Unlike standard weed killer products that kill the undesired plants after they sprout, applying pre-emergent herbicide nips them in the bud (or seed). Due to its very nature there are restrictions to applying herbicide that targets weeds before they grow.
How Does Pre-Emergent Herbicide Work
Applying pre-emergent herbicide keeps weed seeds from germinating. Remember that weeds generally go to seed in summer and fall; these seeds are dormant in the soil -throughout the winter — until the temperatures heat up again. Catching the weeds before the seeds have a chance to take advantage of the warming temperatures ensures a weed-free garden or lawn.
How to Apply the Weed Killer
Gardeners have two options open to them: using a liquid pre-emergent herbicide (a good choice for spot use) or opting for a granular version that also contains a lawn fertilizer (an excellent choice for the grassy areas overrun with crab grass). Prior to applying any of these products, the hobbyist will do well to read the instructions carefully and – as indicated – temporarily suspend any watering or actually employ it for activation.
Timing Weed Control Just Right
The main sticking point when applying pre-emergent herbicide products is the proper timing. Applying it too soon may break down the barrier it forms around the weed seeds before they had a chance to decompose. Applying it too late – after the soil warmed sufficiently to allow for germination – renders the weed killer useless.
As a general rule of thumb, a pre-emergent herbicide should be applied in the latter days of fall to control weed populations in the early days of spring. As outlined by Cornell University(1), the sweet spot for applying the pre-emergent herbicide is a soil temperature that falls between 59 and 65 degrees Fahrenheit. If the ground is any warmer, the odds are good that the seeds already germinated.
Is Pre-Emergent Weed Control always a Good Choice?
For the gardener who is not above measuring soil temperatures but is committed to attacking weeds as they occur, applying a pre-emergent herbicide may be a workable solution. To the hit-or-miss hobbyist, it might just be money wasted, especially when it is applied too late in the spring.
Do not opt for this weed control choice if a lawn is being reseeded. A pre-emergent herbicide does not distinguish between crabgrass and Kentucky bluegrass seeds. It will kill any grass seeds – depending on the weed killer product – for two up to six months upon application.
(1)Cornell University. “Managing Lawn Weeds” (accessed April 29, 2010)