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The presence of weeds does not necessarily mean that they are competing with a crop, especially during the early stages of growth when each plant can find the resources it requires without interfering with the others. However, as the seedlings’ size increases, their root systems will spread as they each begin to require greater amounts of water and nutrients. Estimates suggest that weed and crop can co-exist harmoniously for around three weeks, therefore it is important that weeds are removed early on in order to prevent competition occurring.
Weed competition can have quite dramatic effects on crop growth. Harold A Roberts cites research carried out with onions wherein “Weeds were carefully removed from separate plots at different times during the growth of the crop and the plots were then kept clean. It was found that after competition had started, the final yield of bulbs was being reduced at a rate equivalent to almost 4% per day. So that by delaying weeding for another fortnight, the yield was cut to less than half that produced on ground kept clean all the time.”
Perennial weeds with bulbils, such as lesser celandine and oxalis, or with persistent underground stems such as couch grass (Agropyron repens) or creeping buttercup (Ranunculus repens) are able to store reserves of food, and are thus able to grow faster and with more vigor than their annual counterparts. There is also evidence that the roots of some perennials such as couch grass exude allelopathic chemicals which inhibit the growth of other nearby plants.
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