Chapter 16. Weeds and Weed Killers

Weeds belong in a discussion of soils for several reasons. For one, modern chemical weed controls introduce problems of toxic residues, which may produce either short- or long-term injury to desirable plants as well as weeds. For another, deep-rooted weeds such as bindweed, Canada thistle and nut grass often make a soil unusable for gardening until they can be exterminated. Third, the amazing longevity of certain weed seeds in soils is a factor in gardening.

Longevity of weed seeds is a serious problem and one which often determines how a certain soil should be treated for lawn or garden purposes. It often becomes critical when a home owner, following "curbstone advice" from neighbors, insists upon buying a load of "good black dirt" to start his new lawn, bringing in someone else's accumulated weed seeds to further complicate matters.

My own most striking experience with seed persistence was with crabgrass. In laying a new water line to my home in 1955, plumbers uncovered an old carriage drive which must have been laid down either in 1868 when the house was built, or in 1900 when it was remodeled. Underneath the old bricks was rich black prairie loam. I dug this out and spread it over my vegetable plot. This was done in early spring, before any crabgrass plant could have contaminated the soil with fresh seed. Nevertheless, that spring, a heavy crop of crabgrass sprang up from this long-buried soil. Thus those seeds retained their viability for a minimum of 55 years.

CONTROLS

There is only one 100 per cent control for long-lived buried seed— sterilization of the soil with live steam for several hours. This is feasible only in greenhouses, where special covers are used on benches to retain heat and pressure at a lethal level. Practical con­trol can be had in home gardens or future lawn areas by the use of one of two chemical materials.

Calcium Cyanamid: Used at a rate of 75 pounds to 1,000 square feet of surface (a heavy dose) this has killed out all weed plants and seeds for me, even on fairly heavy clay soil. Calcium cyanamid, called Garden Cyanamid, is a granular material that at first breaks down into substances poisonous to seeds but later converts into valuable nitrogen and lime. It is a grim coffin-gray in color and even looks poisonous to handle, but is perfectly safe if used as directed. The soil to be treated should be plowed or rotary tilled and leveled just before application. After 60 days you can plant seed, but disturb the soil surface as little as possible, to avoid bringing up new weed seeds. The 60-day wait is one drawback to this material. Since soil should be warm during treatment, this means you have an unplanted lawn or garden during the major part of the growing period, which some gardeners find too unpleasant a sight to face.

A lawn I treated in this way four years ago came up without a single weed and, except for a few seeds blown in from the outside, has had no weeds since.

Vapam: This is a fumigant, a liquid used in much the same way as garden cyanamid; it works almost as well but for some reason has failed to control purslane, a persistent and annoying weed in my soil. Otherwise, it cleaned out such nasty perennials as Canada thistle and bindweed, as well as all annual weeds. It is somewhat easier to use than calcium cyanamid; soil fumigated with vapam can be reseeded within a week after treatment.

Other Materials

A second fumigant, methyl bromide, is not generally practical for amateur use. However, some landscape gardeners are equipped with a device that injects the gas into soil. The area is then covered with a plastic sheet. A day later the cover is removed to air out the fumi­gated soil. After a day or two, it is ready for seeding.

Chemical control of annual weeds in flower and vegetable plant­ings is not always easy. Both Sesone or Crag Herbicide 1, and Alanap are quite effective on selected crops. One difficulty is that they will not discriminate between seedlings of weeds and those of desirable plants.

In a commercial truck garden where a single crop occupies several acres, weed control is possible. Several chemicals can be used that will not hurt a given crop, but will destroy weedy plants. Examples are the use of monuron on asparagus plantings and simazine on sweet corn.

PRE-EMERGENCE CRABGRASS CONTROLS

Perhaps the most spectacular use of chemicals for weed control on home properties is in the pre-emergence materials which will prevent crabgrass in lawns. At present, five materials are fairly well dis­tributed in commerce, each with certain advantages and disadvan­tages. These are calcium arsenate, chlordane, dacthal, lead arsenate and zytron. Two of them, calcium and lead arsenate, are also con­trols for three other lawn weeds—knotweed, common chickweed and annual bluegrass (Poa annua).

A common impression is that these pre-emergence chemicals kill crabgrass seed before it germinates. This is not the case. If it were, a single application would eradicate this weed for years, since there would be no viable seeds left to produce plants. Actually all of these chemicals work by killing seed immediately after it sprouts and as the seed coat splits. Calcium and lead arsenate kill by "waiting" until the germinating seed has exhausted its extremely small store of phosphorus and sends out its minute new roots to take up a replacement supply of this element from the soil; the arsenic sub­stitutes for phosphorus and kills the tiny plant. Chlordane works by poisoning the root runner as it starts to make growth, checking any further development of the plant.

Solutions of all these chemicals can be applied to dormant crab­grass seed but if the chemical is washed off before the seed coat splits, a normal seedling will develop.

Signs of Germination

These chemicals work by building up a toxic layer an inch or so deep on the surface of the lawn. To work effectively all must be in place before crabgrass seed germination begins. Calendar dates for germination are notoriously unreliable because of the variation be­tween seasons. Instead, use natural signs which appear when a cer­tain number of degree hours have accumulated, not when a certain date appears on your calendar pad.

Here are some natural signs that announce the imminent appear­ance of the first crabgrass seedlings: Magnolia steUata dropping its petals; Magnolia soulangeana in flower; first buds open on French hybrid lilacs, first blooms on Darwin tulips, and apple blossoms showing trace of pink.

List of the Best

The principal pre-emergence chemicals and their function char­acteristics are as follows:

Calcium Arsenate: In many ways this is the most practical of all pre-emergence crabgrass controls. It gives between 95 and 100 per cent control. Granular types are not as satisfactory as those ad­sorbed on vermiculite to form a heavy powder that clings to soil and does not blow away. Granular forms also tend to wash away from point of application in heavy rains. The granular forms are less de­sirable, too, because birds may pick up the bright particles. The vermiculite-type clings and is not easily picked up, and it does not adhere to the feet of pets and children.

Precautions to observe: For two weeks after applying calcium arsenate, do not fertilize and do not reseed bare spots. After two weeks, if a fertilizer is used, apply one that is low in phosphorus, or use a straight nitrogen product for that season. Most lawns have adequate reserves of phosphorus and potash, so skipping one appli­cation won't hurt.

The question is often asked, "Are these weed killers dangerous to birds?" Calcium arsenate adsorbed on vermiculite contains 4 per cent soluble arsenic which kills earthworms in their burrows so they do not come to the surface. Other forms (and lead arsenate) may kill the worms after they have surfaced. Birds that eat these poi­soned worms can be killed.

Plus advantages: Calcium arsenate kills many harmful soil insects such as white grubs, Japanese beetle larvae, and others. Do not expect control of third-year June bugs which are about to emerge, but all feeding forms will be dead by midsummer. This chemical will also kill common chickweed if applied in fall. Incidentally, even if ap­plied in fall it is equally effective in spring on crabgrass. Calcium arsenate checks knotweed so that it is easier to kill with ordinary lawn weed killers. It checks Poa annua so that it does not form seed.

Residual effects: One application has been known to give three-year control of crabgrass. However, a booster shot of about one-third the strength recommended on the package for initial treatment, applied in either spring or fall of the second and third year, will insure 100 per cent control. But do not continue using either of these materials for more than three years; depend on a thickened turf to control crabgrass after that period.

Lead Arsenate: I have used this chemical off and on since 1936. I have gone back to it despite its defects because other chemicals did not always give the desired control. But I feel that today it has been superseded by calcium arsenate, which is cheaper, more effective and less likely to damage turf. The one place where lead arsenate does have value is in control of knotweed, but for this purpose it must be applied during a February thaw. Knotweed germinates at that time, and if lead arsenate is present, the knotweed seeds will be killed.

One drawback to products containing lead arsenate is their dusti-ness—free white arsenic may fly in the air. Be sure the wind is blowing away from you when applying. A dust mask is a reasonable precaution. All forms of lead arsenate I have tested offer some haz­ard to birds, since worms tend to come to the surface to die.

Chlordane: This product is worth considering if soil insects are a problem; it has no superior as a treatment for such pests. Its control of crabgrass has been somewhat less than satisfactory. I am sorry to see this, as I was the first to publicize it as a crabgrass control and helped launch a commercial chlordane crabgrass killer about nine years ago. This was based on research work in Colorado and Cali­fornia, where chlordane gave outstanding control of crabgrass, equal to that of calcium arsenate in less arid areas.

For some reason—which might be intensity of sunlight, pH of the soil, soil moisture or some unknown factor—the farther east we come, the less effective chlordane seems to be against crabgrass. Chlordane may perform beautifully in one lawn, but fail in another. It can be expected to give about 25 per cent control as a minimum, but this will still mean hundreds of thousands of crabgrass plants in a home lawn. Instances where chlordane has given good results are frequent enough so that it has been kept alive as a crabgrass control, but it is far too erratic in action for an out-and-out recommendation except in the West. For Colorado and California and perhaps as far east as parts of Kansas, I would rate it tops.

A major advantage is that desirable lawn grass seed can be sown four or five days after chlordane is applied.

Dacthal: This relatively new pre-emergence chemical (introduced for the first time in 1960) has proved an excellent control for crab­grass. It has two drawbacks. The first is its residual threat, the most severe residual effect on seeds of permanent grasses of any of the five materials mentioned. It is not harmful to established turf, but it will not allow reseeding of bare spots the same season it is used. In spite of this strong one-season residual, it does not carry over winter and must be reapplied the following spring. Dacthal seems to be best suited for use on a lawn where permanent grasses are thick but occa­sional crabgrass plants are annoying. Its big advantage is cost— perhaps the cheapest material sold for this purpose.

Zytron: This material was test-marketed during 1960 and gave excellent control of crabgrass in eight limited areas in the Middle West. Recalling my experience with chlordane and its strong re­gional adaptation, I would not want to go on record as recommend­ing zytron outside the Middle West. Package recommendations sug­gest a 10-week wait before reseeding with permanent grasses.

The full residual period of zytron has not yet been determined, but it is being recommended largely as a one-season control.

Combining Pre- and Post-Emergence

Because pre-emergence application dates are so rigidly fixed by the appearance of the first crabgrass seedlings, I have been running extensive tests on the advantages of combining pre- and post-emer­gence techniques. At first I thought a late spring application of a pre-emergent (even after the first seedlings appeared) would elim­inate enough crabgrass to be worth while since it would control everything appearing later. Unfortunately, while this did give con­trol of later seedlings, the spring crop was so enormous (about 75 per cent of the seed usually sprouts at that time) that the lawn looked as though 100 per cent of the current year's crop had sur­vived. My next move was to apply both post- and pre-emergence controls. When these materials went on together, however, injury to permanent grasses was often severe. Spacing the two treatments three weeks apart saved the turf but control from post-emergence treatment was not always satisfactory because of low temperatures which often occur in spring.

Solution of this problem came with the introduction of a chemical called Super Sodar, an improvement on the older dry Sodar powder. It is not properly a Sodar product, since it does not contain disodium methyl arsonate but is a mixture of ammonium methyl arsonate and dioctyl methyl arsonate. It is, however, a very effective post-emer­gence crabgrass control and will work at lower temperatures than most chemicals in this class.

The program, then, is to apply calcium arsenate on vermiculite as early as possible in spring, even if crabgrass has germinated, and to follow this three weeks later with a spray of Liquid or Super Sodar (on some containers this may be listed as AMA).

Toxic Residues in Soils

I have gone to some lengths to describe the action of these weed killers because this has a bearing on the residues the chemicals leave in soils. We can dismiss chlordane's four- to five-day residual effect (on plants) as unimportant. This must, however, be distinguished from its four- to five-year residual effect on soil insects.

Both calcium and lead arsenate toxicity (carelessly caused by excessively heavy doses) can be overcome with a heavy application of superphosphate (about 75 pounds to 1,000 square feet) or with ferric sulfate (iron sulfate, but be sure you use the ferric form, not ferrous). The rate on ferric sulfate is quite high—10 pounds to 100 square feet—but for spot treatment might be feasible. This heavy a dose of iron sulfate will, of course, kill grasses and other plants so the remedy may be as bad as the disease.

One of the worst weed killers, in so far as residual effect goes, is sodium arsenite. This is an old material: there has been some kind of a sodium arsenite weed killer on the market since 1888. It will sterilize soil so that nothing can grow there for two to three years. This is excellent on drives and walks, where I use it regularly. I spray a thin line along a fence where no mower or tillage tool will go. It keeps weeds from growing there for years. Fortunately, it does not wash once it has become fixed on soil, so I can spray within a few inches of desirable plants.

Ferric sulfate, 10 pounds to 100 square feet, is the remedy for sodium arsenite toxicity. Next, put on gypsum to neutralize any re­maining sodium particles.

Two Major Herbicides

I recall how, when I was working with 2,4,5-T in 1943 and 1944, I would say to myself, "But what is this doing to soil bacteria?" I couldn't see how this stuff could kill plants and not be equally toxic to bacteria. Yet nothing seemed to happen to the bacteria or other microorganisms, even after repeated spraying. Later, we found that this was not a toxic chemical that destroyed directly, but affected growth abnormally. Since they attacked the cambium layer of broad-leaved plants, bacteria (having no cambium layer) were safe.

Later an even more astonishing fact came to light. When we tried to trace 2,4-D and 2,4,5-T to see what happened to them in soils, they had disappeared. Bacteria had actually used them as food—had "eaten" them completely. With each successive application, the period these chemicals could remain in soil uneaten became shorter and shorter. This meant that the population of chemical-eating bac­teria was increasing and using up this strange food faster and faster. In one series of tests where bacteria of this type were transferred from one flask to another, always with plenty of 2,4-D to eat, they used up over 98 per cent of the chemical after 70 transfers in four days.

Many of the chlorinated phenoxy compounds cause a similar re­sponse. Since their breakdown in soils is linked to bacteria, tempera­ture plays a vital part in their disappearance. For example, if either 2,4-D or 2,4,5-T are applied just before soil freezes in winter, bacteria are inactive and do not consume them. As a result, the chem­icals persist in the soil and if they come in contact with certain deep-rooted, hard-to-kill weeds, will destroy them through prolonged exposure to 2,4-D effects.

The one drawback to this method is that no plants susceptible to 2,4-D can be seeded early in spring in a soil treated in late fall with 2,4-D because it will still be there. After two weeks during which soil temperatures are in the 60s, it will be safe to sow seeds of most vegetables and flowers.

Substituted Urea Compounds. Under such names as Neburon, Diuron, Monuron and Fenuron, these are being used to control weeds under a wide range of conditions. They differ considerably in such qualities as solubility, persistence in soils, species of crops on which they are safe, weeds they kill and in other respects. They are considerably more residual than 2,4-D, so when a weed killer is wanted for a period of weeks, selection of a proper formulation of a substituted urea compound is perhaps the answer. These chemicals are digested and destroyed by soil bacteria but at a much slower rate than 2,4-D.

Aminotriazole: This specialized weed killer is perhaps the best we have for control of nut-grass, poison ivy and Canada thistle. It de­serves special mention because of the furor it raised in the fall of 1959 during the cranberry fiasco. Aminotriazole seems to be con­siderably more persistent in soils than any 2,4-D type of material or certain forms of the substituted ureas. In laboratory tests, only about one-fifth of it disappeared 35 days after exposure to soil organisms.

I feel that because of this persistence, aminotriazole (in the hands of the amateur) should be confined to use around ornamentals where such hard-to-kill weeds as nut-grass, poison ivy and Canada thistle must be destroyed.

Additional Materials: New weed control chemicals are emerging from the laboratory; it is impossible to keep up with them. In addi­tion, there are hundreds of materials already in commerce. There is, however, one fundamental and universal principle which must be followed in handling such materials, namely, read and adhere to the package directions.

If you only knew how many hours of work and testing went into the preparation of directions to protect you from personal injury and to protect your plants, you might appreciate how important it is to put on your bifocals and read all the print on the package. Above all, if a manufacturer thinks it is important to tell you his product has a residual period of 10 weeks, believe him. He isn't anxious to limit his sales; hence any restriction of this kind is put on the pack­age for your protection.

Washing Out Soluble Weed Killers

Many of the less-persistent weed killers are fairly soluble and will move downward in soil if subjected to heavy watering. Most of the 2,4-D products, for example, can be removed from surface soil by applying two inches of water. To know when you have applied two inches of water, set coffee cans at intervals under the sprinkler and when the can with the least water in it contains two inches, turn off the hose. Reseeding can be done immediately.

This washing (as well as rainfall), by the way, does not spoil the effect of the chemical on weeds you want to kill, provided the water does not go on for at least eight hours after application of a tri-ethanolamine salt, a sodium salt or an amine form of 2,4-D or 2,4,5-T. If ester forms of the chemicals are used, only one hour need elapse before it is safe to wash the soil, since ester types penetrate the plant tissues within that time.

Chapter Digest

Weed control is closely connected to soil care. The finest Gardener's Loam is not worth much if blighted by weeds. With the help of a great and ever-increasing selection of chemicals, the home owner can bar many kinds of undesirable plants from his lawn and garden. It can be done in three "stages": sterilizing the soil, killing germinating seed, and destroying mature weed plants. But weed-killing chemicals, too, present potential hazards from the build-up of toxic residues in the soil. In any event, the faithful observance of instructions on the package will assure maximum protection as well as value to the user, to the plantings being treated, and to the soil itself.

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