Herbicides: Preemergent Or Postemergent?
Outlines the differences between preemergent and postemergent herbicides and ways they are used. Includes precautions to take.
What are they?
Herbicides are chemicals that retard or kill plant growth. One of the ways herbicides are categorized is in terms of the time they are applied. Two types are Pre-Emergent and Post-Emergent.
These weed killers are applied before the weeds sprout, which prevents weed germination while allowing desirable plant growth to continue normally. Pre-emergent herbicides are usually applied in a granule form, which permeates the soil to about an inch or so. They are an effective weed control when applied beneath a weed block. Pre-emergent herbicides do not kill existing weeds and the product will lose some efficacy if the soil is often disturbed by feeding birds, digging or cultivation or is used as a pathway. Always check the label to make sure the pre-emergent product you have chosen is safe for your groundcover.
These herbicides will kill off existing weeds. Some of these can be applied without damage to groundcover, while others must be applied only to the weeds you wish to destroy. Otherwise they will kill off both desirable and undesirable growth. Some types of post-emergent herbicides are systemic and will work throughout an entire plant right down to the root. Before choosing a post-emergent herbicide read the instructions carefully. Make sure you are aware if your groundcover is susceptible to damage and if you need to protect other garden plants in the area. If your groundcover is susceptible to the herbicide and is thick, you may have to carefully paint the herbicide onto the weed with a brush. Only use post-emergent herbicides on days of low wind, otherwise chemicals can drift on the breeze.