By Aaron Patton and Vera Vukovic
Smooth crabgrass is one of the most common weeds in lawns and probably the weed species we get the most questions about. We investigated the influence of mowing, fertilization, and light on the development of smooth crabgrass through a combination of field, greenhouse, and growth chamber experiments.
We learned that smooth crabgrass development in lawns depends on the amount of light reaching the soil surface. Nitrogen fertilization, and more importantly, increasing mowing heights, reduced the light penetrating the turfgrass canopy and minimized smooth crabgrass encroachment. In fact, we could almost completely eliminate crabgrass by increasing the mowing height to 4.0 inches, whereas we often had close to 50% crabgrass by the end of the season when mowing at 2.0 inches.
Increasing fertilization in the Kentucky bluegrass turf we studied reduced crabgrass, but it didn’t have the dramatic effect that mowing height had on crabgrass.
Our tests revealed that smooth crabgrass germination is not affected by light quality or quantity. In fact, smooth crabgrass germinated in the dark. This is different than what we anticipated and it caused us to ask the simple question, “If crabgrass can germinate in the dark, why doesn’t tall cut turf have crabgrass?”
What we learned in our greenhouse studies is that crabgrass germinates in shaded turf canopies, but increasing turf density (i.e., increasing shade at the soil surface) greatly diminishes the early development of smooth crabgrass.. Numerous morphological and physiological consequences were noted in smooth crabgrass plants growing in dense shade that prevented their proper development and rooting.
The results highlight the importance of using practices to promote healthy and dense lawns to increase competition against smooth crabgrass by excluding the light it needs to develop.
The above summary is a brief synopsis of the research we conducted. If you are interested in more details and the technical aspects of this research, read the entire published paper in Crop Science by clicking this link: https://doi.org/10.1002/csc2.21351