How Long After Using Weed Killer Can I Fertilize a Lawn?

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When the goal is an expanse of healthy lawn, a routine of weeding, weed killers and fertilizers keeps the lawn lush and green. While there are a multitude of lawn care products available, each is applicable to specific weeds and grasses. Using the wrong product at the wrong time can be disastrous to your lawn. Timing is key when you use a weed killer and fertilize a lawn.


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Weed killers and lawn fertilizers may be used within a few hours or weeks apart, depending on the product. Always follow the package directions to ensure best results.

Weed Killers and Lawns

Weed killers are broken into two general categories: pre-emergent and post-emergent. Pre-emergent products are applied to prevent weed seeds from germinating in spring and fall. While this is an effective method of preventing weeds, it also means that if you're planning a new lawn or renovating a tired lawn, you shouldn't sow grass seed for up to four weeks after applying the weed killer, depending on the product. Since starter fertilizers are applied at the same time or shortly after planting the grass seed, refrain from applying the fertilizer until it's time to plant the new lawn.


Post-emergent weed killers are formulated to kill existing broadleaf or grass-type weeds. Whether a homemade vinegar and salt solution or a commercial product, most weed killer applications should be targeted efforts when the weeds are actively growing, preferably in fall. In addition, weed killers can adversely affect desirable plants, shrubs and trees, so follow the directions carefully to avoid killing more than just the weeds.


Some non-selective herbicides, such as glyphosate products, may be used to spot treat perennial weeds, although they'll also kill grass and ornamental plants. Applied as a spray onto the weed's leaves, results may be seen in a few hours. While watering and rain will not affect the treatment after 10 minutes, wait at least a few hours before fertilizing and watering.


Lawn Fertilizers and Timing

Fertilizing your lawn is a matter of timing and need. Before applying fertilizers, do a soil test to determine what nutrients, micronutrients and other elements should be added to ensure a healthy lawn. Lawn fertilizers contain mostly nitrogen. Select a product that contains quick-acting as well as slow-release nitrogen to green the grass quickly and then keep it green throughout the growing season.


Fertilize grasses when they are actively growing. Water thoroughly the day before you fertilize and then apply no more than 1 pound of actual nitrogen per 1,000 square feet of lawn. When in doubt, apply less, not more. Apply fertilizer two to four times per year at the recommended rate indicated by a soil test.

If you prefer organic methods, use an organic lawn fertilizer or rake 1/4 to 1/2 inch of compost or well-decomposed manure over the lawn in spring. Always leave the grass clippings on the lawn to decompose after mowing.


Alternatives to Chemical Treatments

Practice cultural controls to reduce the need for weed killers. A well-watered lawn that is mowed regularly at the correct height for the species will crowd out many weeds. In addition, mowing removes the flowers of many weeds, thus reducing the number of seeds scattered by wind and wildlife.


Pulling up weeds as soon as they appear also helps prevent their spread through the lawn and garden. Alternatively, allowing a few nongrass ground covers to grow in the lawn, such as Dutch white clover (​Trifolium repens​), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10, improves the health of your grass and provides nectar for bees and butterflies. Clover fixes nitrogen in the soil, making it possible to apply less fertilizer throughout the year.


Safety Precautions and Lawn Chemicals

Before preparing and applying any herbicides or fertilizers, put on a mask, safety goggles, gloves, long sleeves, long pants and closed-toe shoes. Whether mixing a solution or filling and setting the lawn spreader, follow the directions exactly.

Do not apply fertilizers to warm-season grasses in early spring when they're just beginning to revive and green up after winter's cold temperatures. In addition, weed killers and fertilizers should not be applied when temperatures are above 85 degrees Fahrenheit, when the wind is blowing or when the lawn is drought-stressed. Mow two or three days before applying lawn chemicals, water according to the instructions and then wait three or four days after the treatment before mowing again.

After applying the weed killer or fertilizer, sweep any remaining granules back onto the grass. Do not wash chemicals into storm drains. Herbicides can harm wetlands, and excess nitrogen promotes the overgrowth of algae. When the algae die and decompose, it results in a lack of oxygen in ponds, lakes, waterways and the ocean.