Identify weeds to determine best treatment

Out of all the questions I get in the extension office, I can say with confidence that homeowners wanting weeds identified and finding out how to kill them ranks in the top five. Practically every day, someone comes in with a weed or several weeds for me to look at.


As we get late in the season, as we are now, it gets very difficult to control summer weeds. Why? Herbicides just don’t work as well because although the weeds are still green, like the lawn, their growth has really slowed down and so they don’t absorb the herbicide as they would if they were actively growing.

Also, mature weeds are much harder to kill than younger weeds. Many hard-to-kill weeds require several herbicide applications.

For the most part, don’t try to kill summer weeds now. Just wait and let the frost kill them for you. Keep weeds mowed or use a weed trimmer to prevent them from developing seed heads. Most weeds release thousands of seeds over your lawn, making weed control even more of a headache next spring and summer.

There are a few exceptions to waiting for frost to kill a weed. One is a weed called lawn marsh pennywort, brought to me last Monday. It is a summer weed that grows very close to the ground, so you cannot mow it. It is in the same weed family as pennywort (also called dollarweed), familiar to many people. It forms a mat and loves to grow in moist places. I had it pretty bad in my front lawn last summer and fall.

As fall turned into winter, my lawn marsh pennywort remained green. You all remember what a mild winter we had last year. It never got killed. I had a pretty bad infestation of annual bluegrass (poa annua) so I put some Image (imazaquin) in a hose-in sprayer and applied it to my front yard. The Image did a great job on the annual bluegrass and killed the entire lawn marsh pennywort after a few weeks.

This summer and fall, I have continued to get a few clumps of lawn marsh pennywort, but periodically I have treated with Image to kill it.

The moral to this story is that when it comes to weed control during fall and winter, whether to treat or not depends on the summer weeds and the herbicide. I wouldn’t expect us to have as mild a winter as we did last year, so all the summer weeds should die back.

If you have not done so already, apply a pre-emergence herbicide to prevent winter weed germination. If winter weeds have emerged, you can start killing them although most are very small and hard to see until the grass goes dormant.

Keep in mind that you will be much more successful in controlling winter weeds during November through January than in February and March when they become larger and more mature.

Also, the vast majority of herbicides that you can use to kill winter weeds caution against using them during spring green-up of your lawn. The risk of injury from post-emergence herbicides is greater during the green-up process than when the turfgrass is dormant or actively growing. Using them can also delay green-up in the spots you spray.

As winter weeds begin to emerge, it’s best to scout your lawn to find out what types of weeds you have. The more common winter weeds you will encounter are annual bluegrass, wild garlic (most people call them onions), common chickweed, henbit, lawn burweed, Florida betony, dandelions, Carolina geranium and cudweed.

If you have small areas and don’t like to use herbicides, you might just pull or dig up the weeds. For large areas of weeds, this is usually not very practical.

Getting rid of winter weeds will provide your lawn with an advantage when it comes time to green-up next spring. Without weeds, the grass does not have to compete for sunlight, nutrients and moisture and can get off to a better start in the spring.

The weeds and type of lawn you have determine which herbicide you should select.

The question always comes up about using Roundup for winter weeds on the lawn during the winter. This has been done with success on Bermuda and zoysia when they are fully dormant, but if there are any green stolons, you could cause injury. There were plenty of green stolons in these two grasses last winter. When fully dormant, the injury usually amounts to nothing more than a slight delay in spring green-up. The advantage of using Roundup is that it will work quicker and better and will eliminate the need for a follow-up application.

When in doubt, though, don’t use it. You definitely would not want to use Roundup on St. Augustine and centipede.

Always spot-spray your weeds during the winter when the temperatures are at least normal (mid- to upper 50s or, even better, above normal). It also helps when we get a good rain a few days before your application so that the weeds are well-hydrated.

Take control of winter weeds during late fall and early winter so you won’t have to deal with them next spring.