When can I start putting a pre-emergent herbicide such as Preen on my garden beds?

There are two types of chemical weed controls: post-emergent and pre-emergent. Post-emergent herbicide kills weeds that are actively growing. A pre-emergent (Preen being one of these) prevents weed seeds from germinating, but does not kill existing weeds.

Pre-emergents work in the top 1 to 2 inches of the soil by creating a chemical barrier that prevents cell division in developing weed seeds. Different weeds germinate at different times of the year, and pre-emergents last anywhere from 3 to 6 months, depending on the brand you use and local conditions. The rate at which pre-emergents biodegrade in the soil and lose their effectiveness will depend on temperature, soil type, and moisture conditions of the soil. They will need to be reapplied more often in hot, humid conditions.

Depending on the brand, some pre-emergents are applied over the top of mulch, and some are applied to the soil before mulch is applied. To be effective for the spring season, apply pre-emergent in late winter or early spring, before the soil warms up.

While pre-emergents will not harm your plants if they come in contact with them for a short period of time, always wash or brush off stray granules from plant foliage to minimize damage or discoloration that could be caused by long-term direct contact with the chemical. Never use a pre-emergent herbicide in your vegetable garden, and be careful around areas where you intend to sow grass seed. If you spill a little in an area where you intend to plant grass, it will not grow.