Applying post-emergent herbicides in the fall is an important step in controlling difficult perennial weeds that would otherwise reemerge in the spring. While using a pre-emergent herbicide is very effective in preventing new annual weeds, which is also an important weed control measure for those weeds which reproduce mainly by seed, it will not take care of those weeds with spreading roots. In other words, effective weed control is a two-pronged approach, and there are some reasons to focus on post-emergent applications in the fall.

Timing is Everything

In order to effectively control perennial weeds, it is important to understand their life cycle. While spring applications of post-emergent herbicides have the advantage of taking out weeds before they have a chance to build up a thick, waxy surface that is difficult to penetrate, the down side is that the herbicide is not always carried down to the roots, so the weeds can regrow. In the fall on the other hand, the weeds are carrying their own sugars down to their roots for winter storage, so a post-emergent herbicide application will be carried right down to the root with the sugars – killing the weed for good.

Let it Rest

Avoid mowing the turf for two to three days before and after applying a post-emergent herbicide.  Because the herbicide is absorbed through the blades of the weed, having longer, bigger leaves (more surface area) is essential to success.  If you mow too quickly after application, you risk removing the herbicide before it can reach the roots.  Turn irrigation off, and do your best to minimize disturbances to the turf so that the product can be absorbed in a consistent manner.

Not too Hot, Not too Cold

The ideal temperature for applying post-emergent herbicides is between 60 and 85 degrees - when most weeds are actively growing.  Too cold and the plant goes dormant and cannot move the chemical to its roots.  Too hot and you risk volatization of the herbicide in high heat.

Liquid or Granular?

In general, liquid products tend to be more effective than granular.  The liquid formulation absorbs into the leaf tissue and moves through the plant more quickly than granular.  Plus they are normally rain-fast in 3-6 hours.  But a “weed and feed” granular product is convenient and efficient, eliminating the need for a second application and reducing labor costs.  If you choose a granular product, apply in the morning when turf is damp so the product will stick, and avoid irrigation and disturbances for at least 24 hours after application. 

Remember, post-emergent herbicides can cause considerable damage to the desirable turf and plants on the property, so be sure to read the label before applying. Also, take every precaution to minimize drift or runoff.