A pre-emergent is a weed killer (usually granular) that is applied before weed seeds emerge, that is, before they germinate.
What weeds will they control?
Because pre-emergent applications attack seeds before they sprout, the granules are primarily intended for use on annual weeds that complete their life cycle within one growing season. They do not help with perennial weeds that come back from their root systems (nutsedge, Johnsongrass, dallisgrass, etc.).
What are the common pre-emergents I can find?
Products in the consumer marketplace are Team and Dimension (both at independent retail garden centers) and Halts (at national chains). Balan is sold in farm supply centers. All four of those are primarily intended to prevent germination of grassy weeds. Gallery is sold in independent retail garden centers primarily to prevent germination of broad-leafed weeds (non-grasses).
When should a pre-emergent be applied?
Before the weed seeds start to sprout. It’s better to be a week too early than a day too late. The old axiom is, “With pre-emergent weed killers, if you can see the weed growing, you’ve blown it for another whole year.”
• For summer weeds such as grassburs (sandburs) and crabgrass: 2 to 3 weeks before the average date of the last killing freeze for your area. In North Central Texas it would be the first two weeks of March.
• Because these granules are effective for about 100 days you will need to reapply three months later as a “booster shot.” Mark the calendar so you won’t forget. Without this second application the weeds will still germinate later in the season. That second application should be early June in North Central Texas.
• For winter weeds (the ones you can see in your lawn in February and March), apply one of the grassy pre-emergents as well as Gallery the last week of August or the first week of September. (Gallery is not commonly needed in the spring applications, since broad-leafed weeds are less of a hot-weather issue than they are in the winter and early spring.)
What about weed-and-feed applications?
Late February and early March would be prime time for the first application of pre-emergent herbicide, but it’s too early to fertilize turf grass. These two procedures should be done independently. Applications later in the spring and in late summer could theoretically be of a combined product, but some of us still prefer to do the two processes independent of one another.
What if my lawn care professional does this at a different time?
If you have hired a turf professional to apply pre-emergent weed killers or you, put your faith in their experience. They have access to different products and their timing may vary from what is prescribed for the do-it-yourselfer.