Pre-emergent weed herbicides prevent grassy and broadleaved weed seeds from germinating in your flowerbeds. Timing is critical when applying pre-emergent herbicide. This herbicide type does not control weed seeds after they have germinated or started growing. Choosing the right pre-emergent herbicide will also prevent damage or harm to your ornamental flowers.
The best time to spray pre-emergent herbicide in the yard depends on the time of year and soil temperature. Two different types of weeds germinate in flowerbeds: cool and warm season. Cool season weeds are controlled in the late summer to early fall, according to the University of California at Davis. Warm season weeds are controlled at the end of winter or in the early spring when the ground has started warming up. However, gardeners must apply pre-emergent herbicides right before ground temperatures favour weed seed germination.
Broadleaved and grassy weeds germinate at different times. For example, broadleaved weeds, such as dandelions, germinate when ground temperatures remain steady at 10 degrees Celsius. Gardeners must spray their flowerbeds when temperatures reach just under 10 degrees Celsius. Grassy weeds, such as crabgrass, germinate when ground temperatures reach 15.6 degrees Celsius, according to the University of Rhode Island. Spray your flowerbeds with a pre-emergent herbicide formulated for grassy weeds when temperatures hit 12.8 degrees Celsius.
Types of Pre-Emergent Herbicide
There are numerous types of pre-emergent herbicides that gardeners can use to prevent broadleaved and grassy weeds from germinating. Pre-emergent herbicides that contain active ingredients, such as DCPA, isoxaben, oryzalin, oxadiazon, oxyfluorfen, napropamide, dithiopyr, pendimethalin and prodiamine, control both weed types. Gardeners can use a pre-emergent herbicide that contains trifluralin to control broadleaved annuals in their lawn and garden areas. Check the label on your pre-emergent herbicide before using it in the flowerbed.
How to Use
Spray your pre-emergent herbicide evenly over your flowerbed. Cover areas that normally do not produce weeds because you can never be sure where weed seeds will germinate. Water the flowerbed area after applying the spray. You can also spray in the morning on a day where afternoon showers are expected. By watering the pre-emergent herbicide, you allow the chemicals to form a barrier over the soil to prevent weed seed germination. Apply the pre-emergent spray again, according to directions.