Biological clock of plants could help gardeners use less weed killer

Weeds may be easier to kill at dawn
Weeds may be easier to kill at dawn Credit: AFP

Gardeners may be able to cut down on the amount of weed killer they use by tackling invaders at specific times of day, such as dawn, a new study suggests.

Researchers at the University of Bristol have discovered that the circadian rhythms of plants leave them more vulnerable to herbicides at different points in their daily 24 hour cycle.

They team tested Arabidopsis plants and found they were more sensitive to glyphosate herbicide around dawn, when a lower concentration was needed to kill them off.

A second species called proso millet (Panicum miliaceum) was more also more sensitive to glyphosate at sunrise and dusk, but not in the middle of the day.

They believe the findings could help farmers by reducing crop loss and improving harvests. And gardeners could benefit from knowing which weeds respond better at certain times of day.

In recent years, pesticides and herbicides have been implicated in the decline of important pollinating insects, such as bees.

Dr Antony Dodd, Senior Lecturer  in the School of Biological Sciences and senior author of the new study, said: “This proof of concept research suggests that, in future, we might be able to refine the use of some chemicals that are used in agriculture by taking advantage of the biological clock in plants. 

“Approaches of this type, combining biotechnology with precision agriculture, can provide economic and environmental benefits.

“It could certainly be useful for gardeners in future, if we knew about the daily rhythms in the responses of certain weeds. It could reduce the amount of herbicide needed or increase its effectiveness.”

Watering plants is more effective in the morning because their pores are open Credit: Betsie Van Der Meer Taxi

Just like humans, plants have evolved to take advantage of cycles of night and day, with certain biological processes turning on at different times of day.  

Scientists have discovered that many drugs work much better in humans if they are given at specific hours. Aspirin, for example has double the impact on thinning the blood if taken at night as opposed to in the morning. 

The process is known as ‘chronotherapy’ and researchers wanted to find out if the same concept could be applied to plants. 

Many gardeners already know that plants drink in more water in the morning because their stomata, or pores, are open to take advantage of early morning dew and water vapour. At the break of day, plants are also not busy producing food through photosynthesis. 

The open pores could also be the reasons that chemicals are more effective at dawn and also at dusk.  

The air is also likely to be stiller at dawn and dusk, meaning that pesticides or herbicides are less likely to blow away to places where they are now wanted. Pesticide labels often warn against spraying on windy days in case they endanger people or animals.

Many insects are active early in the morning and around dusk, also making very early morning and early evening effective times for insecticide.

A recent study from Oregon State University found that gardeners got rid of more pests spraying less insecticide in the evening than at other times of day. 

Dr Dodd added: “A big question for the future would be of the extent to which the findings might scale to field, or farm, garden environments, and which species this might apply to, and whether there’s timing differences between weed species. 

“The work is a proof of concept that opens a lot of fascinating questions for the future.” 

Commenting on the study, Dr Trevor Dines, Botanical Specialist at the conservation charity Plantlife, said: “This is fascinating research which will be of great interest to many gardeners like me. I suspect I'm like many others and wasn't aware of the effect of the botanical circadian rhythm on the potency of herbicides and pesticides. 

“If anything I thought the opposite was true - that applying herbicides and pesticides late in the day would be better as they'd remain in 'wet contact' with the plant for longer in the cool of the night and therefore be taken up or absorbed more effectively. This research knocks that assumption on its head.”

“Any research like this that allows us to use herbicides and pesticides more efficiently and in smaller quantities is incredibly valuable, both economically and environmentally.”

The study was published in the journal Nature Communications.