Grubbs-Bowling says pre-emergent weed preventers are best suited for grassy weeds and annual weeds that reproduce by seeds. “They don’t work as well on perennials.”
Pay particular attention to the label of the pre-emergent product if you have a new lawn, or intend to reseed. If you pick a variety that kills grassy weeds, likely it will kill any new desirable grass seed as well. Most pre-emergence products lose effectiveness after about six to eight weeks, so wait at least that long before reseeding.
Choosing a Pre-Emergent Herbicide
A selective herbicide is formulated to kill certain kinds of weeds and leave other plant life alone, or at least not damage your grass so much it can’t recover.. A nonselective herbicide will try to kill everything it touches — including your grass.
Whichever product you select, it’s vital to apply it thoroughly and evenly to gain the best weed prevention. You will need to know how many square feet in your yard to mix the right amount. A pre-emergent must cover your target area completely to serve as a barrier against weed growth. Missing a spot could mean trouble because if you give a weed an inch — it’ll take a yard!
Pre-emergence herbicides form the backbone of weed control programs,” says the University of Georgia Extension Service’s guide to weed control. “They do not control all weeds that may be present in a lawn, but they are effective for many of the most common lawn weeds.”
Is the goal to prevent these weeds, to eradicate them, or both?
All weeds have a survival strategy and cannot be completely eliminated because they have different life cycles and methods of reproduction. Seeds can lay dormant for years before they germinate, surviving drought, fire, and herbicide applications. Even if you were to completely clear a property of seeds, seed and vegetative propagules can easily be transported to the property by wind, water, animals or human activity.
Developing a Weed Control Strategy
Most well-known example: Annual Bluegrass (Poa Annua)
Other examples: Shotweed, Chickweed, Mustards.
Life Cycle: 1 year – Germinate in fall. Flower and produce seed quickly, then die in spring.
Pre-emergent timing: Late summer/early fall (rule of thumb is by September 15th)
Weed Classification: Winter Annuals
To know when to apply pre-emergent herbicide, it is important to know how weeds are classified, namely by their life cycles.