Did I apply too early? Am I applying too late? Just when do I need to apply crabgrass pre-emergent? Don’t let crabgrass prevention make you crabby. And with the current unexplained weather patterns it is even more difficult to know when to apply.
Review of the Basics
Annual grassy weeds germinate, grow and produce seeds each year. They resemble desirable turfgrasses in their growth, but differ in texture and color. Crabgrass, goosegrass and foxtail are examples of annual grasses, with crabgrass being the infamous weed we wish to rid our lawns of.
Crabgrass pre-emergence herbicides prevent annual grass seeds from developing into mature plants. They don’t actually keep the seed from germinating; rather, the small, germinating plant takes up some of the herbicide and is killed. Crabgrass preventers are just that — preventers. With the exception of a few products, they will have no effect on existing crabgrass plants.
For most of the area, crabgrass typically begins to germinate around May 1, or a little later. The herbicide product should be applied several weeks before weed seed germination is expected in order to allow time for the active ingredient in the herbicide to evenly disperse in the soil. But this year the warm late winter and early spring has moved the time to apply up by several weeks, so now is the time to apply.
Pre-emergence Herbicides Don’t Last Forever
Pre-emergence herbicides don’t last forever once applied to the soil. Microorganisms and natural processes begin to gradually break them down soon after they are applied. Therefore, if short residual products are applied too early, they may have lost much of their strength by the time they are needed. A second application often is needed to extend the period of control, depending on the product. Most pre-emergence products should be watered in within three days after application with a half inch or more of water.
Long Residual Preventers
Dimension (Dithiopyr) and Barricade (Prodiamine) are the products that will give season-long control of crabgrass from a single application. In fact, they can be applied much earlier than short residual products and still will have sufficient residual strength to last the season. Barricade can even be applied in the fall for crabgrass control the next season. Dimension can be applied as early as March 1.
Because of the added flexibility in timing, these products are favorites of lawn care companies who have many customers to service in the spring.
Though Dimension cannot be applied as early as Barricade, it is the herbicide of choice if you must apply later than recommended. It is the exception to the rule that pre-emergence herbicides do not kill existing weeds. Dimension can kill crabgrass as long as it is young (two- to three-leaf stage).
Short Residual Preventers
Short residual preventers are only effective for up to 60 days. These include, but are not limited to, Halts (Pendimethalin), Team (Benefin + Trifluralin) and Tupersan (Siduron). Normally, April 15th is a good target date for applying these products because it gives the active ingredient time to evenly disperse in the soil before crabgrass germination starts. Based on the 2016 weather pattern now is a good time.
Have you ever been told to apply crabgrass pre-emergence when the Forsythia is in bloom? Weather varies from one spring to the next, and with it, the timing of crabgrass germination. For this reason, it is often better to base herbicide application timing on the bloom of ornamental plants. The Eastern redbud tree is a good choice for this purpose.
When the trees in your area are approaching full bloom, apply short residual crabgrass pre-emergence products. A follow-up application may be needed about six weeks later to prevent any late germinating weeds from establishing.