Weed Killer Before a Rain

Whether or not your weed killer loses its effectiveness when using before a rainfall, depends on the type of herbicide you apply in your landscape. For instance, pre-emergent herbicide must be watered into the soil to be effective. Post-emergent herbicide loses its ability to absorb into foliage if washed off by the rain. Understanding how these different types of herbicides work helps you choose the best time for your herbicide application.

Pre-emergent weed killer works by creating a barrier in the first 1/8 inch of top soil. Weed seedlings that emerge come in contact with the herbicide and die. In order for pre-emergent herbicide to absorb into the soil, it must be watered in at the time of application or on the morning of a day where afternoon showers are expected. In fact, applying in the morning of a rainy day is an effective way to save money on water bills.

Post-emergent herbicide works by disrupting the weed's ability to produce proteins, photosynthesize or grow roots. This weed killer type needs at least 48 hours of dry weather to absorb into the foliage of plants. If rain falls occur, or the irrigation system is not turned off, chemicals get washed off the plants. In fact, these toxic chemicals end up in runoff, which pollutes drinking water. A large percentage of herbicide is found in runoff if it rains within 24 hours after the herbicide is used, according to Grounds Maintenance Magazine.

Gardeners prevent runoff from occurring by checking their local weather. Avoid using herbicide on windy or rainy days, as suggested by The City of San Diego. When applying herbicide, focus the spray on the plant's foliage and not at the base. Also, refrain from spraying plants with herbicide that are growing near storm drains, rivers or ponds. Store your herbicides in a dry area where they cannot fall over and wash away in the next rain.

Timing is critical when applying herbicides. For instance, gardeners should avoid applying herbicides to water stressed grass. Water stressed grass becomes dehydrated and turn brown when herbicides are applied. Also, refrain from applying herbicide during hot weather conditions. Apply herbicide in the spring and fall months. Because cool- and warm-season weeds pop up at different times of year, both pre-emergent and post-emergent herbicide spray must be applied at least twice a year.