Garden Tips: Some weeds grow best when it’s hot

Did you ever wonder why it seems like some weeds grow faster in hot weather?

Growth of cool-season turfgrasses, like Kentucky bluegrass and fine fescues, slows during the sweltering heat of summer. However, some weeds make the most of the heat and sun because they have a different type of respiration, and function better when temperatures are between 85 to 117 degrees. Some of these warm-season weeds are Bermuda grass, crabgrass, foxtail, goosegrass, purslane and prostrate spurge.

I hate crabgrass because it sticks out like a sore thumb with its lighter green leaves and rapid growth, making it obvious that there are major crabgrass infestations in lawns. How could this happen despite applying a crabgrass “preventer,” or pre-emergent herbicide? Even if the herbicide was applied at the recommended time, spring temperatures fluctuated up and down and may have thrown off the timing and resulted in applying the material too late or too early. If applied too early, the material may have lost its effectiveness before the crabgrass germinated.

Failure to control crabgrass could also be because of a lack of good coverage or not using the right rate of herbicide. Always be sure to calibrate your sprayer or spreader before application. For even coverage, apply half the amount of herbicide in one direction, then the other half in the direction perpendicular to your first pass.

A healthy dense turf is the best protection against crabgrass. Last summer’s extraordinary heat was tough on lawns and resulted in thinner turf that is now being stressed again by excessive summer heat, and in some areas, severe drought stress. Keep in mind that the grass in most lawns is composed of cool-season turfgrasses that actively grow during the cooler months (March, April, May, October and November). With the warm fall last year and warm spring this year, many lawns have not fully recovered.

When (and if) cool fall weather arrives, fall fertilization in early September and November will be important for green lawns that have made it through the summer. However, only fertilize if there is adequate water available and the grass is green and growing.

Residents have also been noting the proliferation of prostrate spurge. There are four types of prostrate spurge, with spotted spurge being the most common in this area. These low-growing plants have tiny leaves and form a prostrate mat along the ground. As members of the Euphorbia family, they have a milky sap that can cause skin irritation.

Spurge grows best in dry, open areas and takes advantage of bare garden soil and dry lawn edges. In the garden or landscape, pull or dig up to control spurge. In lawns, use a “spurge killer” herbicide containing triclopyr, but you may find it easier just to dig up the plants if you only have a few.

Of course, the weeds are the only green plants in some lawns where there has been restricted watering this summer. In a few weeks we will discuss what, if anything, can be done to bring these lawns back from the brink of doom.

This story was originally published July 23, 2015 at 10:00 AM with the headline "Garden Tips: Some weeds grow best when it’s hot."