By Jeffrey Derr, Ph.D.


Preemergence and postemergence herbicides can be effective options for managing troublesome weed species in lawns, landscape beds, and other areas.  However, sometimes herbicides do not perform as expected.  Listed below are some reasons that herbicides can provide less than desired results. First, I will discuss preemergence herbicides and then follow with a separate section on postemergence herbicides.


There are application issues, weed species impacts,  as well as environmental ones that can adversely impact weed control with preemergence herbicides.  Keep the following in mind when developing a program utilizing preemergence herbicides.

1. Lack of activation. All preemergence herbicides need to be activated by rain, irrigation, or mechanically to move the herbicide into the zone of weed germination in the soil. Annual weeds will predominantly be germinating in the top inch or two of soil. The activation rainfall or irrigation event should occur soon after herbicide application, ideally within a few days. If a preemergence herbicide sits on the soil surface for several weeks with no rain or irrigation, it can break down from sunlight (photodecomposition) or volatilize (leave the soil surface as a vapor). This reduces the level of herbicide in the soil such that the rate may be now too low to effectively control weeds when rains finally occur. This is especially important for herbicides with low water solubility or herbicides that  are more volatile, such as trifluralin (Treflan, Preen Garden Weed Preventer, etc.). One ideally would like to irrigate immediately after  application for chemicals such as trifluralin or napropamide (Devrinol)  or mechanically incorporate it right after  application. We would like have to a quarter to a half inch of rain for activation.

2. Excessive rainfall. While we need a rain for activation, excessive rainfall will result in deeper leaching of the herbicide, reducing the level in the top inch of soil where weed seed will predominately germinate. Higher rainfall levels will also increase microbial activity in the soil, increasing herbicide break down.  Microbial degradation is an important means of herbicide break down in the soil and microbial growth increases under moist soil and warm temperatures. If we get 10 inches of rain for example in April, that crabgrass preventer you put down in March will not last long, resulting in crabgrass germinating during mid- to late-summer.

3. Wrong herbicide. Each preemergence herbicide will have a certain spectrum of annual weed species that it will control, while other weed species will not be controlled. One good example would be  isoxaben  (Gallery), an herbicide   used for  preemergence  control  of broadleaf weeds such as common chickweed, henbit, and prostrate knotweed.  However, Gallery will not pro- vide acceptable control  of crabgrass. If one wants to use Gallery, a crabgrass preventer should be added for control of weeds like crabgrass or annual bluegrass.

4. Wrong life cycle. As a general rule, with some exceptions, pre- emergence herbicides control annual weeds but not established perennial weeds. One good example of this would be comparing controls for crabgrass versus that for dallisgrass. Annual grassy weeds will be controlled by a crabgrass preventer but established perennials like dallisgrass or bermudagrass will not be affected. So if a client says that your crabgrass application failed, make sure the problem isn’t dallisgrass, which will go dormant (brown) in the winter but regrow in spring. If you see a large grassy weed in March or April, it cannot be a summer annual  grass like crab- grass. It will take a while for an annual grass, that must germinate from seed, to reach significant size.

5. Wrong timing.  Preemergence herbicides   generally   do not control annual weeds once they have emerged. That is why we need to learn when annual weeds start to germinate so we can time an application a week or two before then. If annual weeds are already emerged at time of application, one often needs to apply a postemergence herbicide or hand weed those existing plants. If one waits until May to apply prodiamine (Barricade, others), for example, established crabgrass will not be controlled. Along the same lines, a crabgrass preventer applied in March will not control annual bluegrass germinating in September. One would  need to apply the crabgrass preventer in August for annual bluegrass control since this weed will start to germinate in early September.

6. Long germination period.  Preemergence herbicides often last around 3 months in soil. If a weed species germinates over a period longer than 3 months, then late season breakthrough can occur. For example, crabgrass in southeastern Virginia can germinate from March through August. This is a period of over five months.  So, we may see crabgrass emerging in July after the March preemergence application has broken down. One way to address is through the use of split applications, applying part  of the use rate in March and applying the rest in May.

7. Application errors. I sometimes am asked how much  of a preemergence herbicide to mix per gallon.  Preemergence herbicide rates are based on the amount of area being treated, not on a rate per gallon. The amount of herbicide to add to the spray tank is based on the spray volume. The spray volume applied per unit area depends on nozzle size, pressure, and speed. So, if one guesses at how much to mix per gallon, they may be putting out too low or too high a rate, depending on the actual spray volume. Calibrate your sprayer or granular spreader to apply the correct amount of herbicide.

8. Herbicide resistance. Repeated application of the same herbicide or herbicides with the same mode of action can lead to resistance.  Resistance refers to a weed species that used to be controlled very effectively but is no longer controlled due to the occurrence of a resistant biotype.  One example in Virginia would be the report of oxadiazon (Ronstar) resistant goosegrass. Another example is triazine-resistant annual bluegrass, where biotypes have developed   resistance to simazine and atrazine. We saved seed from a triazine-resistant annual bluegrass project from a few years ago and found that biotype is also resistant to amicarbazone (Xonerate). Learn the mode of action for herbicides you use and rotate modes of action where possible.

9.  Rate too low. We use higher rates on clay soils higher in organic matter and lower rates in sandy soils low in organic matter. A low application rate to a soil high in organic matter can result in considerable adsorption, reducing the amount available for weed control.   In fact, many preemergence herbicides do not provide effective weed control in muck soils, which are high in organic matter (20% or more organic matter level).

10. Applying an herbicide above mulch rather than applying it before mulching. Some of the herbicide can by tied up on bark particles if applied after mulching. Also, one is dependent on rain to move the herbicide through the mulch layer down to the soil, where weed seed is germinating. Applying the herbicide before mulching places the herbicide closer to the site of weed germination.  The mulch will protect the herbicide from photodegradation if spread after herbicide application.