Correlation does not imply causation. That’s a concise statement that biologists use to warn of the dangers of jumping to erroneous conclusions. It means that just because A and B occurred at about the same time, A didn’t necessarily cause B. A case in point may now be visible out the nearest window of your home.
A number of Missourians have contacted me this past fall and this winter regarding the weedy plants that are taking over their lawns and some say “killing the grass.” Unless you religiously watered your lawn last year through the long summer drought, it is likely that you have many clumps of green lawn weeds where you once had grass. Those weeds are called winter annuals because their seeds germinate in the fall and the young plants stay green throughout the winter, only to resume growth, flower and produce seeds very early next spring. They die in late spring or early summer, but by then they have sown the abundant seeds of future weed crops. Those seeds are ready to grow when conditions are again right for germination – bare soil, moisture and plenty of light.
Last year’s severe drought in much of Missouri killed grass in many lawns. The dead spots are usually scattered with some grass remaining. Where the grass died in the summer heat, there was bare soil available in the fall when a little moisture finally arrived. That was a perfect storm for winter annuals whose strategy is to germinate and spread over bare soil areas in the fall. The seeds of winter annuals are always there but a normal healthy cover of lawn grasses doesn’t give them much opportunity to grow. Last fall they got their chance. The weeds spread where there had been grass, so it was natural, but incorrect, to assume that the weeds took over by killing the grass. In reality, the weeds didn’t kill your grass; they just took advantage of the lack of grass.
You can spend your time fighting those winter annuals and you may accomplish something if you can reduce the amount of seed that they produce next spring. But the better use of your time and energy is to work on restoring your lawn grasses. A thick grass cover doesn’t provide much opportunity for the takeover of annual weeds. For assistance with lawn restoration, contact your local lawn care professionals. The University of Missouri Extension also has useful documents and advice.