Taking Care of your Frost-Damaged Trees – Dr. Theunis Smit

With the rapid expansion of the macadamia industry into sometimes more marginal areas, this year’s winter has proven to be rather challenging for most macadamia growers. Not only have growers reported a large number of small trees dying but there have also been some reports on the death of mature macadamia trees. Macadamias, being subtropical crops prefer frost free winters, but can withstand cooler (1-2°C) temperatures during winter months. That being said, the large-scale death of trees during this winter emphasizes the fact that temperatures were cooler than what most of us are accustomed to, and we can’t help wondering why and how our macadamia trees died. 

There are several ways in which frost can kill trees, but the most common of all is the freezing of water within plant tissues. When water freezes it expands and when this expanding water is kept in a confined space the vessel that usually holds this water tends to fail. We have all observed this when forgetting our beer in the freezer overnight and ending up with a mess the next morning.

It is this exact mechanism which causes the death of macadamia trees when exposed to freezing temperatures.One of the macadamia trees’ most well know responses to cold is the burn observed on soft growth during cold winter months. This burn is not usually observed on older flush seeing that older leaves will have significantly less water stored in them compared to younger leaves.

The more extreme consequence of frost is damage caused by the freezing of the tree bark. It is often referred to as the freezing of the stem, but this is highly unlikely as the stem contains a large amount of wood, with fine diameter vessels filled with water. The freezing temperatures required to freeze all the water inside the stem is something that will most likely never be observed in macadamia production areas. Nevertheless, when the bark freezes it will burst open and if the damage is extensive, the tree will most likely die. When trees have this type of frost damage, the trees will die within a couple of days or even weeks following the cold, mainly due to the fact that trees need their bark to move food from the roots to the leaves and the other way around. If the bark is damaged the transport of food ceases to take place and ultimately the tree dies of starvation.

So what do you do when your trees have been damaged by frost?
You have a few options when it comes to managing a frost affected field, of which the easiest and most obvious is to remove the affected trees and plant new trees. Unfortunately, most farmers don’t have the resources to do this and even if they had, the availability of good quality trees is limited. You therefore have a few other options, some of which work better than others depending on the extent of frost damage.

  1. Let the trees re-sprout
  • After damaged has occurred and the tree canopy has died, removed the dead part of the canopy and continue to cut back until you find fresh, green and moist wood.
  • If you have grafted trees, take care not to cut below the graph as these trees will then need to be re-grafted. If the graft has died, the trees can be cut back and after regrowth has taken place the trees can be re-grafted.
  • If you have cuttings, the tree can be cut back up to a point where fresh, green and moist wood is found and can be left to regrow without any need of grafting.

2. Seal wounds and promote root growth:

  • Where trees have only suffered a small amount of damage to the stem, carefully remove the damaged bark and seal the wound as soon as possible.
  • The trees can be sealed with a copper and PVA mix or with specialized sealants such as tree seal.
  • After sealing the wounds, trees can be drenched with kelp containing liquid fertilizers which should promote regrowth and alleviate stress on the damaged trees.

In conclusion, macadamia trees are susceptible to frost and care should be taken when planting in marginal areas as large-scale losses can occur during cold winter months. In general trees which have a small amount of leaf burn during winter should be perfectly fine and with a bit of pruning these trees will be back to normal within a few months. Trees that have extensive damage to the bark will most likely re-sprout from the base of the tree and will grow without any problems. Trees that have re-grown will, in most cases, need to be re-grafted if growers are expected to achieve reasonable yield in the year to come. Taking care or trees grafted in the field requires some skill and growers are advised to make use of a range of experts to guide them through this endeavor.