Gardeners use two types of weed killer on their lawns -- pre-emergent and post-emergent. Both herbicides must be applied correctly for optimal weed control. How soon you can mow after herbicide application depends on the type of weed killer you use. Mowing at the wrong time can cause your weed killer to lose its effectiveness.
Pre-emergent weed killer
Pre-emergent herbicide prevents weed seeds from germinating. Mowing before applying the pre-emergent weed killer can help the spray make contact with the soil. Tall grass blocks the spray from hitting the ground where it will create a barrier between the weed seeds and soil. Also, pre-emergent herbicide must be watered in after its application for it to be activated and effective. Therefore, gardeners must wait for the lawn to dry out before mowing.
Post-emergent weed killer
Post-emergent herbicide kills weeds through foliage absorption. If you cut grass before applying a post-emergent herbicide, you reduce the surface area on the foliage and less herbicide will be absorbed. By waiting two days before and after mowing, you allow enough surface area on weeds for proper absorption. Once the post-emergent spray has killed off the grass, mow your lawn to its recommended growing height.
Organic weed killer
Organic weed killers are becoming more widely used due to their low-toxicity levels. Types of organic weed killers include hot water, vinegar and citrus extract. Mowing straight after using a weed killer like citrus extract or vinegar can spread the weed killers over the lawn. Also, applying lawn clippings that have been sprayed with organic weed killer can damage your lawn grass. If you apply an organic weed killer, wait before you mow that area. Typically, these weed killers lose their effectiveness after 24 hours.
Incorrect mowing can cause weeds. Mowing lower than the recommended height for your grass causes stress. Stressed out lawns thin out or produce patchy areas, which are the perfect breeding grounds for weed growth. To help prevent weed growth, keep you lawn 7.5 cm (3 inches) high. This means cut your grass when it has reached just above this height. Avoid ever taking off more than a third of the grass blade.