During the fall and winter months, you may be thinking more about problems like large patch or other diseases, and weeds might not be the top priority when we have cooler temperatures. However, spring (and prime weed season) is just around the corner. In just a few short months, we will start to see things like spurges, crabgrass, and other weeds really start to take off, signaling the beginning of spring. Before the weed pressure really starts to build during the warmer and wetter months, it is important to develop a solid weed management plan.
Remember that weeds are often “secondary” invaders – they come in and start to wreak havoc when the turf is thinned or stressed. Therefore, the first step in any weed management plan is to make sure the proper cultural practices are being followed. Proper mowing, fertilization and irrigation are going to help the turf remain thick and healthy, and allow it to outcompete weeds. Once you have the cultural practices dialed in, the next step is selecting herbicides. The bulk of most people’s programs is postemergence herbicides, but preemergence herbicides should not be overlooked because of all the advantages they can offer. First, there are many options to choose from that can be used in just about every type of turfgrass that is commonly grown in Florida. Herbicides such as dithiopyr (Dimension®), pendimethalin (Pendulum®), prodiamine (Barricade®), isoxaben (Gallery®), dimethenamid-P (Tower®), metolachlor (Pennant Magnum®), indaziflam (Specticle®), and many others can all be applied to established bermudagrass, zoysia, centipede, St. Augustine, and bahiagrass lawns.
Herbicides should be thought of as tools. There is no tool (or herbicide) that is right for every job, and all of the different options are going to have both advantages and disadvantages. If you are not familiar some of the most common preemergence herbicides labeled for use in landscapes, a brief overview of some of the advantages and disadvantages of a few options may be helpful when starting to look at adding preemergence herbicides to your weed control tool belt.
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Atrazine – most commonly used as a postemergence herbicide for St. Augustine and zoysia grass lawns, atrazine also provides some preemergence control, primarily for broadleaf type weeds.
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Advantages: Post and some preemergence control with one herbicide. Atrazine can provide control of many broadleaf weeds common in Florida such as Florida betony.
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Disadvantages: Cannot be used in bermudagrass or bahigrass lawns, and due to the chances of phytoxicity at higher temperatures (above 85 or 90°F), the application window is Florida is relatively narrow and limited to early spring or late fall applications.
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Dinitroaniline and Group 3 herbicides – This group contains products such as pendimethalin, prodiamine, oryzalin, and although not a dinitroaniline herbicide, dithiopyr has a similar mode of action. These products are generally used for weed control in lawns because they tend to be very strong on annual grasses, such as crabgrass.
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Advantages: As a group, the group 3 herbicides are very strong on annual grasses and labeled for use on the most common turfgrass types in Florida. While their major strength is grass control, they also control certain broadleaf weeds, especially those with small seeds like woodsorrel. Most products are also labeled for use in landscape planting beds.
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Disadvantages: These products are generally not as effective on large-seeded broadleaf weeds such as doveweed, tasselflower, or eclipta. Many of the products in this group are yellow or orange in color and can stain hardscape areas or driveways if sprays land on those surfaces.
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Dimethanamid-P – A preemergence herbicide labeled for use in lawns and landscape planting beds and controls many broadleaf and grassy weeds and provides suppression of yellow nutsedge.
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Advantages: A wide spectrum of control considering it is effective on most broadleaf and grass weeds. It is also one of the stronger options available for spurges and is labeled for use in planting beds in addition to turfgrasses.
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Disadvantages: Purple nutsedge control is generally poor, and as it is an emulsifiable concentration, or oil-based, it can cause injury to some ornamentals when they are pushing new growth. Following label directions will help applicators avoid these issues.
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S-metolachlor – In the same herbicide class as dimethenamid-P, metolachlor is labeled for use in lawns and planting beds for control of grassy weeds, some broadleafs, and sedge suppression.
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Advantages: A great option if yellow nutsedge is problematic. This herbicide also offers control of most annual grasses and several other broadleaf weeds.
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Disadvantages: not as broad spectrum as some other herbicides mentioned here. While metolachlor is one of the stronger options for something like yellow nutsedge, it would be weaker than other herbicides on things like bittercress, chamberbitter (Figure 1), and some other important broadleaf weeds.
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