To minimize adverse environmental impacts while effectively eliminating weeds, proper timing of herbicide application is crucial. When weeds are quite large and mature, greater amounts of herbicide are usually needed and may not be effective.
For example, it is relatively easy to control small, first-year dandelion rosettes with a minimal amount of herbicide. Larger, older dandelion plants have a greater capability to outgrow, and hence recover, from small doses of herbicides. Where appropriate, small dandelion rosettes can be effectively hand removed and eliminated if most of the root system is removed.
Fall is the best time to control perennial broadleaf weeds. At this time of year (mid-September through mid-October), these plants are storing carbohydrates for winter, actively growing and will readily take up the herbicide. Often, a one-time, relatively low rate of application of an appropriate herbicide will be effective. Since much of the other landscape plant material is either going dormant for the winter or has been removed from the garden and flower beds, there is less chance for off-target plant injury.
Some Additional Notes on Timing and Usage
Selective postemergence herbicides may be applied any time the cool season perennial broadleaf weeds are actively growing, but most effective control is usually obtained when applied in early fall (September 15 - October 15) or in spring (late April - early June). Fall is preferred over spring due to greater translocation of the herbicide down into the roots and the reduced possibility of damage to flowers and other vegetation.
Table 10.2. Important Product Label Information
Always consult the product label for information about:
For some weeds, repeated applications at 10 to 20 day intervals may be required for control. Again, always follow label instructions for length of interval between applications. Perennial grassy weeds are usually cool season grasses that are best controlled in the fall. But, they can also be controlled at other times of the year when they are actively growing.
In the case of postemergent broadleaf herbicides, it is usually unnecessary to thoroughly drench an area with the herbicide solution to achieve satisfactory weed control. This may be wasteful of water and herbicides as well as move the herbicide beyond the plants and into the soil where it may be more prone to leaching or affect non-target plants. Spraying only to wet the foliage is usually sufficient to be effective.
Proceed to Post-Application Irrigation