Greg Grant

I’m not a turfgrass specialist, but I sure get asked lots of question about weeds and lawn. Believe it or not, spring is just around the corner, which means that summer weeds can’t be too far behind. Two common lawn weeds are grassburs (sandburs) and crabgrass. Both are warm-season annuals (or occasionally short lived perennials) that germinate each spring from thousands of seed produced the previous year and years past.

Always remember that the best control against weeds in a lawn is having a dense, vigorous, healthy turf. If there are bare spots and sunlight can hit the ground, weeds will germinate. Otherwise the only effective control for these grassy annual weeds is using a pre-emergent herbicide. Pre-emergent herbicides are ones you apply before the weed seeds emerge or start to germinate and sprout. This means you must apply these types of herbicides before you see the weeds. If you see them growing (like the cool season weeds you have now), it’s too late. Pre-emergent herbicides only work on annual weeds germinating from seed, not on perennial weeds germinating from an existing crown, like bahiagrass and dallisgrass.

The timing of this warm season pre-emergent herbicide application is absolutely critical. They are normally applied about 2-4 weeks before our last frost. The last frost here is generally around March 15. This means the typical time to apply a pre-emergent herbicide for warm season annual weeds like crabgrass and grassburs is right now!

Pre-emergent granular herbicides like Team, Halts, Balan, and Dimension are available from retail garden centers, feed stores, and hardware stores. Other independents package and label their own brand. Read the label and follow its directions. They must be watered in to be affective as they form a chemical herbicide barrier at the soil level that kills the seedlings as they begin to germinate. It’s critical to follow label instructions and use only the recommended rate so as not to damage the existing lawn, trees and shrubs.

If the weed infestation has been severe in past years, a second application can be made 90 days later. This is the normal protocol:

1. Mow the lawn to eliminate as many weeds and as much thatch as possible. If you intend to drop the mower down one notch for the first mowing (“scalping”), do so at this time.

2. Apply your pre-emergent granules according to label directions on that same day. Water lightly to form a layer of herbicide across the top of the soil.

3. Wait about a month to apply a 3:1:2 ratio lawn fertilizer (not a “weed and feed”) as our warm season grasses don’t use the fertilizer until the nights are warm and they are actively growing.

Greg Grant is the Smith County horticulturist for the Texas A&M AgriLife Extension Service. You can read his “Greg’s Ramblings” blog at arborgate.com, follow him on Facebook at “Greg Grant Gardens” or read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com). More science-based lawn care information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu. You can also follow Aggie Turf on Facebook.