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Pre-emergent herbicides: The whole story

I’ve been peppered with emails and Facebook posts from listeners disappointed with the effectiveness this year of pre-emergent herbicides like Barricade and Dimension, the two most readily available.

Their complaints are along these lines: “Randy, I used Barricade last year, and I swear I have more weeds than ever. I’m bummed that it didn’t work like you said …”

Photo: Getty Images, Randy Lemmon

As I noted on the air last weekend, pre-emergent herbicides are just one tool we use to keep weeds at bay. They are never 100% effective in preventing all weed seeds from germinating. You also need to follow my lawn care schedule and mow correctly - tall for St. Augustine and using reel mowers on Bermuda, for example. Plus, Mother Nature has to cooperate. Once you’ve applied a pre-emergent herbicide (three times a year, per my schedule), you can’t have a 2- or 3-inch rain wash it away. That happened twice last year, possibly affecting many people’s recommended February and May applications. Following my schedule and having a thick, healthy stand of turf is a “natural” weed-prevention practice. Weeds simply cannot germinate in a St. Augustine lawn that is mowed tall and fertilized appropriately. In my Randy Lemmon Consulting business, I see it all the time: People covered up in weeds usually have the most poorly cared-for lawns. I always see there’s no organic matter in their soil, they mow St. Augustine too short, they mow a Bermuda lawn with a rotary lawn mower, and they say they “try” to follow the schedule, but they really don’t. Plus, they’ve never done a core aeration and they’ve never done a compost top-dressing.

Those are all among the protocols needed for a good-looking lawn along the Gulf Coast. One application of Barricade is never going to keep a yard weed-free. Especially if it’s done right before a major gully washer. And instead of re-applying, as I have recommended in some cases, many just assume the drenched application will still work.

Some emails I’ve received contend that their Barricade application didn’t work because “it didn’t kill any of the weeds.” Remember, pre-emergent herbicides prevent weeds … post-emergent herbicides kill weeds that are already growing. I published detailed look at cool-season herbicides last week, and those are true post-emergent herbicides.

And let me note that pre-emergent herbicides only block typical broadleaf weeds and most grassy weeds. They do not work on Virginia buttonweed, doveweed, basketgrass or nutgrass/sedge.

I’m emphasizing pre-emergents this week because October is when my schedule strongly encourages their use. And of the three annual pre-emergent herbicide applications, October’s is the most important. Turf will go dormant once soil temperatures get below 55 degrees, and that’s when weed seeds become opportunistic. Consequently, we get hit with the likes of clover, chickweed and poa annua in December and January.

And a final note: If you have a thick stand of grass and all is working well and looking good, you may be able to start eliminating the pre-emergent herbicides called for in my schedule. That’s because well-cared-for turf with a deep root system and a dense growing pattern is its own best defense against weed seed germination. Period.


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