If it rains after you apply the treatment , will it still work? Yes, absolutely! Moisture is needed to activate the preemergents, so rain or snow following the 1st app is beneficial, not detrimental. In fact, the longer preemergent remains on soil surface without being activated by moisture, the more it will evaporate into atmosphere. Once activated with moisture, it is no longer susceptible to atmospheric breakdown. Also, the fertilizers we apply also are root absorbed, so moisture helps there as well. The only issue a heavy rain may cause with first app is reduced post emergent weed control. However, we use weed control products that are both leaf and root absorbed and a “sticker” to help hold herbicides on the foliage so the foliar portions can be absorbed. Therefore, the ideal situation is for a rainfall to occur after the treatment has been down for one hour. Thanks for reading this. Hope it helps!