Weeds and Their Control

202. I have heard some people call Kentucky bluegrass a weed, yet I am enjoying my bluegrass lawn. What would be a good definition of a weed?

The definition used most often describes a weed as any plant growing where it is not wanted. This means Kentucky bluegrass is a weed when growing in a flowerbed. And patches of bluegrass in a Bermudagrass lawn would also be considered weeds.

203. What is the difference between an annual and a perennial weed?

An annual weed (knotweed, purslane, sandbur, crabgrass) is a plant that starts its growth from a seed each year. Perennial weeds (dandelions, ground ivy, thistle, quackgrass) come back each year from the old parts of the plant, usually from a stem or root.

Knowing which plants are annuals and perennials is important in ridding them from your lawn. (See Table 7—1.)

TABLE 7-1: Examples of Pre-emergent and Post-emergent Weed Killers

204. Why is it important to know whether a weed is an annual or a perennial?

It is necessary to know the difference in order to properly select a weed killer to control the weeds in your lawn. Annuals die each year and reestablish themselves from seeds the following year. Many chemicals (preemergent) are formulated to stop the seed from germinating and thus eliminating the pesty weed. This type of chemical will also kill the seedlings of the perennial plants, but the original plant will still survive. It takes another type of chemical (postemergent) to control established annuals and perennials.

205. What is a preemergent weed killer?

This type of chemical kills the seeds as they germinate, preventing the plants from becoming established. To be effective it should be applied before the seed-germination process begins. Crabgrass and chickweed can be controlled by this method.

206. What is a postemergent weed killer?

Any chemical that kills growing plants is a postemergent weed killer. The most common chemical in this group is 2, and it is frequently used to control dandelions.

207. What is the difference between a broadleaf and a narrow-leaf weed, and why is it important to know the difference?

The actual width of the leaf has nothing to do with the terms broadleaf weeds and narrowleaf weeds. Narrowleaf is a common term used for grassy weeds such as annual bluegrass and crabgrass. Broadleaf refers to plants such as dandelions and plantain. It is necessary to be able to identify the plants so that the proper chemicals can be used to control them. Many chemicals will control weeds of one group only, either the narrowleaf or broadleaf weeds.

208. What does the term active ingredient refer to on a can of weed killer?

Active ingredient (A.I.) is a term used to show the percentage of the solution that actually kills the weeds. If the A.I. of a gallon of weed killer is 25 percent, then one quart is the actual amount of weed killer present. The other three quarts act as a carrier. Usually the higher the percentage of active ingredient present in a mixture, the more economical it is to use.

209. How do I know which weed killers will control the weeds without damaging any of the grass, trees, shrubs, or flowers?

The safest way is to read the directions on the label and check for a listing of the plants that are susceptible to damage by the chemical. Follow the instructions carefully because some plants will tolerate only small amounts of the chemical being applied.

210. When I’m applying chemicals to control weeds, is it okay to increase the rates of application to insure the eradication of the weeds?

The best policy to follow when using any chemical is to follow the standard directions on the label. Several years of research have gone into the developing of each chemical, and the rates suggested on the label have been found to be the best. Sometimes increasing the amount of chemical applied will be harmful to the grass.

211. When is the best time to control common lawn weeds?

When you are dealing with annual weeds, the easiest time to control them is when the seeds germinate. Perennial weeds present more of a problem but can be controlled anytime they are actively growing by the application of a foliar herbicide (weed killer). Some weeds are more resistant than others to foliar sprays and may require repeated applications before they are killed. (See Table 7—2.)

212. What are some of the lawn-management practices that help control weeds?

The first rule to remember is that chemicals are not a substitute for good lawn care. Mowing, fertilization, and watering, if done properly, will help prevent weeds. Mowing the grass at the correct height allows it to become thick and prevents weeds from getting established in your lawn. This is especially true (mowing height) if there has not been any fertilizer applied or if the lawn is improperly watered. Overseeding, aerification, and dethatching also help to keep a lawn vigorous and weed free.

Three Steps to a Weed-Infested Lawn

1. Never bother to fertilize your lawn.

2. Hand-sprinkle often, just barely wetting the surface of the ground.

3. Always mow the grass as low as the mower can be set.

213. Are there any precautions I can take to keep weed seeds out of my lawn?

Here are four rules to follow for keeping weeds from getting established in your lawn:

1. When buying topsoil, be sure it has not been covered with weeds. Weeds growing on topsoil is an indication that weed seeds are present.

2. Buy grass seed that is free of weed seeds.

3. When using mulches (Straw and peat), be sure they are free of weed seeds.

4. Follow good lawn-care practices to insure a luxurious lawn.

How Weed Seeds Are Introduced into Your Lawn

1. poor quality lawn seed

2. mulching material

3. topsoil used to level lawn

4. animals, especially birds

5. wind carried

6. heavy rains

7. borrowed lawn equipment

8. people (seeds stuck to clothing)

TABLE 7-2: Troublesome Lawn Weeds

214. How can I control broadleaf weeds around trees, shrubs, and flowers when the same chemical that kills the weeds is also harmful to the desirable plants?

Often it is necessary to hand-pull many of the weeds when they are too close to desirable plants. If you want to spray the weeds, pick a day when there is no wind. Use a large piece of cardboard to shield the desirable plants as you spray the weeds. Be sure to read the instructions before starting so you understand what the chemical will and will not kill.

You could also use a wax bar, which has the herbicide impregnated into the wax. This bar can be pulled across the lawn, killing only the weeds it comes in contact with. (See Tables 7—3 and 7—4 for broadleaf weeds common to your region.)

TABLE 7-3: Broadleaf Weeds Common to the Northern States

TABLE 7-4: Broadleaf Weeds Common to the Southern States

215. How can I control the dandelions in my lawn when my next- door neighbors let them go in theirs?

Next tune you are spreading a weed killer on your lawn, have some left over that you do not want to store until next season.

Offer it to your neighbor. Be sure to compliment him or her on how nice the lawn looks later on in the year. When it is time to apply the weed killer again, suggest to your neighbor that you go to the lawn and garden center together and buy some. Try to make it a weekend job when two neighbors get together on a project and have fun doing it. You might even have a barbecue afterward.

216. What meant by the term noxious weed?

A noxious weed is any plant that has been declared by law to be a harmful or dangerous weed to animals or humans. The list of noxious weeds varies, depending on the state. Some examples of noxious weeds are wild carrot, dodder, giant foxtail, halogeton, marijuana, quackgrass, and annual bluegrass.

217. If I spray 2,4-D to kill the dandelions in my lawn, will it stop any of the dandelion seed from coming up later?

No, 2, kills only those dandelions that are actively growing; it does not have any effect on the seeds in the soil at the time you sprayed your lawn. Killing the actively growing plants will, however, reduce the chances of new plants getting started in your lawn.

Three Reasons Why 2,4-D Will Kill Broadleaf Weeds Without Damaging Mature Lawn Grasses

1. The surface of the leaves are different in the external makeup.

2. The growing habits of the plants are different. Grasses tend to grow upward, whereas broadleaf plants spread out over the ground.

3. The growing point of the plant is located in different places. In grasses it is buried at the base of the plant; on broadleaf plants it is located at the tips of the plant and readily exposed to any chemical treatment.

218. I have seen weed killers at the garden center in several different forms. Which should be used?

Weed killers are usually found in three different forms (formulations): wettable powders, granular, and liquid. Wettable powders (WP) must be mixed in water and require constant stirring to keep the chemical from settling to the bottom of your spray tank. For this reason it is usually best not to buy this form. Your choice between a granular and a liquid weed killer depends on the type of application equipment you own. The granular weed killers can be put on your lawn with a fertilizer spreader. A three-gallon, hand-operated sprayer works well for a liquid formulation. Economically, you will usually get the best buy for your money when you use the liquid weed killers.

219. How can the condition of a lawn affect weeds?

A sparsely covered lawn with large bare spots invites weeds and will quickly become a weed patch if you do not work to remove the weeds. If you have a lawn with dense, healthy turf, chances are you do not have many weeds present.

220. How does mowing of a lawn affect the weeds growing there?

Mowing is one of the most important factors in keeping your lawn weed-free. Mowing too short or removing too much growth in a single clipping will seriously weaken the grass, cause it to be shallow-rooted, and open the door for weed encroachment. If you want to keep your lawn weed-free, it is very important that you know the correct mowing height for the type of grass you have.

221. How soon after my new lawn seed has come up can I spray for weeds?

New grass seedlings are killed easily by herbicides, so you should avoid applying any for as long as possible. A rule of thumb is to wait until you have mowed the grass two to four times before you do any spraying. If the weeds are crowding out the young seed lings, then you may have to spray to save the grass. In this case, cut the recommended application rate in half and apply the chemical twice at a two-day to four-day interval.

222. Should I apply a granular weed killer before or after I water my lawn?

If you are going to apply a granular weed killer, do it after you water your lawn. In order for the plants to be killed, the granules must be left in contact with the leaves of the plant. If you wash the granules into the soil, then the weeds will not be killed and you have wasted your money. If your lawn needs watering, do it first, and the water will help the granules stick to the leaves.

223. What is the best way to apply a granular weed killer?

Drop spreaders (gravity flow) are the best applicators to use when applying a granular weed killer. They allow you to get fairly close to the ornamental beds around your lawn, with little chance of damaging the shrubs and flowers. Never use a cyclone spreader unless there is absolutely no chance that you will be near any desirable broadleaf plants.

224. How long does it take for a weed to die after it has been treated with a herbicide?

Usually it takes several days before the plant will die. If you watch the plant closely during this time, you will notice a gradual change in its appearance.

225. When is the best time to control a perennial weed?

The best time is when the plant has been actively growing and using up its stored energy (food) to produce new growth and flowers. Near the end of the growing season is when you will usually get the best results in trying to kill perennial weeds.

226. Can grass seed be sown after a preemergent weed killer has been applied?

This will depend on the type of chemicals used and how long after it has been applied that you wish to seed. Some preemergent chemicals allow you to seed the same day you apply, some require a 6o-day waiting period; and others will kill your seedlings up to a year after the chemical was applied. Always read the label care fully to determine how long you will have to wait before you can start seeding your lawn again.

227. How important is it to get even coverage when applying a herbicide?

Uniform coverage is very important when applying chemicals because a concentration of chemicals in one spot could kill the grass. You should always measure the area to be treated to insure that the correct amount of chemical is applied and the coverage is uniform. Remember, under-application and poor coverage will not give you the desired results, while over-application will concentrate the chemicals and kill your grass.

228. Why does the label on dicamba (Banvel) instruct me not to spray the weeds under the trees in my lawn?

Most postemergent broadleaf weed killers are absorbed and are no longer effective as broadleaf weed killers when they contact the soil. This is not true of dicamba, which moves through the soil and into the plant through the roots. Once it is taken up by the plant, it spreads throughout and usually kills it. This chemical is one you should be extremely careful with when using it around your trees and shrubs.

229. Are there any chemicals available I can use to kill the clumps of grasses growing in my yard?

There are several chemicals on the market that will kill the undesirable bunch grasses (tall fescue, meadow fescue, broom. sedge), but they also kill the desirable grasses. The lack of a chemical that will selectively kill the bunch grasses means you either have to spot-treat the clumps or dig them out by. hand.

Spot-Killing of Weedy Grasses in Your Lawn

To spot-treat weedy grasses in your lawn, use a kill-all chemical and direct the spray only on those areas where the problem exists. Chemicals to use are dalapon, paraquat, sodium arsenite, cacodylic acid, borax, and amitrol-T.

230. Isn’t hand-weeding of dandelions and other broadleaf weeds just as effective as using a chemical spray?

No. Often part of the root, stem, rhizome, or stolon is left in the ground when you dig out a weedy plant. Any of these plant parts is capable of developing into a new plant you will have to remove later. By using a chemical spray, you kill the aboveground portion of the plants as well as the belowground parts.

231. Is there any way to get rid of the weedy grasses growing in my lawn?

You can control the annual grasses (crabgrass, foxtail) without too much trouble by using a preemergent weed killer. The perennial grasses (quackgrass, nimblewill, tall fescue) present a different problem. You can do one of two things to rid your lawn of these pests. Hand-weed or apply a kill-all chemical and re-sod or seed the areas you treated. Remember, the kill-all chemicals will kill the desirable grasses too, so be careful not to treat any larger areas than absolutely necessary.

232. I have often heard the term monocot and dicot used when some weeds are being discussed. What do these terms mean?

These terms are commonly used by botanists when discussing plants as a convenient way to separate grasses from broadleaf plants. The terms nionocot (grasses) and dicot (broadleaf plants) refer to structures (cotyledons) found on young plants after they have germinated. You have undoubtedly noticed that when a grass seed germinates, only a single seedling leaf emerges, while a broad leaf plant has two seedling leaves emerge. Thus the terms mono- (one) and di- (two).

233. How long can a weed seed stay dormant in the ground before germinating?

Studies have shown some seeds to remain viable for as long as 8o years when buried in soil. Most seeds will remain dormant in the soil for long periods of time until conditions are right for germination.

234. Could you explain why some chemicals kill certain plants without harming others?

It is difficult to give an exact answer. There are many different kinds of plants, and each species varies in its internal and external makeup and the way it reacts to any given chemical. Some plants have, over their leaves, a waxy coating that repels chemicals sprayed on them, making them tolerant to the chemical. The same spray will stick to plants without this waxy coating, allowing the chemical to be absorbed and causing the plant to die. Selectivity of sonic chemicals is determined within the plant, where it has the ability to convert the chemical into nontoxic forms.

235. Are there any chemicals that I can use to kill the weeds growing in my pond?

There are several excellent chemicals available for weed control in lakes, ponds, and ditches. Before you select one, you need to positively identify the aquatic weeds present. Another important point to consider is where the water is used after it leaves your pond. Any water treated in the pond will carry the chemical with it, and if the water is applied on lawns or crops it could be toxic to them. Read the label on the package containing the chemical before you use it. If your pond is stocked with fish, be sure the chemical you select is nontoxic to fish.

236. I have noticed crab grass growing in my lawn this fall. Should I try to kill it now or wait until next year?

The best time to control crabgrass is in the spring. Use a preemergent chemical when the seeds are just starting to germinate. Once the plant has matured, there are two chemicals (Phenyl Mercuric Acetate, PMA, and Disodium Methyl Arsonate, DSMA) you could spray on the crabgrass to kill it, but usually the best thing to do is just Wait until next spring. Both of these chemicals will discolor the turf when applied, especially in hot, dry weather. To be most effective the chemicals should be applied when the crabgrass plants are in the two-leaf to three-leaf stage of growth. Trying to kill it in the fall is not necessary because the first frost will do the job for you.

Crabgrass Control

This pesty lawn weed is a menace to homeowners and their efforts to keep the lawn free of any blemishes. Many a well-manicured lawn has seen the invasion of crabgrass ruin its attractive ness. This need not happen if you understand the life style of a crabgrass plant.

Crabgrass, by definition, is an annual grass, which likes warm weather. For this reason it is referred to as a summer annual. (You will never have to worry about it if you live on a mountain peak in the Rocky Mountains!) The new crabgrass plant starts from seed each spring, after the temperature has warmed the soil. The plant begins inconspicuously at first, but by midsummer it is a full- fledged pest and easy to spot. It’s the big clump of light green grass hugging the ground and prolifically producing seedheads. During this stage of its life, crabgrass can be controlled, but it is difficult.

At this point you might as well wait and let nature do the job for you. When temperatures start to drop in the fall, the crabgrass plant stops growing and the arrival of frost spells the end of the crabgrass problem for another year.

Well then, how do you subdue one of the worst scourges of your lawn? If you’ll look at the Crabgrass-Germination Map and locate where you live, you will discover the approximate date to expect crabgrass to make its appearance in your lawn. You must be pre pared to act before this happens! Go down to the local lawn and garden center and ask for a pre-emergent crabgrass killer. This is a chemical that will kill crabgrass plants as the seed germinates, never giving it a chance to become established in your lawn. There are all kinds of crabgrass eradicators on the market. All work equally well, so long as you remember one KEY rule. You must spread it on your lawn before the crabgrass seeds germinate. If you wait until you see the plant emerge from the soil, you are too late and will have to live with it for another summer. The pre emergent chemical works only on the emerging seedling as it is germinating.

Crabgrass-Germination Map

What’s that? You forgot to put any crabgrass killer on your lawn again and now there is more crabgrass than desired grass. Is there anything you can do about it now? Yes. There are a few chemicals that may be used to get rid of the crabgrass without harming the desired grasses. These post-emergent (kills existing plants) chemicals are tricky to use, so be sure to read the directions carefully. Then follow them! Unless your lawn has a really severe case of crabgrassitis, it may be best to live with the problem through the summer and let nature do the work for you. But remember not to be late again next spring with the pre-emergent chemical because each crabgrass plant produces hundreds of seeds that will compound your problems another year.

237. Is moss a weed?

Moss can be called a weed if it is growing somewhere you do not want it. Usually, raking the soil lightly and then letting it dry out will solve your problem. Moss requires moist conditions to survive so watch your watering schedule and don’t keep the soil wet continually. If the soil stays wet for long periods of time, regardless of what you do, then the next step is to put in a drainage system to remove the water from the problem area. Moss grows best in an acid soil, so check the pH of the soil and add lime if necessary to bring the pH to neutral (pH = 7.0).

238. What are some tips on the use and care of a pressurized hand- spray tank?

When using the hand-spray tank, always keep the pressure low and set the nozzle for coarse droplets. These bigger-sized droplets will be less likely to drift onto nearby desirable plants and cause damage to them. Clean the tank and nozzle of foreign matter before and after each use. This will insure that it is in the best possible working condition and the nozzles won’t plug up during use. When you are through with the sprayer, wash it with water; adding one-fourth cup of kitchen ammonia per gallon of water is helpful. This is especially important if you have used a broadleaf weed killer in it and then plan on using the same sprayer to apply an insecticide to your ornamental plants. A trace of 2,4-D left in the tank can kill a very sensitive ornamental plant.

239. Do I need to be concerned with the weather when I am going to apply a herbicide?

Yes. When spraying, pick a day that the temperature is between 55 and 80 degrees and rain is not likely to occur for at least 12 to 18 hours. This gives the chemical a chance to dry on the leaves and to be absorbed by the plant. Remember that 2,4-D is ineffective when washed off into the soil by rain.

240. How can I easily identify a plant that is dying from an application of 2,4-D?

The weed killer 2,4-D is a hormone-type chemical. When applied to a broadleaf plant, it stimulates abnormally fast growth. This accelerated growth causes the leaves to become twisted and curled. The plant literally grows itself to death.

241. Do grasses show different tolerances to weed killers?

Yes, grasses do show different tolerances to weed killers. The use of 2,4-D on an established bluegrass or Bermudagrass lawn will not harm either of them, assuming the recommended rates were used. However, this same chemical used on bentgrass or St. Augustinegrass could be disastrous, resulting in a severe yellowing of the grass to a complete killing of it. Always read the label to be sure the chemical has been tested for use on the type of grass you have in your lawn.

242. How can I control the weeds growing in my dichondra lawn?

The safest way is to hand-pull the weeds if there aren’t too many of them. Since dichondra is a broadleaf plant, none of the broadleaf weed killers can be used on it. Most serious weed problems are caused by plants developing from soil containing an abundance of inactive weed seeds just waiting for the optimum conditions before germinating. The best way to keep the weeds out is to treat the soil with a kill-all chemical (soil sterilant) before seeding the dichondra.

243. How can I properly water my lawn to help keep the weeds out of it?

Mature, healthy grass will have a deep root system that enables it to go without water longer than a young weed plant just trying to get established. With infrequent watering, the surface inch or two dries out, and the immature weed plants die, whereas the grass is able to draw moisture from deeper within the soil.

244. Is it true that knotweed growing in a lawn is an indication something is wrong with the soil?

Any time there is knotweed growing in a lawn it is a safe bet that the soil is severely compacted. Compacted soil weakens the grass, and once knotweed gets established under these conditions it will crowd out the desirable grasses. You have to kill the knotweed, then loosen the soil before seeding or sprigging grass into the area.

245. There is a wide-bladed grass growing in my lawn, and the clumps seem to be spreading more each year. Any idea what it is?

Chances are that you have a case of tall fescueitis. This perennial grass grows in clumps and will remain a darker green color than the rest of your lawn in dry weather. Pick a blade of grass and run your finger carefully down along the edge. If it feels rough, sort of like a saw blade, then it’s probably tall fescue, provided it is a perennial grass.

246. How can I rid my lawn of tall fescue after it has invaded my lawn?

You have the same problem with this weedy perennial grass that you do with quackgrass and nimblewill. If you are dealing with a few clumps, hand-dig them. If the area is too large, then spray with a nonselective weed killer and re-sod the area. Your last choice is to live with it; some people actually prefer it.

247. It’s spring, and I am just starting to put in a new lawn. Is there any chemical I can use that will not interfere with the germination of my bluegrass seed but will kill the crab grass seedlings as they sprout?

Tupersan (siduron) is one chemical you might want to consider. When used carefully (instructions must be followed closely), it will give good control of crabgrass while letting the bluegrass grow. It is a good idea to seed a little more heavily than normal (one extra pound of seed per one thousand square feet) to insure a good stand of grass.

What is the Most Widespread Weed in the World?

In a study of more than five thousand weeds found in the world, the one weed most often found was purple nutsedge, also called nutgrass (Cyperus rotundus). This plant can be easily identified by looking at the stems, which are solid and triangular in shape. The seedheads will be reddish-purple in color.

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