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It is possible to control many weeds just as their seeds germinate with herbicides known as pre-emergent weed killers. These are especially useful for annual weeds such as crabgrass, foxtail and spurge. Some pre-emergent products are labeled for lawn use, others in shrub and flower beds and others in garden situations. Additionally, keep in mind that pre-emergent weed killers do not discriminate between weed seeds and desirable seeds such as newly planted peas and carrots. They additionally do not control already established weeds growing back from roots. Follow the label to ensure products are used safely and to maximize their effectiveness.
Lawn Labeled Products: There are three different products labeled for lawns available from various manufacturers, which are prodiamine (Barricade), pendimethalin (Scott's Haltz or Pre-M), and dythiopyr (Dimension). For residential use, these are usually sold in granular form and bundled with fertilizer. They generally last around two months and should be applied when the shrub forsythia is in bloom. Depending on what specific weeds are being controlled, a second application is often needed in mid-summer. These three are also more effective against weedy grasses, such as foxtail and crabgrass, than broadleaf weeds.
Another pre-emergent herbicide labeled for lawn use that is more effective against broadleaf weeds such as spurge and black medic includes isoxabin (Gallery, Galleria).
Depending on the rate it is applied, it can last for up to six months. In extreme situations, isoxabin can be applied with dythiopyr to produce a synergistic effect against both weedy grasses and broadleaf weeds. This may need to be done by a professional lawn applicator.
Garden Labeled Products: Trifluralin (Preen, American Brands Traflan) is the most common product labeled for garden use. It can be used around many established vegetables but not all of them, specifically not corn. Depending on the rate applied, trifluralin remains effective in the soil for 1 to 6 months.
An organic or at least more earth friendly pre-emergent option includes corn gluten meal. Some research indicates it is very effective while others' research calls this into question. Anecdotally, I have spoken with a few folks who have found it useful but have not tried it myself. An added advantage is that it also adds nitrogen fertilizer to the soil and some organic matter when applied. Corn gluten meal may need to be purchased online and is more expensive than many other choices.
Flower and Shrub Bed Labeled Products: All of the products listed above are usually labeled to be used in landscape beds. The one precaution is near ground covers. Check the label to be sure your ground cover is listed as being safe to apply the particular product around. Some forms of Preen are also sold as being safe for use around ground cover.
