When Is the Best Time to Spray Weed Killer?

When Is the Best Time to Spray Weed Killer?

Weeds represent a common nuisance to gardeners all over the world. Weeds can interfere with the growth cycle of foliage and crops in addition to being unsightly and giving your garden an unkempt appearance. Herbicides are powerful weapons in the fight against weeds, but when is the best time to spray weed killer, and the perfect season to administer the chemicals?

Weed killers should be applied when the weeds are preferably dry and active during the middle of the day. Spring and autumn are the best times of year to apply weed killers to ensure the weeds are consistently destroyed before the peak growing times during the year. 

Applying weed killer properly is crucially important to ensure that the stray plants are effectively killed. Read on to discover when the optimal application times for weed killer are best, and to find out how you can keep your garden or lawn weed-free throughout the year. 

An Overview of Herbicides

A weed killer is a term that is used to describe herbicides, which are a range of various chemicals that effectively kill plants by interfering with the growing cycle. Herbicides can be broken down into two different varieties: pre-emergent and post-emergent. 

Pre-emergent herbicides are designed to be administered into the soil of an area you wish to keep free of weeds. While post-emergent herbicides are the most common and administered as a spray directly on the weeds. Both types of herbicide work well, yet there are some considerations to keep in mind to ensure the efficacy of weed removal. 

Since weed killer is a chemical formulation, and weeds can be helped to survive based upon the surrounding environmental conditions, it is important to keep the following in mind when using herbicides:

  • Avoid excessive moisture. Herbicides cannot work properly if there is abundant moisture in the weeds. Avoid administering weed killer in the early morning when dew is present and wait to apply herbicides within 24 hours before or after expected rainfall. 
  • Monitor surrounding wind speed. High winds can also cause the chemical formulation to blow away and off of the weeds, therefore, be sure to apply weed killer when the atmospheric conditions are calm. Wind can also cause the herbicides to blow to other plants around the garden and lawn and cause harm to foliage or crops.
  • High and low outside temperatures. If the outside temperature is too hot or freezing cold, this can interfere with the herbicide and dry the formulation out or cause it to freeze, rending it useless. 

When Is the Best Time of Day to Administer Weed Killer?

According to the International Journal of Agronomy, weeds are most active during the daylight hours when photosynthesis takes place, which means it makes the most sense to apply weed killer when the weeds are converting sunlight into energy. Peak times for photosynthesis typically occur during mid-day in the time between early afternoon and evening. 

It is not recommended to administer herbicides early in the morning due to the high levels of moisture that has accumulated on the weeds. This moisture can effectively dilute the chemicals and render them weak or useless depending on how much moisture is present. The one caveat here is if you are applying weed killer during the hot summer months.

In this circumstance, weed killers should be applied in the early morning and early evening hours. This is because weeds will not grow actively during hot temperatures, therefore, the herbicides will not be properly absorbed into the weeds. During winter, the reverse scenario is in effect; weeds will photosynthesize during the warmest time of day, typically during the middle of the day. 


When Is the Best Time of Year to Administer Weed Killer?

As equally important as the time time of day to apply weed killer is also the time of year. Each season presents advantages and disadvantages to weed killer application. Let’s take a look at each season. 


The spring season is undoubtedly the best season of the year to apply herbicides. Spring is a temperate season that starts off with cool weather and slowly transitions to the hot summer months. Ideally, you can administer pre-emergent herbicides in the early spring to stop the weeds from germinating, which can then be followed up with post-emergent herbicides throughout the rest of the season to effectively destroy the weed life cycle. 


For maximum results, it is essential to apply weed killers during the spring to knock down the increased weed growth that can occur during the summer. Summer is not ideal for herbicide applications since the intense heat radiated by the sun can quickly dry out the herbicides. 


Like spring, autumn is a transitional season and the conditions for applying weed killers are ideal. Weeds are slowly losing sunlight during the fall, therefore, herbicides can attack the weakened system of the weeds and also work well to keep the weeds inactive during the winter. 


The winter months can be something of an easy target for effectively killing weeds. With this in mind, the application of a weed killer needs to be timed right to avoid the possibility of freezing overnight. Always apply herbicides during the middle of the day when the sun is shining strongly to disrupt photosynthesis. Active spraying during the winter can also ensure that the weeds are not allowed to germinate properly before the arrival of spring. 


A weed killer is an effective tool in ensuring that weeds are destroyed throughout the year. The time of day and year are important considerations to keep in mind when applying herbicides. Spring is the best season to apply weed killer, whereas summer is less ideal. 

Remember to apply weed killer when the sun is shining brightest to disrupt the photosynthesis cycle in weeds unless you are applying herbicides in the summer. With these tips in mind, you can rest assured that the weed cycle will be kept to a minimum and your plants can flourish in solitude.

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1 comment

Thanks for a brilliant article. I had been wondering when to spray, I now know how and when.
Many thanks again.
Cornwall UK.

Brian John Trotman

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