Trial name: Multispecies cover crops in subtropical horticultural plantations
Funding body: Department of Agriculture, Fisheries and Forestry
Farmer: Tom Silver
Main Crops: Avocados, Macadamias
Farm name: Laurel Park Avocados and Macadamias
Laurel Park Avocados and Macadamias is situated on the Alstonville Plateau, halfway between Ballina and Lismore. It covers roughly 15 hectares (nearly 40 acres) of land on red Ferrosol. The farm has 2000 avocado trees, both Hass and Shephard, 500 macadamias and 140 beehives. Owner Tom Silver completed a Forestry degree at Southern Cross University before taking over the family farm in 2000.
Laurel Park became a research site for the University’s trial of ‘multispecies cover crops in subtropical horticultural plantations’ in 2021.
The first cover crop was planted in the interrow in June that year and included: buckwheat, sunflower, tillage radish, field pea, rye corn, vetch, mustard, canola and chicory.
The second cover crop consisting of sunflower, millet, cowpea, tillage radish and buckwheat was planted in November 2021.
The Laurel Park avocado yield is normally between 50 and 100 tonnes per year across the whole farm, but heavy rainfall and flooding in February and March 2022 destroyed one of Tom’s avocado orchards.
Laurel Park has both conventional orchards and high-density planting, which is being trialled alongside the cover cropping.
“Where we used to have one tree, we now have three. It used to be about 54 square metres per tree and now we’re working on about 18 square meters a tree,” Tom said.
The floods which destroyed Lismore in February and again in March 2022 (over 3m of rainfall) resulted in a loss of 560 avocado trees.
The main issue on the farm prior to this was Phytophthora. “There’s good evidence that more organic matter will fight that. So, anything that involves more organic matter is good,” Tom said.
Sourcing the seed was also tricky due to the floods. “I think all of the rural stores were just a bit messed up…” Tom said. “I think they actually had stuff that floated away.”
When asked what he had to do to prepare for the cover crop trial, Tom said: “Very little, besides providing the land”. He found they got a much better strike rate and less competition from weeds when they tilled the interrow. “At certain times of year, it’s very hard to supress kikuyu and not just weeds, but other perennial groundcovers. We got much better results from tilling.”
The winter cover crop was planted in June 2021 and the summer cover crop in November. This crop was maintained until about March 2022. They’ve now shifted to a spring sowing rather than winter.
Dr Karina Griffin took soil samples at the beginning, middle and end of each trial.
Tom also planted a mustard crop on the paddock next door but had to remove it because the bees were preferencing the mustard and not pollinating the avocados.
While it is too early to have any solid data on the effect of the multispecies cover crops, Tom has made a few observations:
> There’s a lot less soil compaction in the cover-cropped areas, which is a good thing.
> He noticed a lot of bee and assassin bug activity when the cover crops are in place (assassin bugs are beneficial insects that prey on pest insects).
> In terms of the soil sampling data, no solid conclusions can be made just yet. “There have been some differences in the cover cropped areas versus the control, but not large differences over a significant time period, so you can’t make any management decisions based on the soil health test,” Tom said, “But you can watch it over time and see what’s happening.”
Pest and disease management
Across the farm the trees are monitored and sprayed (on a hotspot basis only) and managed by pruning.
“Fruit spotting bugs are the biggest issue in avocados,” Tom said.
“That’s what causes the nuggets under the skin. Everyone hates it. The chemistry has improved from the old days of broad-spectrum insecticides. At this stage we’re still having to use it – we monitor and do the hotspots only – but we’re still spraying.
“Fruit spotting bug is an Australian native insect. It tends to like bush; it actually likes macadamias and doesn’t mind camphor laurel. Any sort of rainforest area which provides a year-round habitat.
“In terms of how we manage chemical use, it’s almost impossible to be an organic avocado producer in this area on any scale. With 3000mm of rainfall this year, it’s not so much about insects but more about fungicide.
“We use a combination of cultural and chemical management, as well as good vigorous trees. Cultural in terms of good drainage, good set up, lots of mulching, and that’s where the interrow cover cropping comes into it. If you’ve got more organic matter, you’ve got better drained soil and that’s going to allow all that water to get away before Phytophthora flares up. Phytophthora flares up where it rains a lot. They describe it as a water borne mould.”
As Dr Griffin explains: “It actively swims in water; it’s got little flagella that make it swim along so it will seek roots.”
From an entomology point of view, there’s not enough evidence yet to prove cover cropping has a positive impact. The trial’s entomologist, Dr Christopher Carr, only has one year’s worth of data, which is not currently enough to draw any conclusions.
When it comes to successful avocado farming, it’s all about the three Ds – drainage, drainage, and drainage.
“We go very big on the mounds. Mounding is incredibly important with avocados,” Tom said.
“We’re trying to do this high-density planting, bigger mounds suit the machinery we’re trying to run, but also, just to get the roots out of the wet ground.
“Avocados are quite pedantic about what they like. They like it moist, but not saturated, and we had three to four months of saturation.”
While Tom’s high-density planting leads to good yields, it does require more management and a specific variety of root stock.
“Those varieties don’t ‘eat very well’, but they grow really good roots, so you plant the seed from that tree and then you graft the Hass on top of that,” Tom said.
“In terms of root stock, I’m using two that my nursery guy has found. He calls one Rosebank and one Whian Whian but they’re not industry known. Velvic is probably the most desirable.”
Tom has enjoyed the University’s trial process, saying it “makes a lot of sense” given the research that has been done into avocados and organic matter.
“My joke is when you think you’ve put enough organic matter on, put some more on. You just can’t have enough; you see the soil and the trees respond to it,” he said.
“Organic matter gives you really good drainage in the profile, but you also want to have the trees on mounds. You also want to re-direct water around rather than through the orchard.
“What we’re about to do is sub-soil drain the whole block, put trenches in, put gravel and slotted pipe in. We’ve only done it on one block previously – it’s incredibly expensive but it’s working. It’s a just a very, very big capital outlay, probably in the vicinity of $40,000 to do just one block.
“Even just mulch for that block, I’m looking at about $20,000. Woodchip a few years ago was just a by-product that no one wanted. I prefer to put our woodchip on in a fairly un-composted form, just because it lasts longer. It’s all time and money.”