Using perennial plants to reduce herbicide use

Herbs and forage species have been planted under apple trees in a Nelson orchard to establish if they will out-compete weeds and in doing so, reduce the use of herbicide sprays.

Alyssum is a compact perennial flowering plant

The trial, termed Ground cover plants to replace the weed spray strip, is one of 12 projects to gain funding from the Rural Professionals Fund established in 2020 by the Our Land and Water National Science Challenge to support projects that could lead to significant improvements in farming systems and benefit farming communities.

AgFirst, Willisbrook Orchard, New Zealand Apples & Pears Inc (NZAPI) and Plant & Food Research Ltd in Motueka, are all involved in the trial.

AgFirst Motueka consultant, Aimee Lister, says current orchard practice is to use herbicide sprays in the area under trees, to remove competition for nutrients and water from unwanted 'weed' plants.

“This has been a reliable and cost-effective management practice, but leaves the ground bare, susceptible to erosion and with overall reduced soil health,” Aimee says. “Some chemicals have also been found to harm pipfruit trees. This project aims to contribute knowledge to the goal of eliminating the need for herbicides in future.”

The industry has a goal of significantly reducing chemical use by 2050.

“That date is just a target and it is important to look for alternative effective management practices, now,” Aimee says.

The trial began in October on Willisbrook Orchard at Brightwater when seed beds were established and seven different seed species planted under two rows of apple trees – each row 100 metres long.

The advent of new age canopy systems with trees growing in a 2D configuration, as opposed to traditional vase shaped trees, means more light now reaches orchard floors, making it easier for weeds to grow but also for species used in the trial to become established.

“We selected plant species which were deemed to have the best attributes, including that the seeds were readily available, they were low growing, spread easily and were perennial,” says Aimee. “We planted each species separately because we wanted to be able to measure how each performed and how they interacted with the soil and trees.”

The species selected are sheep's burnet, a deep-rooted perennial herb; bird's-foot-trefoil, a member of the pea family; plantain, a herb with a fibrous and coarse root system; strawberry clover, a perennial clover that performs well in hostile conditions; alyssum, a compact perennial flowering plant; chicory, a hardy perennial; and common yarrow, a flowering, low-growing plant.

Nine months is a short timeframe for the trial, but Aimee says it is hoped enough information can be gained to establish if all, or some of the plants, effectively out-compete weeds – reducing the need for sprays without having any detrimental effects on the trees and the fruit they produce.

“We have the support of NZ Apples and Pears Inc (NZAPI) and Rebecca Campbell from Plant & Food Research Ltd who will help with gathering the scientific information needed to understand the species which perform best,” Aimee says.

The research will include sampling leaves and fruit, both from fresh fruit and stored fruit, to find out if the ground cover plants compete for nutrients with the trees, and if there are any implications for increased pest and disease pressures.

Soil samples were taken before the trial began and sampling will be continued throughout the trial period to find out if there are any differences in soil microbiology. Control samples will also be taken from trees and from the soil of orchard rows managed under the traditional weed spray strip system.

The project met with a few initial challenges, including finding machinery to prepare seed beds and plant the seeds.

“We were able to find a vineyard contractor who had a machine which could prepare the seed bed between the trees, but there is very little orchard equipment capable of working under apple or pear trees,” says Aimee. “However, if the trial is successful and the concept widely adopted, then the equipment needed to prepare the seed bed and plant the seeds will no doubt emerge.”

While the focus is to control weeds under trees, the trial will also monitor how the ground cover reduces soil erosion – which will be particularly relevant for orchards on hillsides.

“There has been quite a lot of interest in the trial from the industry and we will be keeping growers up to date with its progress through regular NZAPI field days,” says Aimee.

The trial fits well with AgFirst’s vision to enhance sustainability within its own business and that of its clients, Aimee says.

“The ideal result of the trial would be that at least a couple of the plant species do well and quickly establish ground cover which excludes weeds, but does not compete too much with the pipfruit.

“If we can prove the use of ground cover species works, the practice is easy and cheap to use, without creating additional labour costs, and it reduces expenditure on herbicides, then ultimately we would like to see the practice more widely adopted within the industry.”