Why Are Tree Trunks Painted White? Tree Whitewashing Explained

The practice of painting tree trunks white to protect them, which is known as whitewashing, has a long history that can be traced back to ancient times.

The use of lime-based whitewash dates back to ancient Greece and Rome and gained popularity during the medieval period in Europe. In the 19th and early 20th centuries, whitewashing became a common practice in fruit tree orchards, and it’s a widespread practice in traditional farming communities throughout many cultures around the world.

The popularity of whitewashing tree trunks has fluctuated over the years, and even though the practice is still carried out today, it’s more commonly used in certain regions of the world for specific tree care purposes rather than being a widespread practice for all tree species and locations.

Reasons Why Tree Trunks Are Painted White

The purpose of whitewashing tree trunks and the lower branches is to provide protection from sunscald, various pests and diseases and frost damage.  

1. Protection from Sunscald  

Sunscald, or sunburn, is the condition where the tree bark is damaged by excessive exposure to hot, direct sunlight. It occurs on the stems or trunks of young trees because their bark is thin, and is less able to tolerate exposure to harsh, direct sun, and they have less foliage to provide shading.

On older trees, the bark may be damaged when it becomes more exposed to direct sun after pruning away leaves that were previously shading it, or if the tree drops its leaves prematurely due to some issue, such as drought.

Other factors that may contribute to sunscald (sunburn) or tree bark are:

  • The removal of structures, other trees or plants that shaded out the trunk or branches.
  • The addition of paving or structures that reflect light or radiate heat placed near a tree.
  • Any other factors that harm root health (such as drought, waterlogging, root pests and diseases) or reduce a tree’s ability to absorb adequate water from the soil.

Typically, bark burnt by the sun splits and cracks on the side facing the midday and afternoon sun direction, when the sun is hottest and most intense. In the northern hemisphere this is on the southwest and west side, while in the southern hemisphere that direction corresponds to the northwest and west side of the tree.

The whitewash reflects sunlight, reducing bark heating and large temperature fluctuations, minimizing the risk of sunscald.

2. Pest and Disease Control

The mechanism by which whitewashing deters pests is not fully understood. It’s believed that the light, reflective colour disrupts the visual cues that certain pests instinctively look for.

Typically, most tree barks are darker coloured, and specific pests may seek out dark-coloured tree trunks preferentially as being more suitable for feeding, breeding, or shelter.

A bright, white tree may confuse the pests, appear less attractive to them, or present a surface that feels unnatural to them, or make it more difficult for the pests to crawl up the tree trunk and reach the foliage, which may help deter them, though it may not be a foolproof deterrent method against all pests.

Some of the pests that whitewashing is used for include:

  • Wood-boring pests that are frequently a problem on trees with sunburnt branches or trunks.
  • Bark beetles that are attracted to volatile compounds released from damaged trees, and whitewashing can help mask the scent and make the tree less attractive to them.
  • Aphids, of which some species are attracted to dark tree trunks, as reflective mulches are used commercially to repel winged aphids by interfering with their ability to find host plants and trees.
  • Winter moths, which are attracted to dark tree trunks when searching for a site to lay their eggs.
  • Cankerworms, that are caterpillars also called inchworms, or loopers because they move with a distinctive looping motion, may find it more difficult to crawl up the tree trunk that’s been whitewashed to reach the foliage.

Additionally, it’s believed that certain fungal and bacterial diseases can be deterred, limited or slowed down in their spread by the application of whitewash.

3. Protection from Frost

Whitewashing can act as a barrier to reduce heat loss and protect the tree bark from frost cracking and other frost damage during cold nights.

What Materials Are Used to Whitewash Tree Trunks?

The primary reasons for whitewashing tree trunks may have not changed much over time, but the materials and techniques used have evolved. In addition to the traditional lime-based whitewashes, we now have modern paints, as well as proprietary coatings designed specifically for tree protection.

Various formulations can be used to whitewash tree trunks, but it’s important that they should be safe for trees, and not contain toxic chemicals that could harm trees or hinder their growth.

1. Traditional Lime-Based Whitewash

Traditional whitewash has been used for centuries and typically consists of hydrated lime (calcium hydroxide) and water. It has a natural, chalky appearance and is breathable, allowing moisture and gasses to pass through it.

To make a lime-based whitewash, use:

  • Hydrated lime, also known as calcium hydroxide (Ca(OH)2), slaked lime, or builders lime
  • Zinc sulphate (ZnSO4) – optional*
  • Water

Mix hydrated lime (50g) and zinc sulphate (4g) into 1 litre water, or

Mix hydrated lime (8oz) and zinc sulphate (0.65 oz) into 1 gallon water, or

Mix hydrated lime (2.5 lbs) and zinc sulphate (0.2 lbs) into 5 gallon water for bulk volume.

Dilute if necessary, then spray or paint on tree surfaces.

*Note – Zinc sulphate is optional, it’s added for its fungicidal properties, which can help prevent or slow down development of fungal diseases.

2. Latex-Based Whitewash Paints

Modern tree trunk paints often use latex-based formulations which provide a more uniform appearance and can offer protection against pests, diseases and environmental stresses. To make a latex paint-based whitewash, use:

  • Interior white latex paint (not exterior paint, and not oil-based paint, which are all harmful to trees)
  • Water

Mix equal parts (a 1 to 1 ratio) of interior white latex paint and water. This diluted mixture is easy to spray or paint onto the tree trunk and major limbs.

It’s critical not to dilute the paint too much, as a strong white colour is needed to reflect sunlight and prevent damage.

3. Commercial Whitewash Tree Trunk Paints

There are commercially available tree trunk paints specifically formulated for tree protection. These products may contain a combination of acrylic or latex polymers, fungicides, insecticides, and other additives for improved durability and pest control.

Commercial formulations may vary between manufacturers, so it’s essential to follow the instructions provided by the specific product.

How to Apply Whitewash to Trees

To use traditional, paint based or commercial whitewash properly, follow these guidelines:

  1. Do not whitewash trees that have been planted less than two years ago.
  2. Do not whitewash the base of the tree where it flares into the root, as it can impair gas exchange and moisture absorption through the area.
  3. Apply whitewash with a brush, spray it on, or wear rubber gloves and wipe it on with a sponge or cloth. Slaked lime is a bit caustic to skin so it’s important to protect your hands!
  4. Apply the whitewash evenly and not excessively. With the paint-based formulations, apply in thin coats to allow the tree trunk to breathe, as a thick layer of paint can trap moisture against the bark which may promote rotting or fungal growth.
  5. Paint the trunk from the ground to at least 45cm (18”) above ground. For additional protection, it’s possible to paint as high as the lower 20-30cm (10-12”) of each lower scaffold branch. This is especially useful when the branches have narrow crotch angles, because the way the bark grows makes at the crotch of the branch makes the region less winter-hardy than branches with wider crotch angles.

When to Apply Whitewash to Trees

Paint tree trunks with whitewash in late autumn to early winter (May to early June in the Southern hemisphere, November to early December in the Northern hemisphere), as this timing reduces the chances of injury from the whitewash application.

Select warm, sunny days when drying conditions are good. Don’t apply the whitewash if the air temperature is below 10°C (50°F), and finish application by mid-afternoon to allow adequate drying time. The faster the whitewash dries, the less chance there is of damage occurring.

The whitewashing on trees requires regular maintenance, as it can deteriorate over time due to weather conditions. Reapply yearly, and before applying a new coat, ensure that the previous layer is not causing any harm to the tree.

It’s also good idea to apply the whitewash on the same day as the pruning is done, since pruning exposes more or the trunk and branches to sudden direct sunlight. Ideally, trees should be whitewashed first, before pruning, so that the whitewash doesn’t cover the pruning wounds and inhibiting their normal healing process.

References

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