Sandburs are annual warm-season weeds that tend to thrive in sandy soils. Gardeners have problems with sandburs not only breaking up the uniform texture of their lawns, but also getting stuck with this weed's thorny seed head. To kill off these weeds, choose a herbicide that is chemically formulated to kill annual weeds but not your grass.
Pre-emergent herbicides containing trifuralin prevent sandbur seed germination, according to the University of Minnesota. This herbicide works by forming a barrier in the first 1/8 inch of soil. When sandburs emerge, they come in contact with the herbicide and die. For this pre-emergent herbicide to be effective, you must apply the spray when soil temperatures reach 52 degrees Fahrenheit. Measure your soil temperatures after the last frost in order to pick the right time to apply pre-emergent herbicide.
Pre-emergent Herbicide Application
Press a screwdriver into the first 6 inches of top soil. Place a soil thermometer inside the hole to obtain a reading. Check the soil temperature in the morning and afternoon. Take an average of both temperatures for an accurate assessment of your soil temperature. Apply pre-emergent herbicide evenly over the entire grassy area. Be sure to skip your flowerbeds, because pre-emergent herbicide can prevent ornamental seed germination. Water your lawn after applying the spray so the pre-emergent herbicide can sink into the soil.
You may use a post-emergent herbicide combined with a fertilizer to prevent sandbur growth. Post-emergent herbicide and fertilizers contain trifuralin and balan. Use this herbicide/fertilizer mix in late April to early May. If soil temperatures favor sandbur seed germination and your grass has not greened up for the growing season, avoid using the herbicide. Grass types such as bermuda should not be fertilized before they have a chance to green up.
Use a post-emergent herbicide that contains MSMA or DSMA on existing sandburs. Choose a wind- and rain-free day to apply post-emergent herbicide. It is important to avoid mowing two days before and after applying post-emergent spray so the chemicals may absorb into the sandbur foliage. In addition, if you're growing St. Augustine or centipede grass, you should not use herbicides that contain MSMA. MSMA damages or kills off these two lawn types.