This tip sheet is a compilation of everything I have written in the past about weed control.
First, here are some very basic points and a few products that work great early in the season.
If you have a preponderance of weeds, it's likely you have very unhealthy soil and/or you're not following THE SCHEDULE, which promotes the importance of pre-emergent herbicides. The healthier the yard, the better natural defense it is against weeds with or without pre-emergents. You get a healthier yard by ...
1. Following The Schedule
2. Mowing at the appropriate height
3. Keeping up good irrigation/watering practices.
Most weeds that crop up early in the season can be hit with a broadleaf weed killer — post-emergent herbicides. The "cool-season herbicides," used while the temperatures are still 45-75 degrees, are Fertilome Weed-Free Zone and Bonide Weed Beater Ultra. There are also a couple new granular products considered environmentally safe replacements for the Atrizine-based weed-and-feeds, and they can be used at any time of the year. Fertilome's is called Dollarweed Control Plus and GreenLight's is a granular version of Wipe Out.
Personally, I've always recommended the liquid versions of broadleaf weed control, because the granular weed-and-feeds with atrizine are so damaging to groundwater supplies and the roots of trees and shrubs. Plus, you can spot treat with the liquids. However, I am impressed by the early research on the atrizine replacements. The active ingredient of the Fertilome Dollarweed Control Plus is penoxsulam. The products are so new, though, that we have scant empirical data to back up their early claims. But I'm confident I'll be recommending such products more and more. Still, I will only recommend them to people with huge weed problems. With a few weeds here and there, it is still environmentally better to spot-treat.
If you wait until daytime highs start creeping into the 80s to control broadleaf weeds, then stick with the three most well-known broadleaf weed controls on the market: GreenLight Wipe Out, Bonide Weed Beater for Southern Lawns and Fertilome Weed Out. The previously mentioned cool-season herbicides become ineffective and stressful to grasses once the high temperatures are consistently in the 80s. And, as always, you should use a surfactant with any liquid weed control. Read this tip sheet for reasons why.
MAJOR WARNING: You will see many other "weed killers" at mass merchandisers and big-box stores. If you'll read their labels, you'll likely see they are not formulated for southern turfgrasses like St. Augustine. That's why I recommend only those listed above.
Most of the obnoxious weeds that appear early in the season (dollarweed, clover, dandelion, thistle, chickweed, henbit, wild geranium, nettle, etc.) are broadleaf weeds. The only annoying grassy weed that appears early is poa annua (wild bluegrass), and that doesn't usually look bad as long as you keep it mowed ... it will burn off with the heat, so I don't pay it much attention. Prevent it altogether in November with a pre-emergent herbicide. If you're not sure exactly what weeds you're faced with, check this handy Web site for identification.
EARLY GREEN-UP TRICKS OF THE TRADE
The best-kept secret in lawn fertilization — one even I was unaware of until two years ago — is the use of a basic 3-1-2 ratio fertilizer in late February or early March. You may recognize the ratio from the GardenLine fertilization schedule. But this 3-1-2 is a basic, immediate release fertilization. And the best fertilizer I've found for this purpose is a 15-5-10. But remember, it should not have the words "slow-release" or "controlled-release" on the bag. Don't don't be lulled into a false sense of security if you experience an early green-up but have not followed the basic fertilization schedule for early April. Also, be sure that there are no freezes on the horizon. All is not lost if you miss the basic fertilization by 2 or 3 weeks, but try to get back on schedule.
I don't recommend weed-and-feed fertilizers because of the atrizine found in most.
If misused or over-applied, atrizine is notorious for moving very quickly through the soil, eventually contaminating ground water. And even the weed-and-feed bags warn against getting it near the drip line of trees and shrubs. But I realize that many people need to solve a winter weed problem before getting busy with the basic fertilization schedule. So, here is your one and only opportunity. But you have to take this pledge:
I DO SOLEMNLY PLEDGE TO USE A WEED-AND-FEED FERTILIZER ONLY ONCE IN A YEAR, AND I FURTHER PLEDGE TO MAKE SURE THAT I FOLLOW APPLICATION INSTRUCTIONS EXPLICITALLY, SO AS NOT TO OVER-USE OR OVER-APPLY, BECAUSE SOME WEED-AND-FEEDS THREATEN OUR GROUNDWATER SUPPLIES AND CAN HARM THE TENDER ROOTS OF TREES AND SHRUBS. WITH JUSTICE AND GARDENLINE FOR ALL!!!
Since most of the weeds that are up now will burn away in the heat of a Texas summer, let's focus first on keeping late spring and summer weeds from being a problem. This is done during the end of February through the first of March with pre-emergent herbicides. Pre-emergent herbicides control the grassy weeds (crabgrass, goosegrass, Dallisgrass and Johnsongrass) that are such a problem May through July.
The only truly effective way to control these weeds is to prevent them. If you do the pre-emergent application right, you'll never need to use a weed-and-feed. But it is critical that pre-emergent applications are put out before the end of March. The best pre-emergent herbicides for February application are Barricade and Dimension. Preen is another brand-name pre-emergent found at big-box stores. Barricade or Dimension from the Nitro Phos and Fertilome are found at smaller independent nurseries and garden centers. Other pre-emergents are Betasan, Treflan, and Surflan. Green Light makes two three pre-emergent herbicides — First Down and Amaze. A third from Green Light simply says "Betasan."
If you have a bunch of weeds already up, they are usually the winter germinating type that could have been prevented with two pre-emergent herbicides back in November. If that didn't get applied or if you have clover, poa annua, chickweed, henbit or others, knock them back with a post-emergent herbicide. Most post-emergents kill only broadleaf weeds, so if your problem is mostly the thin seed-head-heavy poa annua (often looks like rye grass gone bad), don't worry — heat will take care of it. Most post-emergent herbicides are also of the liquid variety. You can either use a concentrate in your own pump-up sprayer or a dial-in hose-end sprayer. The best news in recent years is the ready-to-use sprays you simply attach to the end of a water hose.
The best names for post-emergent herbicides are Green Light Wipe Out, Fertilome's Weed Out, Bonide's Weed Beater for Southern Lawns. The Green Light and Bonide products come in a ready-to-use form that attaches to the hose. In all cases, these products must be used according to the instructions. And here's your final "best kept secret" with regard to broadleaf weed killers: My two favorites are Spreader/Sticker from Hi-Yield and Bonide's Turbo. Also, note that some of the ready-to-use sprays might have a "surfactant" built in. Just read the label. Finally, no matter how bad you think the weed problem is, if you double the dosage of any of the products I've recommended, you will almost assuredly kill your grass.