Best time of day to spray weed killer or risk killing plants

As winter approaches, you might have overlooked the growth of grass in your garden. Neglecting to mow your lawn during the colder months may seem like less of a task, but it can provide an opening for weeds to dominate.

This could result in many plants in your garden being deprived of vital resources such as sunlight, water, and other essential nutrients. This deprivation could weaken the plants and potentially lead to them dying off.

Weeds typically proliferate throughout early autumn, but you can combat them during this period with an effective weed killer, ensuring your garden remains in top condition for the winter season. However, to maximise the efficiency of the weed killer, you should consider the timing and weather conditions when applying the product.

How does weed killer function?

Weed killers are usually available as a spray or liquid and utilise a blend of powerful chemicals specifically formulated to either inhibit weed growth initially (pre-emergence) or eradicate them once they've started growing (post-emergence).

Your choice will entirely depend on whether you can already see weeds sprouting in your garden or if you aim to prevent the issue from the outset. If you haven't applied a pre-emergence weed killer to your lawn by this time of year, you likely already have some weeds causing issues, in which case, you will need a weed killer that targets eliminating weeds that have already started growing.

Best time of the day to use weed killer

If you've got a bottle of weed killer ready to go, it might be tempting to dash out to the garden and start spraying without a second thought. However, it's crucial to consider the weather conditions as they can impact the effectiveness of some weed killers.

For instance, if you apply it in the late morning or afternoon when there's more sunlight and heat, this could diminish the potency of the spray due to evaporation before absorption.

Moreover, if you use the weed killer on a windy day, you risk inadvertently spraying the potent chemicals onto other plants, potentially causing them to wither and die along with the weeds. One study investigating the use of glyphosate, a common weed killer product, on giant ragweed discovered that it was least effective when applied at 6am and 9pm.

However, the research found that the chemical was significantly more effective when sprayed between 9am and 6pm. Similarly, a Cambridge study carried out in 2018 and 2019 revealed that weed killers such as glyphosate and dicamba were most effective from midday to one hour before sunset.

Bear in mind, though, that different weed killer products may be specifically formulated to be used at certain times and under certain conditions. Generally, the product's packaging will provide information on the recommended usage times.