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How To Be Safe And Successful
With Chemical Weed Killers.
Most lawn owners need a weed killer at some point. It is true that a lawn can be so healthy, lush and thick, that a weed doesn’t stand a chance of growing. That probably describes fewer than 5%, and I don’t want to rub it in that your lawn isn’t perfect!
The easiest thing, which most people do, is to grab a bottle of commercial weed spray, or a bag of weed and feed, when they happen to think of it on their way through most any store. That works, to a certain extent. But things can go wrong, especially for those who don’t read the label. Do you prefer to know something about what you’re doing when you use chemicals around your family or pets? Would you like to understand some basic truths about weed killers, the good, the bad, and the exceptions? This information is slanted toward the use of weed killer products on lawns. A lot of it will prove useful for spraying (or not) weeds in other parts of your yard or garden.
Facts About Chemical Weed Killer
Satisfaction with a product comes when you have reasonable expectations about the results. It may seem like the best product should kill everything you want it to kill, in one application, with no negative results, and very little bother. That is not realistic. Here is what you need to know: - Many lawn weeds will die with one application. Additional spraying may be required for more mature weeds, for perennial weeds with stronger root systems, or for some stubborn types, especially those with waxy or hairy coverings. Definitions: mature weeds = the ones in your lawn because you waited too long; perennial weeds = dandelions, and perhaps some of the others that keep returning.
- Weed killer does not work better by using a stronger concentration. It is better to spray again, rather than go with a heavier dose which can have negative consequences. (You will probably need to spray twice, so why waste product that you didn‘t want to spend money on in the first place?)
- Some work quickly, within hours you may see dieback. Others work slowly and may take many days before you notice any result. (Fast is not best. This is not a race.) A fast weed killer is usually non-selective and kills everything. What matters is getting the best killer for a particular weed. Temperature and the rate of growth affect the reaction of many weeds to chemicals. (Cooler weather = slower results.)
- Liquid weed spray usually provides better weed control than a granular product. You can cover more thoroughly or do spot treatment of lawn weeds with a liquid. (Don’t believe everything you see in TV commercials about success with granular products.)
- Liquid weed killer is available as a ready-to-use product or a concentrate. The concentrate can be used in a spray bottle, a pump-up tank sprayer or a hose-end sprayer. Do not use a fertilizer sprayer like the Miracle-Gro type. A hose-attached sprayer must have a dial setting to adjust the concentration. (The price for ready-to-use products is anywhere from 4 to 10 times the cost of mixing your own, so consider what quantity you will eventually use, and what convenience is worth to you.)
- Sprayers used for chemicals that kill weeds should not also be used for fertilizers. Failure to clean out all residue may kill good plants later. (Leaving mixture in the tank leaves residue that can clog the nozzle, and the mixture may lose potency. It is best to use the entire mix within a day or two, and mix fresh when needed.)
- The weed killer found in weed-and-feed fertilizer is for convenience, not effectiveness. For early control of a few weeds, it can be helpful. For a serious infestation of lawn weeds, it will have limited results. You would need to reapply soon, but your lawn will not need additional fertilizer at that point. So concentrate on one task at a time. (Fertilizer should be watered into the ground. Weed killer has to sit on the leaf and not be washed off. Figure that one out!)
- Caution: be certain that the weed killer specifies on the label that it is safe to use on all lawns, or on your grass variety. Some grass lawns are more sensitive than others. (Some grasses, like St. Augustine, can react as if they are a broadleaf plant instead of grass.)
- Read and follow all directions. Some people are not aware that many garden chemicals have only the basic info for use on the back of the bottle. You can peel back the label for additional useful details. Each chemical is different. Don’t assume that the mixing amount is the same for different brands of the same product or for similar products. (Products like Round-up can come with differing percentages of active ingredient.)
- Many weed killer products for lawns specify that it is safe for people and pets to go on the lawn once the spray has dried. Use your judgment here to reduce chemical exposure to a minimum. (Also avoid placing treated lawn clippings with your compost pile, if the compost will then go on fruit or vegetable plants.)
Selecting The Correct Weed Killer
The chemical industry is continually introducing new garden products, and regularly removing old products, often due to environmental regulations, sometimes because it is not profitable for them to maintain registration on a label. It pays to look around at the options when you are in a garden department with an extensive selection. Some weed killer products will do double duty. For example, you can buy a broadleaf weed killer that is safe to use on most lawns, that won’t hurt the grass. It is now possible to get the same broadleaf killer with a crabgrass killer added. This may cost more than the original, but not as much as getting two separate products. Be cautious, however, since chemicals that are combined can increase phytotoxicity (plant sensitivity and reaction to chemicals). A mixture might be harmful to a plant even if both chemicals alone would not be. This is another reason why you should not increase the dosage of chemical in your spray mix. Here are the basic types of chemical weed killer: - Broadleaf weed killer. This will kill most broad-leaf weeds without hurting grass. Different brands have 2 to 4 active ingredients geared toward different types of weeds, or with a different mode of action. Always verify that it will be safe on your type of grass.
- Grassy weed killers. These will NOT be safe to use on your lawn. If it kills a weed grass, it will kill your lawn. It is great for any grassy weeds in your flower bed or ground cover areas. Grassy weeds in your lawn must be hand-pulled, or prevent them next season by using a pre-emergent weed killer.
- Sedge or nut-grass killer. This is a very specific killer. One called SedgeHammer, formerly Manage, should be safe to use on most grass lawns and is very effective. Some weed killer products claim to kill nutgrass, but the active ingredient is MSMA, which is more commonly used to kill crabgrass. It may work alright, but check carefully about the safety of your grass variety. (Nut-sedge reproduces from corms, little bulbs, that grow on the roots. These are difficult to kill except with repeat sprayings, and difficult to hand-pull without digging up and sifting through the soil.)
- Non-selective weed killer. The well-known Round-up is the best example of this. There are many generic versions that offer the same active ingredient, though not the same surfactant which helps absorption. All of these non-selective weed killers will kill your grass. They may be useful to you if you plan to kill everything prior to replanting a lawn. Also, if the weed density is so heavy that the grass has been overtaken, you can use this type of weed killer, avoiding overspray as much as possible, and plan to reseed the bare area later.
- Special Mention: Bermuda grass is a creeping grass that is used by some as a lawn grass. Other people consider it a weed when it invades their lawn of fescue, rye or bluegrass. A particular weed killer developed to kill broadleaf weeds was found to be effective in controlling Bermuda grass in turf. This product, Turflon Ester, is great for those who need to remove Bermuda. It will take numerous applications, 30 days apart, so start early in the growing season. But it would otherwise be impossible to eliminate Bermuda in regular turf, so check this out (and read all limitations and warnings about some grass types).
When selecting a weed killer, take the time to determine if it makes the necessary claim to be both effective and safe for your situation.
Final Tips For Using Weed Killer On A Lawn
Grass endures constant stress, and keeps on growing. That is why we love it. What other plant can handle constant pruning, being walked on, having the soil around its roots compacted, endure heat, cold, too much or not enough water, and still keep growing! Spraying chemicals is one more added stress, so reduce the impact as much as you can. - Avoid spraying in extremely hot weather. Temperatures above 85 degrees will amplify the stress on the grass and cause damage. It is best to wait for temperatures below 80 degrees.
- Some apparent damage to lawns is not unusual after spraying chemicals, especially in hot weather. This wonderful plant will usually grow right through it, and the damage should no longer be visible after a couple of mowings.
- Don’t apply any weed killer to young grass. It does not have the same resistance to chemicals as mature grass, and will probably be burned, possibly die. Three months is the absolute earliest to try it, and don’t go heavy on the concentration. Waiting 4-5 months would be a better approach.
- Many weed killers need to remain on the weed without rain or irrigation for 24 hours. Check to be sure, then plan your spraying accordingly. In hot weather, consider watering your lawn the day prior to spraying to give it a boost.
- Weed killers have to be absorbed into the leaves of the weed. Mowing prior to spraying weed killer reduces the amount of chemical that can be absorbed. Wait at least a few days after mowing, and do not mow for several days after spraying, which can allow more chemical to get down to the root.
Remember, the earlier you attack the weeds in your lawn, the easier it is to get rid of them. If it is later in the season, just be persistent. Make plans to prevent weeds next year with preemergent and building up healthy soil and grass. Check out the rest of the Lawn Care site for help with doing that! Go to Top of page Go to Lawn Weeds general info page.
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