The series of questions below (first suggested by Kopf et al in 2000) can be used as an aid to deciding whether or not a direct weed management action needs to be undertaken.
A weed is generally defined as a plant growing where it is not wanted, so the definition of a weed will depend on each individual and the situation. Weeds compete directly with a sown or planted crop for space, light, water and nutrients and are estimated to be responsible for around 10 per cent of all crop losses. To prevent yield losses they need to be removed when they are actively competing with the crop.
Some things to consider when deciding if weeds are a problem and if control is needed:
- Will marketable yield be affected by the weeds? Yield reductions occur when weeds compete directly with the crop. Without some form of weed control marketable yield can devastated: reductions in fresh weight of as much as 96 per cent have been reported in trials with salad onions. Growth and size requirements for marketing many field vegetables may be seriously compromised by weed competition.
- What other negative effects are the weeds having? The physical presence of weeds may impair the harvesting process, shed seeds may affect the quality of the crop if, for example, seeds fall into lettuce heads or contaminate crops such as peas, weeds may encourage crop pests or pathogens
- What are the long term consequences of infestation? Lack of control may result in a build-up of the weed seedbank or spread of perennial weeds even if there are no short term reductions in yield in which case immediate weed control may be justified.
- Are there any benefits associated with the weeds? Some weeds in grassland may be a useful source of trace elements, self-medication or extending the grazing season, although many suppress grass yield by a significant percentage. Some weeds cover might deter pests like aphids or root flies although once again there will be some competition effect on yield.
Two tips emerging from the knowledge on organic weed management are:
- Remember that removing all weeds is not only difficult, it is not always desirable and it does not necessarily make sense economically. Many pieces of research have shown that the cost of removing weeds completely can financially outweigh the resulting yield benefits.
- Thinking long term is crucial in organic farm systems. Weed management should be considered throughout the rotation and as a property of the whole farm system. Weeds may not threaten the current crop, but may have an impact on later crops if allowed to set seed or propagate. Perennial crops that will be in the ground for more than one season may be affected because competition in the early stages of growth may affect crop productivity in the longer term. The weeding of each crop in the rotation needs be planned and the schedule of weeding operations in different crops in one season needs to be co-ordinated