Managing the orchard floor for optimal soil health

Summary of a presentation given by Charles Merfield, Biological Husbandry Unit (BHU), Lincoln University as part of the winter 2019 AGM/field day weekend

Originally published in the HGA newsletter, August 2019

Merf delivered a fascinating presentation on the key aspects of a healthy soil and how common orchard management practices can destroy soil health.


Merf’s key point was that it’s possible to achieve a healthy orchard and clear floor for harvesting while also maintaining biodiversity in the orchard floor, rather than using herbicides that kill soil microorganisms.

In any orchard, we want to optimise tree performance, maximise soil quality and manage pests and diseases. Most of us are also keen to improve biodiversity. So a management regime needs to take into account tree, soil, pest/disease and biodiversity considerations and allow us to manage them as a system.

Common practice is to use all sorts of chemical inputs – agrichemicals for pest and disease control, herbicides for ‘weed’ control and mineral fertilisers for nutrients. However, there is plenty of evidence that this approach is failing. There has been little innovation in pesticide/herbicide chemistry in the last 30 years and resistance rates are growing exponentially, as is consumer opposition to chemical-heavy production methods.

Healthy soil = healthy trees

If we consider the soil as the foundation of any orchard, it becomes obvious that a healthy orchard can only be achieved with healthy soil. Improving soil health has multiple positives and improves many negatives.

Healthy soils are biologically extremely active. Soil is the most complex ecosystem on the planet and is typically 10 times as complex and contains 10 times the amount of life (weight and species) than the above ground biomass.

A long-term trial by Plant & Food Research over 11 years discovered that soil maintained in a permanent herbicide fallow had higher rates of soil carbon loss and much worse structure than areas left in permanent pasture. Bare soil is also more vulnerable to run-off and erosion.

On the positive side, looking after all this complexity is surprisingly straightforward. Soil needs to be fed, with organic residue such as animal dung and dead animals (freshly dead, living things). The more diverse the ‘fodder’ the healthier the soil will be. Living plants not only contribute leaf fall, but their roots actively move energy and photosynthates into the soil. The outcome is highly complex food webs, especially with involvement from mycorrhizal fungi.

Understorey planting

Although understorey planting offers considerable theoretical potential for soil improvement, there has been very little actual research into what works. A critical issue is the need to balance the competition between the underplanting and the main crop trees, for both nutrients and water. Careful selection of the understorey plants is the key to successful co-existence. For example, blueberry (main crop) is very little affected by the presence of twitch (couch grass) or California thistle. The level of competition is also likely to vary across the season. Trees in their dormant phase are unaffected by potential competitors.

In practical terms, it’s important to balance the benefits of the understorey against the effort involved in establishment, management and maintenance. Over time, perennials require less work (one-time effort to establish, typically mowing to manage during their life). Annuals are more flexible – different species can be deployed at different times to match needs across the season – but require repeated effort to establish and terminate. A combination of both may be best, to accommodate the different needs of midrow and under-tree locations.

Managing the understorey for hazelnuts

A specific requirement in hazelnut orchards is the need for a ‘clean’ flat orchard floor for nut pickup. In one sense, a short dense vegetation cover (ie lawn grass) is ideal, but grasses are extremely competitive and might not be practical to establish as an annual. What if other vegetation could be used to enhance soil throughout the growing season, but be cleared from the orchard in time for harvest? And how could this be achieved without use of herbicides?

Merf discussed electrothermal weed control, a high voltage/low current system which effectively cooks plants from the inside out. It’s systemic (kills the roots), broad-spectrum (kills grasses, broadleaf species and anything else), and plants cannot develop resistance. There’s no possibility of spray drift, and with careful operation it is a highly selective solution for controlling weed species amongst desirable plants. This isn’t new technology – the first patents were issued in the 1890s and a range of machines were brought to market in the 1980s.

Additional benefits of understorey plants

In addition to the soil benefits, understorey plantings can provide biocontrol of pests and diseases. An ecosystem that supports growth of beneficial species allows them to out-compete pest species. For the orchard floor, beneficial plants such as buckwheat, alyssum (Cv Benthamii) and phacelia may provide conservation biocontrol. Obviously, this would need to be tested and customised for each pest and setting. An obvious first pest to attempt control of would be aphids.

There has been less research into the possibilities for biocontrol of diseases, but the little research there has delivered promising results. Lots more research is required to quantify the potential of biocontrol in this area.


  • We need an ag/hort revolution to solve our global challenges.
  • The orchard floor is a nexus of issues.
  • We can potentially improve production while addressing:
    • Soil health
    • Agrichemical use
    • Pest & disease control
    • Biodiversity