How Can I Prevent Weed Growth? (Pre-emergent vs post-emergent herbicides)

As the air warms up and we transition to springtime, you may notice some weeds beginning to “spring” up in your lawn. A pre-emergent herbicide is the most efficient way to rid your lawn of pesky weeds. Pre-emergent herbicides allow the weed to germinate but attach to it before it emerges. Once the cap root shoots up, the weed takes in the chemical and dies. As Mr. Allen Woerner, owner of Woerner Farms, said ”Rather than getting a sip of water, [your weed] gets a sip of chemical.” This is a better option than a post-emergent herbicide because they can only kill 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. For pre-emergents, by contrast, the requirement is only that the seed must be active in the soil before you can effectively disseminate your herbicide.

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 does recommend, however, that you spray a post-emergent herbicide 2 days after pre-emergent treatment, to catch any escapees. As for any weed-and-feed product, he argues that this is 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.” 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 I be applying 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.