Is it really better to spread out pre-emergent before it rains?

Question

Why is this or isnt it?

Answer 1

Preemergent herbicides are soil applied and mechanically mixed with the soil OR are irrigated into the soil before the weeds emerge. They are effective against germinating seeds before they emerge from the soil. Make sure you get proper coverage when spraying the soil to prevent any escapes.

These kinds of herbicides work best if they are applied when soil moisture is adequate for proper seed development. Preemergent herbicides are effective against germinating seeds, NOT dry seeds.

By spraying OR applying it in granular form, they usually require a means to move slightly into the soil. In most cases, this is done through irrigation or by rain. Not sure what kind of pre-emergent you're talking about but keep in mind that some of them are broken down quite rapidly by Ultra-violet light and must be moved into the soil within 7 to 21 days after application (depending on the product). You usually need about 1/2 inch of water to properly incorporate the herbicide into the soil after an application.

When there's too much water or too much rain after treatment, you have to be concerned about movement of the chemical into unwanted areas. Many times the chemical will bind onto the soil after heavy rains but some will move with the water and cause damage.

Hope this answered your question and better understand how pre-emergents work. GOOD LUCK!

-Certified Professional Crop Consultant with over 30 years of experience and a Degree in Plant Science

Answer 2

Ideally you want about 1/2" of water to spread and activate it. If you're expecting severe rains, you may want to hold off. If you don't get at least 1/2" of rain, turn the sprinklers on and set out some tuna cans to measure how much water you get on it.

Answer 3

The rain is what activates it. If you spread on a day when it isn't going to rain, it needs to be watered in. I've done it DURING a rainstorm!

editing: #3, A pre-emergent is not an herbicide. It's a large dose of nitrogen, usually from corn gluten meal, that simply inhibits seeds from sprouting. It doesn't kill anything.

Answer 4

needs to be watered in within 3 days of applying, would try to avoid heavy rain as it could dilute chemicals. Whatever you do not disturb surface as this will create a hole for the weeds to come through