When Should I Apply Pre-Emergent Herbicides?

As the temperatures begin to warm up in the spring or cool off in the fall, it becomes imperative to apply pre-emergent herbicides to your lawn to prevent weed growth. A pre-emergent is an herbicide that works by preventing weed seeds from germinating or killing seedlings shortly after they germinate, subsequently preventing it from becoming a full-grown plant. As Mr. Allen Woerner, owner of Woerner Farms, said, "Rather than getting a sip of water, [your weed] gets a sip of chemical." Since they go into effect before weeds sprout, correctly timing the application of pre-emergents is critical. Warm-season (summer) weeds begin to germinate when the soil temperature consistently reaches 55 degrees F. Cool-season (winter) weeds germinate in the fall. According to Mr. Graham Simmons, former director of Turfgrass Operations at Woerner Farms in Foley, AL, pre-emergent timing is critical because, “once the temperature is right for weed seed germination, you are fighting from behind.” One option to estimate your location’s soil temperature is to use this online map tool.

If it's too late to apply pre-emergents, you might consider a post-emergent herbicide, which kills weeds after they have germinated. Post-emergents must be used when the plant is actively growing and not simply green, and a heavy application can easily kill nearby desirable plants. Because of this, planning ahead to use pre-emergents is a better option to defend against weeds. However, pre-emergent herbicides can harm new grass seedlings and prevent grass seed from germinating, so avoid their use in recently planted areas or areas where you plan to seed.

According to Mr. Woerner, 93% of your lawn’s weed problem can be controlled by pre-emergents. When you rely on post-emergent herbicides, it is often too late to tackle the problem, because the weed is already out and growing. Mr. Woerner and Mr. Simmons do agree, however, that spraying a post-emergent herbicide 2 days after pre-emergent treatment will help catch any escapees. Many pre-emergents are mixed with a nitrogen fertilizer, which can be problematic for your plants when nitrogen works with spring frost to cause harm. Mr. Woerner argues that these weed-and-feed products are not a viable solution for weed control. “One is trying to kill and the other is trying to nurse back to health. It is not the most effective way to manage weeds.”

Mr. Woerner’s recommendation for the best pre-emergent herbicides is Ronstar, which he says is “the most favorable [and the] safest pre-emergent out there that can be used on all grasses.” Some other suggested options for the he best pre-emergents include Hi-Yield Weed & Grass Stopper, Sta-Green Crab-EX, and TURFGRO Professional (the latter two of which can be purchased at your local Lowe’s). Some chemicals like Atrazine have both pre and post emergent control. The aforementioned pre-emergents do not contain nitrogen, and are therefore our favorite picks. As for how much to use, you should be able to find this recommendation on the label! You can spray your lawn using a backpack sprayer or spread the granular version via a spreader.

Spray a post-emergent herbicide 2 days after pre-emergent treatment, to help catch any escapees.

So, when should you apply pre-emergents to be most effective in eliminating spring weeds? Mr. Woerner said it all depends on temperature, but it is safe to say that “any time late February you need to be rockin' and rollin'” here in south Alabama. You should spray your lawn with pre-emergents between the transition from spring to summer. Then, you should wait until fall goes into winter to spray again. Mr. Allen explained that this gap period is when you should be using post-emergent control.

Humidity and temperature are some other major factors that must be considered while trying to figure out when to spray. According to Mr. Allen, it is best to spray in high humidity. Additionally, there is a specific temperature range at which you should spray your lawn with pre-emergents. That range is 65-75 degrees. If you spray when the temperature exceeds 85 degrees, the pre-emergent will burn your lawn on contact. This is why it is important to “water in” your pre-emergent within 24 hours to prevent burning. “Watering in” essentially means that you provide your lawn with plenty of water following a treatment. Mr. Woerner continued, “[You need] at least a quarter inch of water or a half inch of rain within 24-48 hours to activate [the] pre-emergent, and prevent your lawn from burning.”

If you use too much pre-emergent on your lawn, you may experience some “jumpers.” Mr. Woerner explained, “If you get over a pint and a half per acre, the roots are not able to peg into the ground and they will shoot skyward and make an arch. These are known as jumpers.” One downside to using pre-emergents at incorrect rates is that this will usually set back your root pruning. Mr. Allen argued, however, “[It] is way more favorable to hurt your root than to have a yard full of weeds during the summer; [and] post-emergents will actually be more harmful.” Over-spraying your lawn is usually difficult and expensive to remedy. Applying a large amount of carbon can sometimes help by absorbing the chemical. Mr. Allen explained that carbon “will attach itself to the active ingredient and neutralize it," but "it requires an extremely large source of carbon and is not ideal.”

So, as springtime approaches and you anticipate weeds, have your pre-emergents ready to tackle the problem before it arises.