Imaginary Call to the Master Gardener's Office

    "I have this weed that grows only in the winter in my St. Augustine grass yard! It's really annoying because it grows real low to the ground and forms mats.  The leaves are like little ears (lobed) and I don't see any flowers.  When I try to pull it up, it sticks me in the hand!"

I recently took an online Master Gardener continuing education / re-certification course in which the previous scenario was given to us.  Before taking this class, there is good advice I could give this person without knowing exactly what the weed is.  It is a winter annual so, a properly-timed application of preemergence herbicide will control it.  Hand-pulling larger plants (with gloves) is an option.  Also, a broad-leaf herbicide for St. Augustine lawns would work if you missed applying a preemergence.  But we were given an additional resource to identify this weed - Turf and Weed ID.

I've only played with it a few times, but this site helps identify a number of grassy and broad leaf weeds and sedges, if you can describe them accurately.  Some of the questions you needs to answer are whether it is a grass, broadleaf or sedge, its growth habit, bulbous or fibrous roots, leaf characteristics, etc.

So, let's use this tool to solve our current problem.  The first decision we have to make is the type of plant.  The fact that the leaves are lobed tells us that it is a broadleaf weed - we're down to about 50 choices.  The fact that it grows only in winter suggests that it is a winter annual - that cuts our choices in half.  We used the fact that the leaves are lobed to determine that it is a broadleaf, which narrows our possibilities down to about seven.  This person said that it grows low to the ground, so, after choosing "prostrate" as its growth habit, we are left with only one plant - lawn burweed (Soliva sessilis).

I didn't know what this plant looked like, but I was familiar with it nevertheless.  I used to walk around barefoot a lot and I won't forget the feeling of stepping on burweed.  Over the years I avoided certain yards and wear shoes outside all the time now, so it's not an issue anymore.  It is a winter annual, but as the weather warms in the spring, it begins to grow rapidly, producing spiny burs.

An ounce of prevention goes a long way.  A healthy, robust lawn will outcompete this weed, reducing the amount your yard.  Burweed can become established in areas where the turf isn't very thick, so good lawn care is important.  A number of different herbicides will control the spread of lawn burweed.  Postemergence herbicides are more effective on this weed during the winter, when the plant is small.  If you use them on the plant during the spring, it may be harder to kill.  It may also give the plant a chance to produce its burs, in which case, a preemergence herbicide will be necessary to further control the spread of this plant.  For more detailed information and herbicide recommendations, please see HGIC 2323 Lawn Burweed.