How irrigation affects your applications
Irrigation can provide an array of benefits to any turfgrass site. Beyond merely supplying water to the grass plants, irrigation can remove dew and frost, apply nutrients and provide a cooling effect, to name a few of its benefits. One of the most important effects of irrigation is that it can enhance or, at times, reduce the efficacy of pesticides.
Weed management Managing weeds successfully depends on manipulating plant competition to favor turf over weeds. Clearly, keeping the turf in a healthy, actively growing state improves its competitiveness. Therefore, you should always seek to minimize moisture stress, which can reduce turf growth and give certain weeds a competitive advantage.
Just as important, however, is to ensure-through irrigation, if needed-that any weeds that have managed to invade the turf are growing actively enough to be susceptible to herbicides.
* Pre-emergence weed control. For the most part, pre-emergence herbicides control weeds by interfering with the germination process. They work most effectively when activated by water. Therefore, it is important to irrigate sites treated with pre-emergence herbicides immediately after application.
Apply about 0.5 inch of water. This will allow the applied herbicide to form a complete chemical barrier at or near the soil surface (where the majority of the target seeds are). Moving the herbicide into the thatch/soil upper root zone lessens the possibility of active ingredient losses via volatilization or ultraviolet (UV) light decomposition.
In addition, dense turf stands may intercept a significant proportion of spray-applied materials. The sooner you can apply irrigation to the turf, the easier it is to wash it off the leaf blades and into the soil. Once the spray dries, it is relatively difficult to dislodge. (The perfect time to make a spray application of a pre-emergence herbicide is while a gentle rain is falling.)
* Post-emergence control of annual grassy weeds. Herbicides that control annual grassy weeds, such as crabgrass and goosegrass, are primarily contact materials. You should apply them to the leaf canopy with enough volume to coat the leaf surface thoroughly, but with a minimum of drip-off. You must allow the herbicide to dry. Do not irrigate until drying is complete.
Weeds should not be under moisture stress prior to treatment, because this will compromise control. For example, annual grassy weeds under moisture stress are extremely difficult to control with fenoxaprop (Aventis' Acclaim Extra). Therefore, you should irrigate sufficiently to alleviate moisture stress before you apply the herbicide.
* Post-emergence control of broadleaf weeds. The labels of practically all post-emergence broadleaf herbicides make some reference to "actively growing" weeds. Although this statement may seem mundane, it is critically important for a couple of reasons.
First, when weeds are under moisture stress, most will produce more waxy substances on the leaf to reduce atmospheric moisture losses. The increase in waxy substances reduces the ability of foliar-applied herbicides to stick to the leaf surface and to penetrate the leaf.
Second, weeds that are not "actively" growing do not translocate materials as effectively as when they are actively growing. Many of the post-emergence broadleaf weed-control products will not provide satisfactory control if they are not translocated sufficiently to meristems (areas of active cell division) throughout the plant.
Translocation throughout the plant is of particular importance for controlling weeds that spread vegetatively (by stolons or rhizomes). Insufficient translocation will result in regrowth of the weed from nodes on the vegetative propagules.
Granular formulations of broadleaf-weed control products use vermiculite as a carrier. Vermiculite is a lightweight material, and, with the addition of "sticker" materials, can adhere to the leaf surface of weeds quite well.
However, if the weed leaf surfaces are dry, the vermiculite formulation will not adhere well and control will be poor. The labels of such products always state that, if the leaf surface is dry, you should irrigate prior to application. This provides the leaf wetness the product requires for adhesion, allowing the active ingredient of the herbicide to move into the leaf and translocate throughout the plant.
For granular formulations of broadleaf herbicides to be effective, several things have to "go right," not the least of which is having a wet leaf surface. Though granular formulations have their place, spray applications of broadleaf herbicides, as a general rule, are more effective.
* Non-selective herbicides. The use of non-selective herbicides to control perennial grassy weeds or to renovate turfgrass sites is, again, facilitated when the weeds are in an "actively growing" state. As with broadleaf-weed-control products, the leaf surface must be conducive to adherence and subsequent absorption of the herbicide. Translocation to meristems is also important for successful plant kill when using these products.
Avoid any irrigation following application to allow for adequate drying, and avoid application if rain appears imminent. Labels often state the number of hours required for a product to become "rainfast," meaning that rainfall or irrigation should not substantially reduce control after that interval.
Insect management Irrigation's role in successful insect management is similar to its role in weed management. Avoiding moisture stress enhances overall turf vigor, often to the extent that treatment threshold levels for insect pests are higher for irrigated sites than for non-irrigated sites.
Also, turf sites not exposed to moisture stress may harbor fewer insect pests because conditions are more favorable to predators and fungi that can attack the pests.
* Surface feeders. You can effectively control insects that attack turfgrass plants by feeding at or above the soil surface when the insecticides you apply remain near the target after application. Therefore, when treating for these kinds of pests, the spray volume should be just enough to cover the aboveground portion of the turf stand. Avoid irrigating within a day after application to let the insecticide dry and to minimize any dilution from applications of additional water.
Proper irrigation management can also reduce the numbers of certain surface feeders, such as chinch bugs. Often, sufficient irrigation alone can reduce the number of chinch bugs to a level that does not require treatment.
* Root-zone feeders. For most turf managers, the most frequent insect problem is the root feeding of white grubs. When grubs attack the root system of turf, its ability to absorb water and nutrients is reduced. This often leads to significant turf losses.
I cannot over-emphasize the importance of proper irrigation management for grub-infested turf. Proper irrigation can provide the turf with enough moisture to survive, even with a reduced root mass. That is why irrigated turf can tolerate higher numbers of grubs per square foot than unirrigated turf.
Irrigation also is important for moving insecticides to where the root-feeding pests reside in the root zone. Therefore, in addition to requiring a high-volume insecticide spray, insecticidal treatments-whether sprays or granules-for root-zone pests benefit from irrigation in the same way as applications of pre-emergence herbicides. The water will move the pesticide to the location of the target pest (the root zone) and reduce losses of active ingredient caused by UV light degradation and volatilization.
Disease management Irrigation management is crucial to effective disease management. A healthy, actively growing turfgrass plant is less susceptible to fungal attack and more tolerant of low inoculum levels of various pathogens.
Some diseases that attack turfgrasses are in what is known as the "water mold" group of fungi, such as Pythium and snow molds. These pathogens thrive in high moisture. Therefore, when environmental conditions favor the development of water-mold fungi, carefully manage your irrigation to avoid excessive or poorly timed irrigation.
Many of the most effective and economical fungicide products are contact materials. They protect the leaf surfaces of turfgrass plants from invasion by fungi. To maximize protection, the applied fungicide must dry on the leaf surface. Therefore, delay or minimize irrigation after application as much as possible to reduce wash-off or dilution.
As with granular herbicides, granular contact fungicides have the same requirement for wet leaf surfaces when you apply them. Therefore, it is often necessary to lightly wet the leaves prior to applying granular contact fungicides.
During the past several years, several systemic fungicides have become available which are highly effective against an array of diseases. The root system as well as the leaves can absorb these materials which translocate throughout the turfgrass plant. Fungicides that are internal and mobile within the plant have important advantages for controlling diseases, but they are also relatively costly.
Systemic products work best when you irrigate them into the root zone. Sometimes, the target disease actually attacks the root system, similar to a grub feeding on the roots. In such cases, it is critical that the active ingredient of the fungicide be positioned where the pathogen is found. In other cases, the primary benefit of irrigating the fungicide into the soil is that it increases the amount of active ingredient available for root uptake and subsequent translocation throughout the plant to provide protection from disease. Either way, the irrigation is key.
With most pest-control products, it's not just what you apply, it's also how you apply it. Irrigation is a key aspect of the "how" for many products. Proper irrigation management can facilitate or enhance the activity of pesticides, but irrigation at the wrong time can defeat your pest-control efforts. Most importantly, proper irrigation is essential in maintaining good turf health and vigor, which is the best pest control of all.
Dr. Thomas L. Watschke is professor of turfgrass science at The Pennsylvania State University (University Park, Pa.).
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