Pre-emergent Weed Killer and Crabgrass

The principal pre-emergent weed killer chemicals and their function characteristics are as follows:

Calcium Arsenate: In many ways this is the most practical pre-emergent weed killer. 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 for many years. 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.

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-emergent weed killer (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 recommend 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 pre-emergent weed killer.

Crabgrass Control - controlling crabgrass for a lawn free of the dreaded stuff.

Crabgrass Killer Strategy - my strategy from extensive post and pre-emergent testing.

What Does Crabgrass Look Like? - you can't control it if you can't recognise it.

Weed Killers - recommended chemical weed killers and the approach to using them.

Toxic Residue In Soil - it pays to be aware of not only the short term, but long term effects of weed killing products.