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.