Timing is Everything
Pre-emergence herbicides can be useful chemical tools for you as you attempt to manage weeds before they can compete for valuable space in your turf. Unfortunately, the level of control you desire does not always happen. There are a host of possible reasons for not getting the control that you want; but, by far and away, the most probable reason is improper timing of application — usually that they are not applied soon enough.
You should have your pre-emergence herbicides on the ground and activated prior to the initiation of weed seed germination. Activation is important because you want to have an “active” chemical barrier present in the soil solution when the target weed seeds imbibe water. You can insure activation easily by irrigating immediately after herbicide activation. Be sure to irrigate at least equivalent to a half inch of rainfall. Actually, if you plan to use sprayed formulations of pre-emergence herbicides, a good way to insure activation is to apply them when it is raining. In circumstances where irrigation is not available and you must depend on rainfall for activation, you should apply pre-emergence herbicides earlier than you might when you could irrigate, to insure there is ample time for rainfall to occur.
GETTING IT RIGHT
There are a number of tactics you can use to insure that pre-emergence herbicides are on the ground early enough. If the target weed for control is crabgrass, some prefer to use phenotypic indicators such as forsythia bloom, in locations where it exists. However, proper timing with regard to forsythia is petal fall, not bloom. Often times, forsythia will bloom significantly in advance of proper timing for pre-emergence herbicides. If you apply too early, you can run the risk of “break through” later in the season because the material does not have sufficient soil residual to last throughout the seasonal germination period. An easy an effective method for determining proper timing is to monitor soil temperature. The key is to monitor the temperature in the upper half-inch of the soil profile. Take the soil temperature measurement early in the day (before direct sunlight hits the site) and track the temperature on a daily basis. Make sure to take the temperature measurement in the site you intend to treat or, at the very least, one that is similar in turf density and soil type. When the temperature is above 50°F for three consecutive days, you should make and activate your pre-emergence applications. As an aside, look for germination of the crabgrass in bare ground areas, as it will germinate there earlier than in the turfgrass stand. Often times, you will see germination in bare ground areas at about the same time you find successive days of 50°F temperatures in the upper half-inch of the soil profile in turfed sites.
If the target summer annual grassy weed is goosegrass, you can apply the pre-emergence herbicide a little later, as goosegrass germination begins about three weeks after that of crabgrass. However, once goosegrass begins to germinate, it continues to do so throughout the summer months and even into the fall season.
Crabgrass, on the other hand, tends to stop germinating sometime during the month of July. What this means to you is that goosegrass is much more difficult to control using a single application of any pre-emergence herbicide. In fact, in most states, it is recommended that you make a follow-up application, about seven or eight weeks after the initial application, at a half rate of the first one. This second application boosts the soil concentration of herbicide to the threshold level needed to sustain control longer into the season.
If you are located where you need to make early season applications of pre-emergence herbicides in order to sufficiently control crabgrass, then split applications will usually work better for you as well. You should make the first application earlier than you would make a single application, and you should make the sequential application seven to eight weeks later.
If part of your weed management strategy is to also target control of summer annual broadleaf weeds (i.e. prostrate spurge, yellow woodsorrel, etc.), should they appear on the label of the pre-emergence herbicide you have chosen, then split applications are necessary to obtain the best control possible for these species.
Having stated that, the best control of these broadleaf summer annuals is through post-emergence application of wide spectrum broadleaf herbicide combinations.
If you apply your pre-emergence herbicides at the right time and activate them prior to the imbibing of water by the weed seeds, then control is usually very good to excellent. However, environmental conditions can still impact the level of control. Pre-emergence herbicides must have a threshold level of concentration in the soil in order to be effective. If you experience a growing season with abnormally high rainfall, it is possible that the herbicide chemistry will undergo significant dilution and the soil concentration could fall below the threshold level needed. Also, on sites that are sandy, through natural circumstances or through soil modification, the irrigation intensity is always higher which can lead to dilution effects as well. You should always consider split applications in such situations.
At times, you might find it convenient or advantageous to use pre-emergence herbicides that are formulated on fertilizers of various types. Fertilizer carriers are generally quite good for purposes of delivering pre-emergence herbicides. You can obviously kill two birds with one stone, so to speak, by using such combinations, but there are limitations. The primary limitation being that when used at the label rate (which is driven by the rate need for the herbicide) the commensurate rate of fertilizer is fixed. Should the rate of the fertilizer and the sources of nutrients fit into your fertilizer program (primarily with regard to rate and timing of application), then the use of a combination product is a sound agronomic practice. However, if your desired fertilizer program is compromised if you use a combination product, it may not be a wise choice.
There is ongoing debate with regard to whether sprayed formulations of pre-emergence herbicides provide more consistent control than formulations that are spreader delivered. It is not prudent to generalize because there are too many extenuating factors that can play a role in the outcome. It is known that both methods of application are capable of producing acceptable results. In either case, the issue of proper timing to get the best control possible clearly overrides any differences associated with formulation.
If your target weed is annual bluegrass, timing of application is just as important as it is for summer annual grassy weeds. Annual bluegrass, being a winter annual grassy weed, germinates predominately in the late summer and early fall. For example, you should apply pre-emergence herbicides in mid-August if germination is expected sometime in early September. Again, soil temperature plays a role in that annual bluegrass does not germinate, in any significant amount, until after the soil temperature is consistently below 70³F. Water is again necessary for the activation of the herbicides labeled for the pre-emergence control of annual bluegrass. It is important that you assess the annual bluegrass population that you have because no pre-emergence herbicide is able to control the perennial ecotypes of annual bluegrass. Your most effective use of pre-emergence herbicides for the control of annual bluegrass is when little or no annual bluegrass exists and you want to keep it from invading. In such circumstances, the only way encroachment will occur in your turf is through germination of seed where voids such as ball marks have occurred. You can provide sufficient protection from annual bluegrass invasion by using highly efficacious herbicides and applying them with optimum timing. Of course, you want to be sure that you are doing all of this with regard to those activities that might be creating voids in your turf.
One pre-emergence herbicide (Gallery) has activity for the control of perennial broadleaf weeds, such as dandelion and clover. You can use this material for keeping these two weeds from re-invading your turf, but it is necessary to control the existing perennials through post-emergence herbicide means before the pre-emergence approach can be efficacious.
Clearly, you have the chemical tools at your disposal to get the level of pre-emergence control you desire for a number of annual and perennial weeds. The herbicide chemistries work very well, provided you introduce them to the site at the correct time and make sure they are activated. Hopefully, Mother Nature is reasonably cooperative throughout the year and you do not experience too many environmentally stressful times that could adversely affect your weed control program.
Dr. Thomas L. Watschke is professor of turfgrass science at The Pennsylvania State University (University Park, Pa.).
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