Preplant herbicides are used to prevent weeds from emerging from the ground. Preplant herbicides reduce gardeners' work. However, there are still weeds that manage to grow despite preplant herbicides. It's best to use multiple weed-killing treatments in combination to protect your garden from unwanted plants.
Preplant herbicides must be used within a particular time frame to be effective. Atrazine must be used within 30 days of planting. Axiom must be used 45 days before planting. Alachlor must be used 30 days before planting, according to Iowa State University.
Preplant herbicides can inhibit the growth of all plants if you don't want plants growing in a certain area at all. However, if you're trying to avoid killing the plants you want to grow, some specialized herbicides can be applied. Some selective preplant herbicides have to be added prior to planting, while other herbicides can be applied at any time.
Some preplant herbicides can cause soil degradation and dissipation.
Annual vs. Perrenial
Preplant herbicides are more effective in stopping annual weeds than in stopping perennial weeds. Perennial weeds won't reproduce when a significant amount of preplant herbicide remains in the soil, according to the Ontario Ministry of Food and Rural Affairs. However, the existing perennial weeds will continue to survive themselves. Targeted herbicides can be used in combination with preplant herbicides.
Some herbicides designed to kill plants after germination can also prevent plants from germinating in the first place, such as with foliar-applied glyphosate, according to Arboriculture & Urban Forestry Online.
Home remedy versions of preplant herbicides exist. One is salt. Salt in high concentrations in the soil can prevent plants from growing and surviving. However, salt can be a little too effective and can be difficult to get rid of when you decide that you want to grow plants in that same area later on.