THE MYSTERY OF WHAT’S BEING SPRAYED ON YOUR PROPERTY  PRE, POST, SELECTIVE, AND NONSELECTIVE  AND WHAT IT ALL MEANS


JIM YANNIE OUTSIDE UNLIMITED, LLC


First, let’s disnguish between a preemergent and postemergent herbicide. The simplest denion is that a preemergent stops a plant from growing beyond the seeding stage, whereas a postemergent herbicide is a product that eliminates plants already acvely growing in the landscape.


Preemergent herbicides in planng beds


The term preemergent is a bit misleading. These chemicals don’t actually stop germinaon but interrupt the growth of a very young plant. As kids, many of us put seeds in a jar of water with some paper to watch them germinate. At the point where the root and shoot brake out of the hard seed shell is the point at which most pre emergent herbicides start to work. The purpose of a preemergent herbicide, whether it is applied in your planng beds or turf, is to stop the growth of unwanted annual weeds such as crabgrass, goose grass, fox tail chickweed, and many others. To see a more inclusive list go to:


hp://extension.psu.edu/pests/weeds/ weedid/weedidandmanagement.pdf


Preemergent herbicides can be applied any me during the growing season to prevent unwanted plants from growing. Whether it’s in the spring during spring cleanup and mulching, July aer the installaon of new plants, or in your fall pansy beds to stop winter chick weed from growing over your owers during the winter. The main purpose of the planng bed preemergent applicaon is not only


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prevenng weeds from germinaon but also to eliminate the need to do signicant hand weeding. Most bed weeds germinate between the middle of April through the end of June. To have your bed pre emergent be most eecve, it should be applied in the spring just before your beds are mulched.


This accomplishes three important goals: First, you’re placing the preemergent on the soil where it will be most eecve. Secondly, you’re geng the preemergent down during the peak weed seed germinaon period. And thirdly, the most commonly used bed preemergent is photo sensive, meaning that when it’s exposed to sunlight it loses some of its eecveness. Covering it with mulch assures geng full benet of the product.


Preemergent herbicides on turf


Preemergent herbicides applied to your turf act in the same method in killing unwanted weed seeds. Unl the early 1990s, the rule was to make sure that your lawn preemergent was applied before the soil temperature in the top inch of soil was above 55 degrees (mid April). Since the mid1990s, products have been developed that will kill crab grass and other annual weeds up to the three leaf stage (small plants).


These chemicals have lengthened the window in which annual grasses can be controlled. Most contractors will use a combinaon of both types of products. The sites that can be sprayed before mid April will use one of the older products which work very well. Aer midApril, they will use a pre / post material.


Postemergent herbicides


Postemergent herbicides are products that eliminate plants already acvely growing in the landscape.


Planng beds


There are two categories of post emergent herbicides. Selecve and non selecve. Selecve postemergent herbicides are products designed to eliminate a very specic plant group. For example, one parcular herbicide may control white cover, but have no control on dandelions. Whereas nonselecve herbicides are developed to kill almost anything that is sprayed with them.


The use of a nonselecve herbicide for weed control in your mulch bed is a cost eecve approach to having weedfree beds throughout the season. Although you have applied a preemergent in the spring when mulching, there are always a few perennial weeds like dandelion, cover, Bermuda grass, and other perennial weeds that are already established in your beds. These products (nonselecves) will eliminate a wide range of weeds with a single applicaon. If you were to use a selecve herbicide in your beds, it might take several applicaons to control the dierent types of weeds.


Postemergent herbicides applied to turf


Post–emergent herbicides sprayed over your turf are Selecve herbicides. Selecve herbicides target very specic plants. That’s why your lawn care company can spray all your grass and only kill the dandelions. Unless you are trying Connued on page 28


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