I've been peppered recently with emails and Facebook posts from listeners who are disappointed with their results after using pre-emergent herbicides like Barricade and Dimension, the two most readily available.
Their gripes are usually along the lines of, "I used Barricade, and I swear I have more weeds than ever. I'm bummed that it didn't work like you said, Randy."
As I noted on the air last weekend, pre-emergent herbicides are just one tool used for trying to keep weeds at bay. They are never 100% effective in blocking germination. But along with following my lawn care schedule and mowing correctly (tall for St. Augustine, reel mowers for Bermuda), using pre-emergents is an important part of the plan. However, Mother Nature also has to cooperate. Once you put down an application - three times every year, per the schedule - you cannot have it immediately washed away with a 2- to 3-inch rain. That may have happened in your yard twice this year, in February and May.
Staying true to the schedule will help develop a thick, healthy stand of turf, and that's a natural weed prevention. Weeds simply cannot germinate in a St. Augustine lawn that is mowed tall and fertilized appropriately. In my consulting business, I see it all the time ... people covered up in weeds almost always have the most poorly cared-for lawns. And I always see there's no organic matter in the soil, the lawn has been mowed improperly, they say they try to follow the schedule but they don't, they've never done a core aeration, and they've never done a compost top-dressing.
Those are all part of the protocol that one needs to follow to get a good-looking lawn along the Gulf Coast. Just one application of Barricade, for example, is never going to keep a yard weed-free, especially if it's done just before a major gully-washer. Depending on when the pre-emergent was put down and how much rain ensued, it may need to be reapplied. Otherwise, don't assume it will work as it should.
Some emailers have complained that their Barricade application "didn't kill any of the weeds." Remember that pre-emergent herbicides prevent weeds. To kill weeds that are already up, you need a post-emergent herbicide. More on that is detailed in this tip sheet.
I'm emphasizing pre-emergents today because my schedule strongly encourages their use from the end of September through October. And of the three pre-emergent applications called for annually, the October application is probably the most important. That's when the turf will go into dormancy as soil temperatures get below 55 degrees. (Don't Laugh - it will happen!) It's also when weed seeds become opportunistic and result in us being hit with clover, chickweed and poa annua in December and January.
If you prefer to go organic in your lawn care, I'm perfectly fine with commingling organic fertilizers with synthetic pre-emergent herbicides. Manic organics might feel that's a no-no, but research shows that soil will not be negatively affected by mixing the two. If you want to be as weed-free as possible, use both. But if your goal is to stay 100% organic, then just avoid the synthetic pre-emergent.
Finally, if you have a thick stand of grass, and all is working well and looking good, you can start eliminating the pre-emergent herbicides called for in the schedule. That's because well cared-for turf that has a deep root system and dense growth is its own best defense against germinating weeds, period!
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