Guidelines for using postemergence herbicides

Most preemergence herbicides can be used after the soil is settled around the transplants and are applied before weeds emerge. This prevents weed seeds from germinating from several weeks to months. As with any other tool, each herbicide has unique characteristics which should be considered when planning a weed management program:

Since preemergence herbicides will not control emerged weeds, they should be applied before weeds germinate. In field production, preemergence herbicides should be applied after transplanting to weed-free soil then irrigated. Frequency of herbicide application will depend upon the herbicide’s residual. Residual weed control will increase with increasing herbicide application rate; control decreases with increasing amounts of rainfall or irrigation, temperature, and organic matter. The proper herbicide for each situation will be dictated by the plant species, weed species, and future use of the field.

Postemergence herbicides are applied to weeds after they have emerged. Characteristics of postemergence herbicides that should be considered before selection and use are:

Postemergence herbicides can be classified as systemic or contact and selective or nonselective. Systemic herbicides are absorbed and move through the plant. These are useful for controlling perennial weeds. For best control, the weeds must be actively growing so the herbicides can move throughout the plant. Contact herbicides such as diquat dibromide (Reward) and paraquat (Gramoxone Estra) kill only the portion of the plant that is actually contacted by the herbicide. Contact-type herbicides control small annual weeds but only burn-back perennial or large annual weeds. Good spray coverage is important. Selective herbicides only kill specific plants. For example, clethodim (Prism), fluazifop-P-butyl (Fusilade II), and sethoxydim (Vantage) are selective, systemic, postemergence herbicides which only kill grasses while leaving broadleaf weeds unharmed. Nonselective, systemic, postemergence herbicides such as glyphosate (Roundup and many other trade names) and glufosinate (Finale) have the potential to kill or injure any plant that it contacts.

All postemergence herbicides have a specified drying time ranging from 30 minutes to 8 hours for maximum effectiveness. This is the length of time that needs to pass after herbicide application before irrigation or rain to ensure that the herbicide has had adequate time to affect the plant. Although postemergence herbicides labeled for field production remain in the soil for a short length of time after application, they have no residual and little or no soil activity; therefore, multiple applications are needed for perennial weeds. The majority of herbicides registered for postemergence weed control in field production are used either for grass control or for nonselective weed control. Products that provide nonselective weed control should not be applied to the foliage of ornamental plants as severe injury or plant death may occur.

Herbicides can not always be used, nor are they effective in controlling all weeds. In these situations, cultivation and hand pulling may be the only available options. Cultivation works well on small annual weeds; perennials will often regrow from the roots even if the top is removed. Also, remember cultivation can stimulate successive flushes of germinating weeds by bringing new weed seeds to the soil surface. You will need to check for emerging weeds on a two- to three-week cycle if you are routinely cultivating. If preemergence herbicides have been applied and activated, they form a herbicide barrier that must be left undisturbed to be effective. Cultivation disrupts this barrier and lessens the effectiveness of the herbicide. Therefore, cultivate sparingly if you use preemergence herbicide.

Cultivation is not without other drawbacks. Cultivated soil is very susceptible to erosion since there is little to no vegetation to hold the soil in place. In addition, implements such as in-row weeders, which cut off weeds 1 inch below the soil surface, can build up ridges, which are detrimental to growth of nursery crops. Ridged soil around the stem collar of newly set liners tends to suffocate them just as if they had been planted too deeply. Cultivating nursery stock in subsequent years causes considerable root pruning and delays growth, and can lead to a soil buildup around the collar of nursery stock. Many landscape contractors have had problems with trees grown with soil thrown over the surface of the roots. The surface of the root ball may have several inches of soil with few roots above what was the original soil line. Landscapers think they are planting at the correct height (Figure 5). In reality, they are planting several inches too deep. Thus, cultivation has received considerable attention in landscape magazines and has been identified as poor practice for growing nursery stock.

It is important to develop a weed management strategy that encompasses all 12 months of the year and uses all available options. These include preventative measures such as preemergence herbicides, as well as sanitary practices that prevent weed seeds and vegetative parts from spreading.

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