Bushfire recovery two years on

Most of the fire-damaged trees were pollarded at five foot high to enable management of the shoots from the ground as they regrew.

In the March 2020 edition we shared the experiences of two South Australian growers whose groves had been impacted by the devastating 2019-20 Cudlee Creek bushfire. The worst affected was Ron and Ethne Baker’s Woodside Farm, with nearly half of the 6400-tree grove heat-effected and 50km of irrigation destroyed.

We followed up in the September 2020 edition, reporting on progress at Woodside Farm nine months after the fire via from observation and discussion during an Olives SA field day. In late November 2021 another field visit was held at the grove, providing an update on progress after another year of growth and grove maintenance.

Grove observations and discussion: November 2021

Overview

At the time of the latest field visit almost two years had passed since the bushfire in late December 2019.

As it was an extremely hot grass fire, not a crown burn, the trees were heat-affected rather than destroyed: the leaves were brown and dead but the trunk and main branches appeared to be okay. It was hoped the damaged trees could be saved so they prioritised getting water back onto the grove. The massive task was achieved in just two weeks thanks to a working bee by fellow Olives SA volunteers.

Knowing the fire would stimulate shooting all over the trees, the Bakers decided to cut all the badly heat-affected trunks off at 5ft (1.5m) to keep the re-growth manageable. From there the plan was for selective re-shooting over two years and hopefully a crop in the third year.

On some of the trees, however, the bark had ‘cooked’ and the intense temperature had damaged the trunk down to ground level. Where no signs of regrowth occurred, these trees were later cut further to a sixinch stump and are being re-established from basal suckers. They weren’t grafted, so there’s no risk that the re-growth is root stock.

Around 400 of the most severely burned trees were also removed immediately and have since been replaced.

Regrowth

Many of the pollarded trees have shot from both the branches and the base, others from the base only, and many of those have also shot from the roots. Of those coppiced at ground level, some have grown their new trunks from the roots. These are stronger, as they’ve grown a full heel, while those grown from the trunk often have weaker joins.

It was noted that the trees which have shot from the base will be the best in the long term, in practice creating a new tree on old, established root-stock.

Overall, most of the damaged trees are doing well and the two-year-old regrowth on some has already created a new, well foliaged (small) tree.

Very few have died, however the regrowth on a small number is very weak, so they will also be pulled and replaced. While it was worth trying to preserve them for a faster new crop, it doesn’t always work, and some trees were obviously more damaged than was outwardly visible.

Varietal differences

In the recovery phase, it’s been noted that Barnea is softer wood, so rots easily. The Mediterranean varieties are very hard and less susceptible to rot.

In the initial stages the Coratina trees survived the fire better, followed by the Frantoio and then the Leccino. The latter weren’t doing particularly well, and it was expected that most of them would eventually die.

With more time, however, the Leccino have shown the best response in terms of

regrowth. It was noted that the variety has really reliable crops, fruiting well nearly every year in the Woodside Farm grove - sometimes a bit less but always good. It’s also a very hardy tree with strong bark, and is therefore highly resistant to pests and disease.

The Coratina continue to do well but the Bakers don’t plan to remove any growth for a number of years. Unlike many varietals which need very good airflow, their experience is that it’s not good to open Coratina up too much.

Wind damage

After two years of TLC and careful selection, there had been wind damage to a number of the chosen new branches shortly before the field day. The new branches had been ripped from the trunks and a new lead branch now had to be established.

This was noted as a reason not to remove all other lead branch options too early, however comment was also made that the wind is taking out the weak growth which would no doubt break later anyway.

Grove maintenance

At this stage of regrowth maintenance of the grove involves a general ‘tidy-up’ of the trees, knocking off long shoots, crossovers and unbalanced growth.

Comment was made that you don’t want the growth to get too big and high too quickly or it will be weak and when the trees fruit in another two years or so, the shaker is likely to rip the branches off.

Crop predictions

The 2021 harvest saw a good crop on the trees that didn’t burn. The coming harvest will see the first fruit on the burned trees: they expect to get “a bit but not much”, while the replants will take several years to fruit.

It was noted that it will be many years until the trees produce at anywhere near the production level they were achieving when burned, and that overall the grove is unlikely to get back to its full pre-bushfire potential.

Harvesting regrowth

Discussion was had around harvesting methods for regrown trees. At this relatively early stage of regrowth, the Woodside Farm trees were deemed too fragile to harvest via shaker but they are expected to progress to the point where they’re strong enough to take it within several years. It was noted that at four inches in diameter the trunks are usually solid enough for successful shaker harvesting without incurring damage.

When they’re at this size, the Bakers will leave the fruit to ripen more than they would

By August 2020, there was strong, healthy regrowth on the majority of the pollarded trees and prolific sprouting on those coppiced at six inches.

have previously and will give them a short, gentle shake. A time delay of four to six weeks later than normal should see a substantially higher abscission rate. They’ll also get a bit of extra oil from the ripe fruit, which will be good to blend with the very green oil from the undamaged trees. Valuable learning The observations from the latest grove visit show that, while there are guidelines and expectations around horticultural recovery practices, trees are living things which OLIVE HARVESTERS ANDPROCESSING LINES don’t always follow the ‘rules’. And with re-growth and cropping now well underway in the Woodside Farm grove, there’s a lot to

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Nearly two years after the bushfire, the pollarded burned trees had lush, strong regrowth and were flowering prolifically.

Sucker solution

One of the biggest management issues from the post-bushfire regrowth is the proliferation of suckers around the base of the trunks. While in some cases a dominant sucker will eventually become the new tree, most are just using valuable energy which is better utilised by the shoot regrowth.

At Woodside Farm there are thousands of rejuvenating trees and tens of thousands of suckers to deal with, so manual control is not considered a viable option. Instead, they’ve put years of experience and experimenting together, and come up with their own tried and true spray for suckering.

At the field day they shared the ‘recipe’ for their spray and also a number of critical elements for success and efficacy, including timing, treatment concentration, type of water and equipment.

Timing: pick your weather – ensure calm conditions to minimise overspray. Spray at the right growth stage: if your suckers are 500mm long, you’re wasting your time. Do it monthly after coppicing or the suckers will get out of control.

Safety: follow all relevant OH&S practices for spraying.

Regulation: Check all labels and ensure that the chemicals you’re using have been registered for that particular purpose – i.e. for removal of suckers on olives. Ensuring the use of only registered chemicals is the responsibility of the grove grower, who is liable should a negative incident occur.

Concentration: Ensure you use chemicals at the recommended rates as per the product label, as different brands are commonly at different concentrations.

Water: good quality water is essential; rainwater is ideal.

Equipment: ensure the equipment you use suits your grove and is in good condition, that the spray is calibrated correctly and the chemical properly agitated. Your spraying equipment doesn’t need to be expensive, just well maintained. Apply using a hand gun from the tractor.

Other comments/notes: • check labels for the lifespan of the spray mix as some will keep for a period of time once mixed, others will deteriorate in the tank and lose their efficacy; a mix made with rainwater will last longer than one made with bore water; • Woodside Farm's spray is specifically for suckers and will work on growth up to 300-400mm high but a full wet-up is critical; • visual impact on the suckers will take 4-5 days; • rainfall makes suckers grow prolifically; irrigation has minimal effect. • spray removal of suckers is a huge labour and time saver. Chemicals are expensive but so is labour, and the process is cost productive if you have a medium to large grove.

Manual works too

While this practice works well at Woodside Farm, not every grower is keen on using herbicides and other chemicals. And for those growing organically, it’s simply not an option. Many of your groves are no doubt also much smaller and suckers can therefore be managed manually.

If that’s your situation, prompt removal of suckers by hand when very young, or with good quality, well-sharpened hand-tools, is a viable alternative. General consensus is that hand removal or grubbing with a mattock is preferable to pruning, as the latter may in fact promote increased sucker growth.

Many of the trunks bear obvious signs of the fire, proof of the resilience of olive trees given the right care post-trauma.

learn from the Bakers’ experiences - and a valuable opportunity for all growers to gain preparatory knowledge should a bushfire impact your grove.

We’ll update on the rejuvenation progress again in the future to see how the ongoing management decisions and practices are working out.

More information

Full details of the initial fire damage and management activity to date at Woodside Farm are available in the previous Olivegrower articles, which appear on pages 14-15 of the March 2020 edition and pages 18-20 of the September 2020 edition.

The March 2020 edition feature on Olives & bushfire also includes a technical article on post-fire care of olive groves, information on the effects of bushfire on soil microbes and discussion around the issue of smoke taint. The September edition provides additional technical information on the recovery of soils following bushfires.

Longer-term subscribers will hopefully still have their print copies of these editions but don’t forget that all subscribers also have 24/7 access to the digital version of the magazine via the OliveBiz website. Just go to www.olivebiz.com – Back issues - and sign in for access to all editions back to June 2012.

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