It’s time to start thinking about that pre-emergence…

Published: February 8, 2013

Crabgrass

Goosegrass

Remember last year. A number of folks out there followed the calendar for their applications of pre-emergence herbicides and were too late. Typically in Missouri, we recommend those applications take place toward the end of March to the middle of April. Some like to follow the phenology of forsythia blooms dropping to dictate their applications of pre-emergence herbicides. In all cases, with a year like last year; you would have been too late.

Pre-emergent herbicides are so-named because they must be in place before annual weed seedlings emerge. Pre-emergent herbicides will not kill weeds that have already emerged. The optimum time to apply your pre-emergent is when the soil temperature reaches 55 F for five consecutive days in the top inch of soil. You can be assured that annual weeds will begin to emerge at that time. Applying a pre-emergence herbicide at this time will provide the optimum length of control. However, length of control is also dependent on other factors such as amounts of rainfall, temperatures, etc. It is imperative that the pre-emergent be applied at the right time and watered down into the soil surface either by light irrigation or rainfall (approximately ½” of water).

If using an organic pre-emergence, such as corn gluten, applications should be anticipated seven to ten days prior to these soil temperature requirements. Organic products will require a little more time to breakdown and become active. Organic products should be watered-in lightly as well in order for particles to come in contact with soil microbes to begin activation. As the plant proteins of the organic fertilizer (corn gluten) breakdown, a natural plant toxin is released which serves as a partial pre-emergent.

Many effective products are available, many of which are impregnated into fertilizers or used alone. Regardless of the combination, both need to be watered-in to be activated. Upon weed seed germination, that first root comes in contact with the herbicide ladened soil and dies.

Culturally, it is still best to maintain a good quality, dense canopy to compete against weeds. If at all possible, try mowing at the highest possible height. Mowing at a height of 3.5 inches versus 1.5 inches can reduce annual weed populations up to 80%. Creating a shade effect with good density is the best defense against annual weeds. This combination will also reduce the number of perennial weeds you find, such as dandelion, plantains, etc.

Prostrate Knotweed

Pre-emergence herbicides are often referred to as crabgrass preventers. This should not be common terminology. Pre-emergence herbicides and corn gluten control far more than just crabgrass. If you read the labels of these products you will find annual grasses, such as annual bluegrass, barnyardgrass, crabgrass, goosegrass, foxtails, fall panicum, etc. Pre-emergence products also control several annual broadleaf weeds, such as carpetweed, henbit, purslane, knotweed, spurge, etc. Always check the product label to determine which product may benefit you the most and read label directions carefully. Do not over apply, either by excessive overlapping or by applying more material than recommended to a specific area. Applying too much herbicide product could result in damage to turfgrass roots.

The following table 1 includes a list of current pre-emergence herbicides and provides active ingredient, trade name, turfgrass species and a condensed weed list.

Spotted Spurge

All herbicide information is presented with the understanding that no endorsement of named products is intended by the University of Missouri, nor criticism implied of similar products that are not mentioned. This article is not a substitution for any product label. Before using any herbicide please read the label carefully for directions on application procedures, application rates, first aid, storage and disposal. Make sure that the herbicide is properly registered for the intended use. Carefully read all precautions on turfgrass sensitivity to products and timings.

Company websites readily allow access to and the downloading of product labels and MSDS’s. Manufacturers and suppliers maintain toll-free numbers for the purpose of providing answers to any technical question you may have on their specific products. You can also contact your local extension office.