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.
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.