Recovering from fire

By Simon Newett, Queensland Department of Agriculture and Fisheries

Australia’s fire season has started hard and early, and it has impacted on rural communities, and farms of all descriptions, including avocado orchards.

In the Best Practice Resource you can find information and resources on irrigation, mulching and nutrition. To provide some advice on how to prepare for and recover from fires, we turn to our colleagues in California, who have faced their own fire crises in recent years.

We thank Dr Ben Faber, University of California Cooperative Extension Farm Advisor for sharing his work, and allowing us to slightly alter his advice to fit Australian conditions for both recovery from and preparation for fires. We have linked to his original works throughout.

Image: Simon Newett, QDAF. 2019
Avocado trees written off for dead after the devastating ‘Thomas’ fire in California are showing good recovery 20 months after the fire”

After fires

Growers need to be patient and observant to bring the trees back into production, Dr Faber explains in this November 2019 article (read the original here).

Although injury to foliage and young growth is visible within a few days of the fire, the full extent of the damage may not be known for several months or possibly the next growing season. In the case of severe injury, die-back may continue to occur for several months after the fire. New growth that occurs after the fire may suddenly collapse the following year if there are more adverse conditions.

The important rule to follow after a fire is to do nothing – don’t prune, don’t fertilise and maybe don’t water. Or rather, water very carefully. Dry winds may have sucked the water out of the ground and may need to be replenished. The fire may have burned the irrigation lines and need to be replaced.

In the meantime, if the tree has been defoliated by the fire, it has lost its ability to transpire water. Watering a tree with no leaves will set up those conditions that are conducive to root rot. Until the tree begins to leaf out, watch soil moisture to decide how much water the trees are pulling out of the soil. The emitters should be capped or plugged on some leafless trees. Then as the tree puts on new growth, shallow, infrequent irrigations should start. This may mean replacing mini-sprinklers with drippers if only a portion of the orchard has been burned and the rest of the trees need their usual amounts and frequency of water.

The avocado has a tremendous ability to come back from fire and frost damage. However, the tree will tell you where it is coming back. It will start pushing growth where the tree is still healthy. It may take 3 to 6 months for this growth to occur.

Delay pruning until the tree clearly shows where it is going to regrow. By waiting, you save the expense of having to return sometime later to remove more wood.

An activity the grower can perform is painting exposed limbs with white acrylic paint. The defoliated tree can be further damaged by sunburn after it has lost its protective cover of leaves. The upper surface of horizontal limbs and the north and west sides of exposed trunks are the most affected. The paint can delay the appearance of new growth, but it does not affect total growth. There is usually no value in applying paint to small limbs.

Avocado trees have a great ability to recover after fire damage (see the image at the top of this article taken by Australian growers during the recent industry tour in California). Even trees killed below the bud union will frequently develop into good trees if they are rebudded and given good care. Trees which do not put out vigorous sprouts should be removed. However, you may end up with a very uneven orchard so the stumping or staghorning option described below may be a more practical option. Interplanting avocados would rarely be advisable because of their rapid recovery.

As Dr Faber notes in this article, every fire is different, so experiences vary. It may take several months to be able to observe whether the bark is killed all the way to the cambium or not. Thus, he recommends against pruning until new growth appears to indicate where the wood is alive. According to Bender (click here to read the 2012 paper on fire recovery, and click here to read his sucker grafting article), an alternative method which is rather unique to the avocado industry, the burned trees can all be “stumped” – referred to in Australia as “staghorned” (cut back to the graft) immediately and allowed to re-grow. “Staghorning” is a normal practice in the industry when avocado trees have reached such heights that fruit is high off the ground and picking becomes difficult. In many of the groves that were burned in California, staghorning was probably needed anyway; therefore after a burn staghorning would be a reasonable alternative for many growers. If the goal is to reduce the size of all trees in the irrigation block to a manageable size, then staghorning the block immediately after the fire is the best solution. Under Californian conditions, trees will be out of production for two years and have about 50% production in the third year, and some re-grafting may have to be done, but fertilising and watering properly is manageable.

In Australia, “stumping” is called “stag-horning” and you can read more about that here in the BPR (you will need to be logged in to access this resource).

A third option could be to scaffold all trees in an irrigation block to two to three metres in height. This would get rid of a lot of deadwood immediately, and might allow the trees to come back in production faster than the staghorning the trees.

Before fires

Dr Faber prepared this article earlier this year, based on the 2015 work of Sonia Rios, Henry Herrera and Gary Bender.

These preventive measures can be suggested for groves in areas of high fire hazard are often counter to other measures which might reduce erosion or improve root disease control, so a balance should be strived for (Goodall 1965):

  1. Remove all combustible material (including mulch) from around the trunks of the trees for a distance of up to one metre (two to three feet).
  2. Prune off low-lying limbs, those that are low enough to trap leaf mulch below them.
  3. Remove from the orchard all broken limbs, deadwood and other combustible debris.
  4. Clear brush, trees and other heavy vegetation away from edge of orchard for a distance of at least 15 metres (50 feet). Keep in mind any local regulations regarding fire break widths.
  5. Do not pile brush or other combustible material in gullies, or around the edges of orchards.
  6. Apply sprinkler water for as long a period in advance of the fire as possible so as to have everything wet. Water during the fire would obviously be desirable but often is lacking because of lack of pressure, power or speed of the movement of the fire.
  7. Use steel pipe and risers for above-ground sprinkler systems.

This article has been prepared for the 13 December 2019 edition of Guacamole. We thank Ben Faber for his generous sharing of his work in this area.

Author: Simon Newett, QDAF
Date Published: 13/12/2019