Once a weed has found a foothold in your lawn, especially if it has deep roots like dandelions, getting rid of it can be difficult. Pre-emergent herbicides interfere with the development of a germinating seed's root system so the seedling withers before establishing itself in the ground. Know which weeds you're dealing with, the time of year and the nearby plants before you choose from among the many chemicals approved for application to home lawns and gardens.
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Siduron is the only pre-emergent herbicide that will kill weedy annual grasses such as crabgrass, foxtail and barnyardgrass without damaging germinating cool-season lawn grass plants. It has no effect on broad-leaved weeds such as dandelions and can't be used on Bermuda grass or some strains of bentgrass.
Trifluralin is a product used by homeowners in garden beds rather than on lawns. It is also applied extensively to soybeans, cotton, vegetables, fruits, nuts and nursery crops. Toxic to both broad-leaved and grassy weeds, it is a general-use herbicide that varies in effectiveness.
Benefin is primarily used to control annual grasses in turf, alfalfa, clover and some vegetables such as lettuce. It is not as effective against broad-leaved weeds. One shortcoming is its quick decay, sometimes necessitating a second application in a few months. It is also available in combination with Trifluralin for use on a broader range of weeds.
Isoxaben is a weak grass killer, but controls broad-leaf weeds such as chickweed, henbit and bittercress well. It is also effective against perennial weeds such as dandelions, plantain and white clover. One drawback is the wait required before reseeding with lawn grass, at least two months.
Dithiopyr is an excellent choice for pre-emergent control of crabgrass, even eliminating small crabgrass plants, and it also kills a wide variety of broad-leaf weeds. Reseeding must be delayed for at least three months after application.
Pendimethilin is also useful on crabgrass, annual grasses and broad-leaf weeds but it has a longer wait time before reseeding, at least four months. It is often used in corn, potatoes, cotton, soybean and tobacco fields and has the advantage of being effective both as a pre-emergent and an early post-emergent herbicide.