Pre-emergent herbicide works on weed seeds rather than on weeds. Applied before the seeds germinate, it forms a barrier at the soil surface that stops new seedlings from establishing. It has no effect on weeds that are already growing, which are handled with post-emergent weed control.
Timing is the whole treatment
Because it has to be in place before the seeds sprout, pre-emergent is applied early in spring. An application made after germination does very little. For that reason, contracts received after March 1 cannot be guaranteed an application in the current season.
Moisture
Pre-emergent needs moisture to move into the top layer of soil and form the barrier. Its effectiveness depends on that moisture arriving after the application, from irrigation or precipitation.
What to expect
Pre-emergent reduces the number of weeds that come up during the season. Complete eradication is not realistic. Some seeds will germinate outside the treated window or in areas the barrier did not fully reach, and those are treated as they appear.
All applications are made under Brookstone Outdoors' Colorado Department of Agriculture commercial pesticide applicator license.