Pre-emergence Herbicides on Annual Flowers

A preemergence herbicide can be used to prevent many weed seeds from germinating. Some preemergence herbicides are applied before planting, others are applied after planting but before weeds emerge. The flower bed should be weed-free when the herbicide is applied. Only a few postemergence herbicides can be used to control grassy weeds after flowers have become established.

Preemergence herbicides are applied before weeds emerge and provide residual control of weed seedlings. As with any other tool, each herbicide has unique characteristics which should be considered when planning a weed management program.

Remember two things when choosing a preemergence herbicide:

• First, no single preemergence herbicide will control all germinating weed seeds. Product labels list the weed species the product has been shown to control. Consult previous scouting notes to determine if one weed is particularly problematic, and be sure the product you choose will control it.
  
  
• Second, herbicide labels list the landscape plants that the product can safely be applied over. If a particular plant is not on the list, damage could occur if that plant comes into contact with the herbicide. New plant species are added to most product labels each year, so check them carefully.
  

The following characteristics are important in utilizing preemergence herbicides:

• Rate of application.
  
  
• Residual activity (Length of time the herbicide will provide effective weed control).
  
  
• Activation - For maximum effectiveness, each herbicide needs to be watered (1/2 inch irrigation or rain) into the soil surface within a specified number of days.
  
  
• Mechanism of action (How the herbicide kills weeds.)
  
  
• Weed control spectrum (Which weeds the herbicide will and will not control).
  
  
• Potential losses (leaching, volatility).
  
  

Since preemergence herbicides will not control emerged weeds, they should be applied before weeds germinate or all existing weeds must be killed prior to herbicide application. With annual flower beds, preemergence herbicides should be applied after transplanting to weed-free soil and irrigated.

Frequency of herbicide application will depend upon the herbicide's residual time. Residual weed control will increase with increasing herbicide application rate; and decrease with increasing amounts of rainfall or irrigation, soil temperature, and organic matter. The proper herbicide for each situation will be dictated by the plant species located in the bed, weed species, and future use.