Crabgrass is an annual weed which invades lawns all over the country every summer. No matter how thick your lawn, crabgrass always seems to able to rear its ugly head. A lot of homeowners are trapped in what seems to be a never ending cycle : you spray it, some of it dies, some of it powers through, and the following year the problem is worse.
Effective control of annual weeds like crabgrass is only possible if the seed cycle is broken. Pre-emergent herbicides are designed to do just that: prevent the seed from turning into a viable plant. Pre-emergents work by creating a shallow “barrier” in the soil that the germinating plant must go through to reach the surface. It is important to realize that pre-emergent herbicides do not prevent the seed from germinating, they kill the germinating plant thus depleting the seed bank.
There are various pre-emergent herbicides available to homeowners, many of which can be found at your local home improvement stores. Two of the most popular (and effective) pre-emergent herbicides are dithiopyr and prodiamine. Proper application of either will provide nearly 100% control of most annual weeds, including crabgrass. Prodiamine has the advantage that it does not leach and does not begin to break down until the soil warms up, which means it can be applied early in the season and provides long lasting control (up to 8 months with one application). For this reason, it is my pre-emergent of choice.
The effectiveness of pre-emergents is highly dependent upon timing and proper application. Observing the following rules guarantees success and a weed free lawn for the year:
- Since pre-emergents do not affect plants which are already established, the pre-emergent barrier must be in place before the seed germinates. Pre-emergent must be applied when soil temeperatures reach 50 degrees @ 4″, or when forsythia plants bloom.
- Even and complete coverage is crucial. Missing spots during application will cause “holes” in your barrier where weeds can germinate and produce more seed for the following year. A good backpack sprayer will help you get good coverage with minimal effort, but for smaller lawns even a 2 gallon pump sprayer will do the job.
- Most pre-emergents require watering in within a short period of time. Applying before a rain means Mother Nature can do that part for you, or you can irrigate immediately after application per label requirements.
- More is not better. Applying too high a rate can cause damage to your existing grass roots and also extend the pre-emergent into your overseeding window (if applicable), meaning none of your grass seed will grow.
- Avoid applying pre-emergent during periods when your lawn is stressed. It can make things worse.
- Follow all instructions on the product label and maximum yearly application rates.
A good pre-emergent plan is the foundation of any great lawn. It guarantees a weed free lawn and helps your grass thrive. Master it, and the grass will always be greener on your side.