Orchard Outlook: Drought FAQ Part 2 - Single Tree Replacements and Unharvested Fruit

I am posting a collection of frequently asked questions to help as you consider management decisions going into next season. There are many questions, and to keep the information digestible I’m breaking it into several parts. This article is Part 2 with answers to three questions. For Part 1, visit Part 1 Fall Weed Control and Fall Pruning.

Quicklinks:

Q1 I’m planning for tree replacement, so how do I know when a drought-affected tree is likely to die?

Q2 Does it always make sense to replant individual trees?

Q3 Is there any effect on tree health by leaving unharvested fruit on trees to overwinter?

I'm planning for tree replacement, so how do I know when a drought-affected tree is likely to die?

My theory is trees defoliated early in August/September from drought stress are at high risk of dieback next year. The defoliated trees were the most affected by the lack of water such that they shed their leaves to conserve what water was left. If some of these trees reached a critical water status, air bubbles could have broken the continuous chain of water molecules known as the water column. Water can no longer flow from the root to beyond the broken water column, and plant tissue above the breakpoint is expected to die. The outcome may vary from branch failure and treetop failure to tree collapse.

We can speculate, but we will only know for sure which trees are dead by next year at bud break to bloom. Trees can often survive on their final reserves until they kick into production to supply new shoot growth. For now, document your worst affected areas on a map to help with future interpretation. Good work mapping and being proactive about estimating replacement trees.

If you’re planning for tree replacement in a new planting or young block, consider that even if severely affected trees survive, they are weakened and could have long term health issues. If you have the opportunity and finances to replace the questionable trees early in the orchard life, then it will be best in the long run for achieving early, max yields. Replacing trees later in the orchard life has less impact, as I discuss in the next answer.

References:
https://link.springer.com/article/10.1007/s00271-025-01017-w

Does it always make sense to replant individual trees?

No, it does not always make sense to replant individual trees. I know it looks better to fill in the gaps, but it might not always be fruitful. Single tree replacements have the best chance of success the earlier in the orchard lifetime they are replaced. As the orchard ages and the canopy fills in - causing shade - the single tree replacements can struggle to succeed.

Have you ever replaced a tree in an orchard only to find that it didn’t reach full production size? I think everyone has. That’s because in highly shaded situations, it doesn’t matter how much you care for a tree, it is horticulturally impossible to grow that tree. High density systems have a lot of internal shading from within and between tree rows.

Consider the age of the orchard. A young orchard has gaps of sunlight exposure throughout the day and replanting might be a good option. An older orchard with tree canopy coverage that shades the orchard floor more than 75% at midday creates a low chance of success for single tree replacements. Another source suggests a low chance of success at even 60% shading. In an older orchard that has reached its maximum height, tree rows cast a shadow at the base of the neighbouring row for a portion of the day. A single tree replacement is short and would struggle to grow in the shadow being cast.

Then consider if it’s practical from a management perspective. For single tree replacements to succeed, the soil needs to be prepared, trees need to be hand fertilized, trained, and pruned, and fruit or flowers removed. If a residual herbicide like Alion, Chateau, or Authority was applied in recent years, then the soil should be replaced for the replanted tree. The investment in the single tree might be greater than the potential return on investment, depending on the situation.

More commonly known, the orchard lifespan also limits the feasibility of replants. General orchard decline means that old age or unproductive blocks have a limited lifespan. Single tree replacements wouldn’t become productive by the time the orchard is replaced anyway.

The best approach for shaded and older high-density orchard is to let existing trees grow to fill in single tree gaps. Modify pruning practices to let neighbouring trees take over the gap.

If you replant in a young non-bearing orchard, you should have a good opportunity for success. However, consider the following:

• It is critical to watch the bloom time of newly planted trees in relation to fire blight risk.
• Remove roots when removing the dead tree and excavate the area to loosen the soil.
• Rebuild the soil berm so the tree is not planted in a sunken area where water pools.

It may be common to fill in the gaps, but let’s pause and consider whether it’s worth the expense of time and money. How much sunlight is available to the new tree? If we look at the shadow cast by the canopy, it should be no more than 60-75% ground coverage at midday. Young orchard is the most promising candidate for single tree replacements. Don’t over-invest time and resources where returns are not expected.

References:
https://www.sacvalleyorchards.com/walnuts/orchard-development/replanting-individual-trees/
https://www.sacvalleyorchards.com/walnuts/cost-and-expense-considerations/replanting/

Is there any effect on tree health by leaving unharvested fruit on trees to overwinter?

I’m coming up short on answers to this question. I’ve reached out to contacts from other regions and so far there is no strong experience with this question. Some of the leftover fruit will fall off in the wind or rot and fall off. However, varieties like Gala have a tough stem and will not fall off easily. Locally some growers have made it a priority to remove fruit and others have not had the resources to do so.

There have been local reports that unharvested fruit can discourage return bloom the following year. However, I wonder if past experiences were related to heavy crop loads that didn’t meet market grade and went unharvested. The reported effect on return bloom might have been a direct result of a heavy crop load that contributed to biennial bearing, rather than an influence of unharvested fruit. I can’t confirm one way or the other.

One research study from New Zealand concluded that removing fruit during harvest induced changes in the tree by transporting more resources into storage organs including carbon, nitrogen, potassium and phosphorus. The study also found that photosynthesis declined slowly after harvest and then more rapidly after the first frost. Fruit harvest might encourage trees to store resources and prepare for winter but ultimately dormancy is triggered by frost and decreasing light and temperature. I would hazard a guess that if fruit are unharvested by the first hard frost, then trees would proceed with dormancy regardless.

Given that we don’t know the influence of unharvested fruit, this year presents the opportunity to answer our questions by doing on-farm comparisons. You could do a few side-by-side comparisons by marking trees that you defruit now versus trees you do not defruit. Choose trees at a similar state of health in the same planting. Return to check the marked trees at bloom time if you question the return bloom of the block.

If leftover fruit are not removed by spring, then eventually the fruit could be a disease risk. The dead fruit tissue mummifies and is a site for the black rot fungus to live. When possible, workers could knock off this fruit during winter pruning. If you find an efficient method (Darwin string thinner? Hedger? Cider poles?), please let me know and we could spread the word. I would discourage shaking the trees too much as it could damage fragile root systems.

References:
https://www.ishs.org/ishs-article/557_62