Unlike most types of garden, your lawn is one you probably never associated with gardening.
However, caring for a lawn can take a lot of work and many (or even more) chemicals to maintain when compared to a flower or vegetable garden.
Herbicides are practically essential for maintaining your lawn, and you’ll likely need to use a combination of different weed killers for them to be truly effective.
But while you may be familiar with terms like pre-emergent or non-selective, you might not be aware that the temperature can have a significant effect on how long it takes for your weed killer to work, not to mention how effectively.
What Is The Best Temperature To Apply Weed Killer?
Temperature ranges for herbicides depend mainly on the type of herbicide was used.
However, no herbicide should be used when the temperature is outside of 40° to 85° degrees Fahrenheit.
Contact (AKA Burndown) Herbicides
Contact weed killers can be some of the most useful post-emergent herbicides, but only when used for spot treatments.
These weed killers work on direct contact, absorbing into the victim plant and killing it down to the roots.
Unfortunately, the method of application means contact herbicides ar probe to evaporation.
Contact herbicides can generally be applied in temperatures between 40° and 60° degrees Fahrenheit but may also be applied in temperatures up to 85° degrees Fahrenheit on an overcast day.
Speaking of overcast, avoid using contact weed killers such as glyphosate-based herbicides when the temperature will dip to 40° degrees Fahrenheit for an extended period, or there’s a combination of cloud cover and lower temperatures.
These cause weeds to become sluggish, which will cause your herbicide to take longer properly absorb and circulate down o the weed’s roots.
Post-Emergent Herbicides
Post-emergent herbicides are designed to tackle existing weeds.
As a result, rather than lingering like pre-emergent formulae, they tend to accomplish their job and vanish.
This means your average post-emergent formula will require a warmer temperature range to be at its most effective.
Except for contact herbicides, post-emergent herbicides work best in a temperature range of 65° to 85° degrees Fahrenheit.
When applied at lower temperatures, these weed killers may become less effective.
Meanwhile, applying a post-emergent to your lawn or garden in temperatures above 85° degrees Fahrenheit can make the herbicide work a little too well, resulting in potential damage to crops or other desirable plants.
Some post-emergent weed killers who become especially volatile in excess heat include 2,4-D, dicamba, and MCPA.
Other post-emergent formulas are affected in the opposite manner, especially ALS, Sencor, or fenoxaprop-based herbicides.
These products can harm crops at temperatures below 60° degrees Fahrenheit, and freezing temperatures can render them ineffective at killing weeds while damaging your crops.
To help ensure maximum effectiveness against weeds and minimal risk to desirable broadleaf plants or crops, try to use post-emergents on a day when the temperature is at least 60° degrees Fahrenheit.
The daytime temperature should not drop below this level for several days to a week, although the nights can be slightly more relaxed.
Pre-Emergent Herbicides
Pre-emergent herbicides do the exact opposite of post-emergent formulas.
These weed killers attach seeds as they germinate and remain active in the soil for up to 5 months. Because they’re soil-based, there’s less risk of evaporation.
Before applying a pre-emergent, stick a thermometer into the ground about 2” inches deep.
For summer weed protection, apply in the spring when the ground temperature is between 65° and 70° Fahrenheit at the time of application and for a few days afterward.
For the second application (to control fall and winter weeds), the ground should remain around 55° degrees Fahrenheit for at least five days.
Some Final Tips
When applying any weed killer, keep an eye on the weather forecast.
Do not apply if the temperature is due to go outside the safe temperature range for the pesticide type you’re planning to use.
You will also want to avoid overcast weather unless you’re applying a contact herbicide in temperatures at or above 80° degrees Fahrenheit where they’ll be at risk of evaporating too fast in the sunlight.
Pay attention to the types of plants near your treatment area, especially broadleaf plants or grass-like food crops (maize, foxtail, wild oat, etc.), as certain herbicides can end up damaging the crops instead of the weeds when temperatures pass outside of a safe range.
Finally, don’t forget that herbicides require time to take effect, and you will need to stick in the proper temperature range until you’ve passed both the rainfast period and the minimal number of days for that product to take effect.