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 stellata 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.