Greg writes
Princep Liquid Herbicide is a pre-emergent that will prevent weeds from germinating in the soil and will last about 4-6 months in the soil. It will not kill any actively growing weeds. Timing is always key when applying pre-emergents and will vary by region and the best person to ask about application timing is your local master gardener who works at your local cooperative extension office. There is not a specific temperature but if it gets close to 85-90 degrees it is possible that those seeds have already germinated.
Answer last updated on: 02/21/2018