicides. Applying recommended soil amendments and implementing conservation tillage practices will enhance soil health .
The stale seedbed technique can help manage a large weed seedbank. It is based on three premises: cultivation promotes the germination of weed seeds; only a small percentage of weed seeds are non-dormant and able to germinate quickly; and the vast majority of weeds only emerge from seeds in the top 2.5 inches of soil. Prior to planting the crop, the stale seedbed technique involves intentionally creating an environment that is ideal for the germination of weed seeds and killing emerging weeds without disturbing those seeds that are deeper in the soil. Begin a few weeks prior to cash-crop planting by preparing a firm seedbed that is free of competing plants or weeds, typically by using tillage. Adequate moisture near the soil surface is necessary, so irrigate if the soil is too dry .
Emerging weeds are terminated using either flame weeders or herbicides. A variation of the stale seedbed technique is the “false seedbed,” which uses shallow cultivation to terminate weeds. If using cultivation to kill weeds, soil disturbance must remain shallow so that weed seeds deeper in the soil are not brought to the surface where they can germinate and compete with the crop during the growing season.
The amount of time between preparing the stale seedbed, terminating weeds and planting the crop will depend on a few factors. Most annual weeds germinate quickly, and this will happen faster in warm soils compared to cool soils. In addition, using a stale seedbed approach, the crop can be drilled into the emerging weeds, which shortens the delay. In this case, give very careful attention to the timing of weed termination. The aim is to terminate weeds as close to crop germination as possible but not after the crop has germinated, which could cause severe damage. In a false seedbed system, or if using transplants, planting must wait until after weeds have been killed with cultivation. In general, weeds will be terminated about two weeks after preparing the stale seedbed, and crop seeds can be drilled one week prior to termination. The process may take three weeks in cooler climates .