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Time to Weed

After the seedlings emerge, it is important to manage the young weed seedlings. In the past, we have known that crop and weedy plants compete for light, moisture and nutrients. Hence, the more weeds, the more it reduces crop yield. Weeds are easier to kill when they are small, so it makes sense to control them early by harrowing, cultivating and hoeing them at the appropriate stages.

However, it has been difficult to explain how early weeds, even at their tiniest stage when the crop is barely out of the ground, can have large effects on final yield. New research at the U. of Guelph, by Prof. Clarence Swanton has found that the light quality at the soil level has a significant effect on yield. Light at the far end of the visible spectrum is called red/far-red (R/FR). The chlorophyll in plants either absorbs or reflects this part of the spectrum. Crop plants can detect the level of weed pressure around them, even at a very early stage, by how much R/FR light reaches the leaves from that reflected off neighbouring weedy plants.

In a weed-free situation, lots of red and less far red light results in a shorter plant. In a weedy situation, more far-red light will promote stem elongation. Swanton’s work with corn indicates that the shorter plant will yield more than the taller plant caused by early weed pressure. These taller leafier plants put more energy into vegetative growth, which doesn’t translate into more grain yield.

Another interesting finding from Swanton’s research is that corn plants in weedy situations tended to orient more of their leaves parallel with the rows. That’s important because this means it takes longer for the canopy to close, and permits more light to get to shorter weeds that are trying to grow.

Another reason to control the early germinating weeds is that weeds that germinate early, grow the largest, and produce the most seeds. Weeds germinating at the end of the critical period tend to produce far fewer seeds and tend to have poorer germination to cause you future problems.

Table 1: Critical Period for Weed Control

Bottom line is to control weeds during the critical period which for most crops is from just after emergence up to early July.