Some weed killers work best when applied at certain times of day, but for others timing has little or no effect. Herbicides are post-emergent or pre-emergent. Post-emergent herbicides control existing weeds, and they can be systemic or non-systemic. Systemic herbicides work by moving through plants to their roots, and they're most effective when the plants are actively growing. Glyphosate is a commonly used systemic herbicide. Non-systemic herbicides damage plant tissues, and they can be applied at any time of day.
Pre-emergent herbicides are also effective when applied at any time of day. These herbicides create a chemical barrier in the soil that prevents weeds from appearing.
Systemic weed killers work best during the growing season, and they don't work well when weeds have stopped growing and become dormant.
Wear long pants, a long-sleeved shirt, gloves and safety goggles when using herbicides, and apply them on a dry, still day. Many herbicides are harmful to plants you wish to keep, so either don't spray near them, or protect them with cardboard.
Applying Systemic Herbicides
Applying systemic herbicides at the peak plant growth times during the day increases the herbicides' effectiveness. Plant growth speeds up and slows down over the course of a day, depending on light level, temperature and the local climate.
Early morning and late afternoon are the best times for applying systemic herbicides in summer in warm climates, such as U.S. Department of Agriculture plant hardiness zone 9 and higher. Don't apply herbicides around midday or early afternoon in summer. Midday heat causes plant growth to slow down, and the herbicides quickly dry out on the leaves, which reduces the amount of herbicide weeds absorb.
In winter in warm climates, weeds often continue to grow. At this time of year the best time of day to apply systemic herbicides is the middle of the day, when the temperatures are highest.
Cool or Cold Climates
Systemic herbicides work best when applied late morning, midday and in the afternoon in cool or cold climates. Plant growth slows down at dusk and speeds up again as the sun rises the following day. Heavy early morning dew can cause herbicides to run off, so it's best apply them after dew has evaporated.
Applying Herbicides for Broadleaved Weeds
Systemic lawn herbicides for broadleaved weeds work best within certain temperature ranges. Herbicides for broadleaved weeds control many lawn weeds but usually don't harm grass. When temperatures are too low, these herbicides may not be effective. When temperatures are too high, they can damage turf grass. During cold or hot times of year, wait until the daytime temperature is within the correct range before applying lawn herbicides for broadleaved weeds.
Dicamba, mecoprop (MCPP) and 2,4-D are some common herbicides for broadleaved weeds. These work best at temperatures between 50 and 85 degrees Fahrenheit. In cool or cold climates in spring or fall, wait until the warmest part of the day to apply them. In warm climates in summer, avoid midday heat, and apply the herbicides in the morning or late afternoon.
A graduate of Leeds University, Jenny Green completed Master of Arts in English literature in 1998 and has been writing about travel, gardening, science and pets since 2007. Green's work appears in Diva, Whole Life Times, Listverse, Earthtimes, Lamplight, Stupefying Stories and other websites and magazines.