Preemergent herbicides prevent the germination

Germination

Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...

 of seeds by inhibiting a key enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

. In some areas of the world, preemergent herbicides are used to prevent crabgrass from appearing in summer lawns.

Brands

Brand	Ingredient	Prevents	Notes
Pendulum (herbicide)	pendimethalin Pendimethalin Pendimethalin is an herbicide used in premergence and postemergence applications to control annual grasses and certain broadleaf weeds. It inhibits cell division and cell elongation...	crabgrass
Surflan	oryzalin Oryzalin Oryzalin is an herbicide. It acts through the disruption of microtubules, thus blocking anisotropic growth of plant cells....	crabgrass
Dimension (herbicide)	dithiopyr Dithiopyr Dithiopyr is a chemical used as an preemergent herbicide used to prevent crabgrass seeds from sprouting in the spring. Dithiopyr may alter microtubule polymerization and stability by "interacting with microtubule associated proteins and/or microtubule organizing centers rather than interaction...	crabgrass
Barricade (herbicide)	prodiamine	crabgrass
Tupersan®	siduron	crabgrass

Crabgrass

Preemergent herbicides must be applied at a critical time. If they are applied to the soil too early they get washed too deep into the soil by rainwater. If they are applied too late the key enzyme inhibited is no longer active. Depending on where you live, one rule of thumb is to apply when the local forsythia

Forsythia

Forsythia is a genus of flowering plants in the family Oleaceae . There are about 11 species, mostly native to eastern Asia, but one native to southeastern Europe. The common name is also Forsythia; the genus is named after William Forsyth.-Growth:They are deciduous shrubs typically growing to a...

blooms are wilting.