If you seed a new, bare area of soil, as opposed to just over-seeding to thicken the lawn, be advised: Do NOT Disturb the soil AFTER you spray the pre emergent.
So you must calculate the timing of your application with that in mind if the area will be tilled or cultivated before planting.
The other consideration is if you were planning to broadcast the pre-emergent weed killer over the entire lawn area, or just in the bare spots that you need to replant.
QUESTION 1: When Should I Apply Pre Emergent and Reseed?
An alternative would be to wait and observe the area after you sod it, for new crabgrass coming up in the grass and hit it with the pre-emergent at that time.
Which pre-emergent weed killer will be effective and safe for your new sod? Tupersan… (…remainder of paragraph deleted, as it duplicates information from previous questions about this product.)
(Editor’s note: a slit-seeder is a machine with multiple blades that slice into the soil; and it is able to dispense seeds at different rates.)
Pre-emergence weed killers are used before you see the weeds to prevent them from showing up in the garden or lawn. This doesn’t mean the chemicals interfere with germination but rather they stop the formation of new root cells in baby weed plants.
Established plants have nothing to fear, as their root system is already developed and the plant is hearty and healthy. Pre-emergent info indicates that it is the sensitive root tissue of newly germinated seedlings that is killed off, resulting in complete plant death.
What are Pre-Emergent Herbicides?
The chemicals in pre-emergent weed killers are not effective on vegetative buds that sprout from existing roots or rhizomes. They also cannot be used on a prepared grass seedbed because their root stunting action in young plants will also affect sprouting grass.
Perennial weeds develop thick persistent adult roots that re-sprout in spring, which makes them difficult to control with a pre-emergent formula. Annual weeds are in two classes: winter and summer annuals. The timing of a pre-emergence weed killer for each must match the germination period for the variety of weed. Biennial weeds, like dandelions, are not controlled by a pre-emergent because they produce seed that germinates nearly year around.
Without weeds, the seedlings cannot continue to feed and grow and they just die back. This whole process happens at the soil level under the blades and thatch of the grass so you don’t ever have to see the sprouted weeds. Timing, weather, and the type of weeds that are problematic in the garden will dictate the exact formula and application for using pre-emergents.