Spray Weeds or Fertilize Grass First?

Weed Control

• As a rule, best results come from applying herbicide first. Fertilizing first actually encourages weed growth, because weeds steal nutrients intended for your grass. If you have few weeds and lush, healthy grass, herbicides may be unnecessary. If your lawn is already weed-infested, however, herbicides can help if used safely. There are two major categories of herbicides: preemergence and postemergence. Use either prior to applying fertilizer. Preemergence herbicides kill weed seeds before they germinate and should be applied three weeks prior to germination. Use postemergence herbicide sprays to kill weeds that have already started growing. Make sure to use a product that is appropriate for the types of weeds you have.

Herbicide Timing

• According to Cornell University, herbicides work best when applied in fall rather than spring. During fall, grass is moving nutrients into the roots for winter survival. If herbicide is applied in fall, it will also reach the roots, causing fewer weeds the following spring. For milder weed problems, spot spray weeds in fall rather than covering your lawn in herbicide. If you have weed trouble during the growing season, apply herbicide on rain-free days when temperatures are between 60 and 80 degrees Fahrenheit. Repeat every 20 to 30 days.

Fertilizer Use

• What keeps a lawn healthiest is a strong root system, and overapplying fertilizer can make grass grow fast while sacrificing root growth. David J. Robson, Extension Specialist at the University of Illinois, recommends applying 1 pound of actual nitrogen fertilizer per 1,000 square feet, to allow both tips and roots to thrive. If a green lawn is important, choose a product that contains fast-release and slow-release nitrogen. The fast-release nitrogen is inorganic and works quickly to green your grass. The slow-release comes from organic material and nourishes your grass in the long term. Choose a product with at least 30 percent slow-release fertilizer. To have a clearer picture of your lawn's specific needs, contact your county Extension service for a soil test for nutrients. Select a fertilizer that corresponds to your lawn's needs.

Fertilizer Timing

• Contrary to popular belief, fertilizer also works best when applied in late spring and fall rather than at the beginning of spring. Grass roots grow most actively in temperatures between 55 and 65 degrees F. Weed first, and wait to apply fertilizer until after the roots have stopped growing, whenever the temperature rises above 65 degrees F. Fertilize again in late fall once temperatures drop below 55 degrees F. This will nourish grass during winter dormancy, resulting in a healthier spring lawn. Reapply every eight to 10 weeks.