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.