Pre-emergents and Crabgrass

Sunday, February 17, 2019 9:10 AM

When is the best time to apply pre-emergent herbicide to help control crabgrass?  Johnathan W., Tulsa

Pre-emergent herbicides are definitely helpful in preventing weed establishment. They are used in early spring (mid-February through mid-March) to prevent crabgrass and other summer weeds. They also are useful in the fall (mid-August through mid-September) to prevent winter weeds such as henbit. Many of the herbicides will need a second application in spring 60 days after the first application for complete coverage of crabgrass. The product label will indicate if this is needed.

Many people are reluctant to use herbicides of any sort for weed control. That is a reasonable choice for those who are willing to tolerate some weeds. The best preventative is to maintain a healthy, well-established lawn as this will help prevent much of the weed invasion. A healthy thick lawn depends on good soil, proper turf grass for the area, adequate sunlight and supplemental irrigation. Most lawns need some fertilizer, and there are organic and synthetic sources available for nutrients.

The Tulsa Master Gardener website contains several helpful lawn maintenance calendars indicating what to do, what to use, and when to use it for Bermuda and fescue lawns.  Specifically, see the “Turf” section of the Master Gardener website for complete details.

Master Gardeners are often asked if there are any “organic” pre-emergent herbicides, as opposed to commercial or “synthetic” ones. Unfortunately, while there are other organic pesticides, there is no effective organic pre-emergent herbicide.

Corn gluten is an organic sold as crabgrass prevention. Some reports state that if it is applied during a narrow window in spring, there may be some benefit. OSU turf grass specialists cite studies that show little benefit.

For those wishing to use a synthetic pre-emergent herbicide, OSU has some recommendations. While there are several varieties of pre-emergents available on the market to prevent weeds, especially crabgrass, OSU feels that one of the many commercial brands containing the chemicals dithiopyr, pendimethalin or prodiamine are very good choices. These pre-emergents cost a bit more than other types but last a lot longer and, in many cases, can kill crabgrass and other weeds after they have sprouted.

The labeled directions of all such products must be followed. These herbicides usually come on a dry particle such as fertilizer or other inert material. They may also be found less often as liquids. They must be washed onto the soil with at least ½ inch of water after application. After washed onto the soil, they form a barrier for weed prevention, which may last for months if undisturbed.

One of the benefits of these three products is that they are not soluble in water and, thus, do not leach into groundwater or spread from where they are applied. They are broken down in nature by sunlight and soil microorganisms.