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 But I feel that today it has been superseded by calcium arsenate, which is cheaper, more effective and less likely to damage turf.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.