MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Recent scouting of the landscape has revealed some pesky weeds invading turf. I’ve noticed some really healthy patches of both white clover (Trifolium repens) and black medic (Medicago lupulina). Thinking of the recent TV show about being smarter than a fifth grader, hopefully everyone knows how to identify white clover as it is one of the most commonly observed weeds in turfgrass. However, black medic could be a little trickier and may be confused with oxalis (Oxalis stricta). Oxalis is commonly referred to as yellow woodsorrel. Oxalis is similar to white clover in that it has the three-leaf cluster, but the leaves are almost perfectly heart shaped and have smooth edges. Oxalis can also be easily distinguished from clover by its yellow flowers. The flower could be described as resembling a miniature yellow daisy with petals. Black medic also has the three-leaf cluster, but its center leaf is on a small stalk and it lacks the water stain mark that is on the clover leaves. Another distinguishing characteristic of black medic that is currently present is the small ball-like yellow flowers. The flowers are about the size of a pencil eraser and have no distinct petals. If you’re looking for further help in identifying the weeds go to http://www.msuturfweeds.net

Weeds are typically indicator species. They may indicate compact soils, shady conditions or low fertility just to name a few. So what stories are white clover, black medic and oxalis trying to tell. White clover is a natural nitrogen fixer, and although it is certainly wide spread throughout may different turf situations, I recently drove by a commercial building with an under fertilized turf and was not surprised to see the turf dominated by large patches of flowering white clover. Simply providing adequate nutrition for the turf will help the turf compete with the white clover. Black medic is very similar to white clover in that it often invades turf that is under fertilized. Oxalis is a little different from white clover and black medic in that it is a late germinating summer annual, so if you applied any herbicides earlier this spring to take out clover, the oxalis would have escaped simply because it was not present. It seems to have developed a nice niche for avoiding the early season broadleaf herbicide applications and then germinating in the voids created by killing those weeds. Controlling oxalis with the traditional 2 and 3 way broadleaf herbicide formulations can be difficult. Products containing triclopyr should be effective for controlling oxalis, black medic and white clover. In many cases, spot treatment of trouble areas can be conducted in lieu of treating the entire turf area.

Keep in mind that treating these weeds at this time of year can be difficult and if you can sign some sort of truce with the weeds to make it through the summer, your best time for treatment is in the fall. With our mild and wet weather we’ve been experiencing lately, the weeds are really enjoying life and any herbicide application may initially damage the leaves, but there is so much food stored up in their roots that they can recover quickly and fight on. The best time to treat is in the fall when the weeds are starting to store carbohydrates in their roots for next year and any herbicide application will be more readily transported to the roots and effectively kill the weed once and for all.

Fertilizing and red thread

The abundant rainfall and prime growing conditions throughout many areas of Michigan has resulted in the turf using up any spring fertilizer applications rather quickly. I’ve noticed red thread disease popping up on golf course roughs and in home lawns. Red thread is typically indicative of low fertility, just like the clover and medic we mentioned earlier, applying a fertilizer application will help the turf out-grow the damaged areas. At this time of year, I would either apply a fertilizer that has slow release nitrogen, or if you’re going to use a fast release nitrogen fertilizer, apply it at a rather modest application rate such as 0.5 lb. N/1000 ft.2. The weather is sure to heat up sooner or later, and you don’t want to create a flush of growth from a fertilizer application just as it gets really hot and humid. That’s a recipe that will make the turf susceptible to heat stress and disease infestation.