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& Landscape supply

Postemergent Herbicides Can Keep Lawns and Your Business Healthy

The most common operation in the turf and ornamental arena is weed management. It is estimated that herbicide applications make up slightly more than 72% of applications made by companies. It also represents one of the highest reasons for call-backs, complaints and discontinued service.

Weeds are often very visible, leaving customers with the feeling that measures to control them have been improperly carried out, leading to dissatisfaction. Granted, property owners are a good reason why weeds appear in the first place. It is true that the No. 1 defense against weeds is maintaining a thick, healthy lawn. This not only refers to fertility but also to mowing heights and frequency, clipping management, airflow and sunlight penetration, soil management and irrigation practices.

As turf canopy thins, sunlight will be allowed to reach the soil surface where many weed seeds may lay dormant until the proper conditions present themselves, then they are off. It is true that some weed species can lay dormant for decades waiting for the right opportunity to germinate.


Identification is Critical to Weed Management

 This discussion can be very lengthy and goes all the way back to the definition of a weed. In many cases the old adage of a weed being any plant growing where it is not wanted is sufficient in this case. More specifically, it is important to identify the weed’s species to best plan for control. Identification is key to knowing when to take action.

Most weeds will fall into one or more of the following four life cycles, which plays a huge role in application timing.

  1. Winter annuals (WA): These weeds appear in fall and through the winter as the weather allows. They begin to germinate as soil temperatures cool back down from the summer. They will persist through the spring until temperatures get too warm, then they will die off. Examples of WA weeds include common chickweed, hairy bittercress, annual bluegrass and many others.
  2. Summer annuals (SA): These weeds will germinate as soil temperatures rise in spring and summer. They will continue to grow throughout the season until frost and cold temperatures kill them. Common examples of SA weeds are black medic, tallow woodsorrel (oxalis), spotted spurge and of course, crabgrass.
  3. Perennials: These weeds are best defined as those that will live longer than two years. These plants do reproduce, some by seed and some by vegetative means (stolons, rhizomes, corms, nutlets, etc.) and can be a big problem in turf and landscapes. Some common examples include Canada thistle, yellow nutsedge, curly dock and buckhorn plantain.
  4. Biennials: This category represents the smallest group of common turf and landscape weeds. These plants will grow for two years, often vegetatively, or leafy rosette growth in the first year then reproductively in the second. This means that the second year is when they will commonly flower. The most well-known weed in this category is wild carrot.  

What Type of Herbicide Should You Use? What’s Growing Will Guide You

Knowing and understanding specific weed life cycles will greatly improve your ability to deliver adequate control. A good example is the use of preemergent herbicides. These materials are often applied in the spring to prevent summer annual weeds. However, if the weeds are already present, such as a perennial or winter annual, don’t expect any activity. Most preemergent herbicides have very little to no postemergent capabilities.

These herbicides are often soil-applied and create a chemical barrier to manage weeds. They do not prevent weed germination but are taken up by the first roots and prevent emergence, so they are in a sense never visible.

These products are selective, meaning that they control a finite number of weeds, and are most commonly used in the spring and the fall on turf and ornamental beds. The strength of many of these products is annual grassy weed control, which is helpful because these weeds become more difficult if allowed to become established. Common examples of preemergent herbicides include prodiamine, dithiopyr, oxadiazon, isoxaben, flumioxazin, and others.

Postemergent herbicides, on the other hand, function as the name states—following emergence. These types of products typically rely on some sort of foliar contact for uptake. This means that many of these herbicides must be applied to the leaves of the plant for them to be effective. This in itself, can create some issues depending on application techniques and physical plant traits.

There are many classes of chemistry and often you will find these chemistries blended into two-, three- or four-way types of products, or as I like to call them multi-AI herbicides. This means that you can have multiple active ingredients working for you, which most often will broaden the spectrum of weeds controlled and some will have additives that accelerate the activity.

Some chemistries will perform better on grassy weeds while others are more effective on broadleaf species. Blending these together will, of course, act as more of a catch-all or shotgun vs. rifle approach. Common examples of these include Triplet, Speedzone, Q4 Plus and many others.


When to Use Non-Selective Herbicides

Last are the non-selective herbicides which more or less kill just about anything that they touch. Most will require foliar contact, and some will be systemic while others not. This means that some will be absorbed into the plant and translocated to the roots for a more “complete” kill of the target weed.

Although these are not commonly used in turf care with the exception of spot applications on difficult weeds or lawn renovation, they play a giant role in landscape bed and bare ground situations.

The most common example of a systemic, non-selective herbicide is glyphosate. When applied properly and according to label recommendations, these products perform as intended and are extremely effective tools in the war on weeds.

Each product will have specific application instructions which need to be followed to achieve the goal of weed management. That said, there are a number of things that can come between you and the excellent control expected following the application. In this case, the focus will be on postemergent herbicides and while at least 15 potential reasons for failure come to mind, it is best to contract that list to five.

Some of the following pitfalls will be grouped to make things a little easier to read.

  1. Product selection and application techniques—READ THE LABEL. This is a broad reason, but it starts with using the right product for the target weed. As mentioned, there is different sensitivity to active ingredients across weed species. This reason continues on through application equipment, nozzle size and spacing, water delivery rate, mix rate. These products are most often calculated in ounces of product per area whether it is 1,000 square feet or acre. This eliminates the ability to determine how much product goes into a 25-gallon ATV sprayer for a blanket application without knowing the coverage of that tank. The exception is for the purpose of spot spraying. This means the applicator must calibrate their equipment prior to application to ensure proper product delivery for control and prevention of damage to the desirable species. Water delivery rate is crucial in that too little water per 1,000 square feet may cause interruption in coverage and too much water may move the products from the leaf surface.
  2. What’s in the tank? This begins with mix water quality, pH and water hardness can degrade a herbicide solution or cause the active ingredient to become bound up, rendering them useless. Likewise, tank mix partners or other components and products added to the tank may change the chemistry of the AI, causing it to lose efficacy. Another idea that fits here is the use of adjuvants or products that enhance spray applications. These can be as technical as a surfactant that can help spread water droplets on the leaf or improve adherence to the leaf or may come in the form of a colored dye to improve coverage and reduce skips or excessive overlap.
  3. Timing: As mentioned previously, there are many aspects of timing; however, the time of the year tends to be one of the more important factors. Ordinarily there is a call to action when dandelion blooms in the spring. The yellow flowers alert us that they are present and are often a source of complaints. While this may seem like a good time for control, there are again some pitfalls, and one would have to first look at what the plant is doing to understand. At that point, energy is being used in great amounts for flower and seed production, so uptake may be minimized and full translocation to the roots may be compromised. While one may see reasonable control of the current plant, it may regenerate. Also, early applications may miss some of the later germinating summer annual weeds. Summer applications can fail for a couple of reasons, including the fact that many weeds are mature and will require higher rates of product, and some summer annuals may not have germinated yet and will be missed. This leaves us with fall applications when typically, weather is more conducive to active weed growth. Also, plants begin to store more carbohydrates in the roots so translocation may be optimized.
  4. Mowers and traffic: The human effect. Most herbicide labels will recommend an extended period of contact with the leaf for best results, mowing and other disruption will greatly decrease the efficacy. It is a good idea to defer mowing and traffic for a period of time specified by the label. Also, timing the applications when rainfall is not predicted, and delaying irrigation will aid in prolonged leaf contact. Sometimes just dragging the hose over treated areas will reduce control, so it is best to plan patterns such that the hose stays clear of those areas.
  5. Mother Nature: It is vital to monitor the weather if you want to be successful. Wind can cause excessive drift, wrecking spray patterns, or may even lead to potential harm to not-target plants. Temperature also comes into play, whether it be from a perspective of being too warm or too cold. Certain formulations, for instance, esters or alcohol-based materials, are more readily absorbed and more active during cooler temperatures but generally should not be applied over 85 F to prevent turf damage or volatility which can damage non-target species. If it is too cold, amine (water-based) formulations may not be as rapidly absorbed, leading to slow or inferior control. Last but not least is water, which can alone have the biggest impact in failure. Rain clouds, showers and irrigation can dilute or wash off product that was meant to dry on the leaf surface, greatly decreasing control. The other side, many areas have been dealing with drought like conditions which can diminish performance to a greater degree. If the plant is not in an active state of growth, uptake will be reduced leaving little to no control. Adequate soil moisture can be one of the biggest factors in weed control performance and is often missed when applications are made.

Just like applying any other pesticide, the take-home message is the same for herbicides. Read and fully understand the label before making any applications. Pay attention to critical failure points and stay focused when mixing and applying. Most of all, be careful—for your safety and that of non-target plants.


Lean on Us for Turf-Care Solutions

For more information on identifying specific weeds and weed control in general, visit our Turf & Ornamentals blog library at EwingOutdoorSupply.com.

Ewing’s Tech Team, myself and Pat Gross, are here to answer your turf-care questions. Email me at klewis@ewingirrigation.com or call/text 480-669-8791. Email Pat at pgross@ewingirrigation.com or call/text 714-321-6101.


TAGS: Herbicides, Post-Emergent Herbicide