There are two types of weed killers that gardeners use 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 herbicide prevents weed seeds from germinating. Mowing prior to applying your pre-emergent weed killer can help the spray make contact with the soil. Tall grass growth 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 their lawn to dry out before mowing.
Post-emergent herbicide kills weeds by being absorbed into their foliage. If you cut grass before applying a post-emergent herbicide, you reduce the surface area on the foliage and less herbicide will become absorbed. By waiting two days before and after to mow, you allow enough surface area on weeds for post-emergent herbicide to become absorbed, as suggested by Grounds Maintenance Magazine. 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, dish soap and citrus extract. Mowing right after using a weed killer like citrus extract or vinegar can spread these weed killers over the lawn. Also, applying lawn clippings that have been sprayed with organic weed killer can hurt your lawn grass. If you apply an organic weed killer to an area, wait to mow that area. Typically, these weed killers lose their effectiveness in 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 bare areas. These bare areas are the perfect breeding grounds for weed growth. To help prevent weed growth, keep you lawn 3 inches high, as recommended by Cornell University. This means cut your grass when it has reached 3-1/3 inches. Avoid ever taking off more than 1/3 of the grass blade.