Horticulture Magazine

What Is Defoliation And How Can It Be Treated?

the leaf of a currant eaten away by Small gooseberry sawfly Pristiphora rufipes

A tree or other plant suddenly and unexpectedly losing a lot of leaves can be shocking.

Identifying the cause of defoliation can be challenging, but this article may help.

In this article, we will introduce you to defoliation and its common causes, help you to identify the cause of the problem, and give you some tips that might help you save your plant – or, at the very least, reduce the chances of you having the same issue again in future.

What Is Defoliation?

a tree that has lost the majority of its leaves against a blue sky background

Defoliation is the name given to plants losing all, or a large proportion of their leaves.

This is sometimes also known as leaf loss, or leaf abscission.

Leaf loss can be due to natural plant processes, stress factors which cause the plants to drop leaves as a form of plant defence, or external factors.

If a tree, shrub or other plants in your garden suddenly becomes defoliated, you’ll need to play detective to try to determine the cause of the issue.

What Causes Defoliation?

a deciduous tree that has lost all of foliage in autumn

First of all, remember that some plants will naturally lose their leaves as winter approaches.

Deciduous plants lose their leaves in autumn, enter a period of dormancy over the coldest part of the year, then burst into new growth in the spring.

So make sure that this defoliation is not simply a natural part of the lifecycle of the plant.

If defoliation has occurred on plants that would not usually lose their leaves, or has occurred at a different time of year, there are a wide range of different things that could be to blame. Common causes include:

  • Environmental issues (to do with temperatures or water).
  • Grazing animals.
  • Insect infestation.
  • Diseases (fungal or bacterial).
  • Fertiliser or herbicide contamination

The Results Of Defoliation

sawfly larvae that have defoliated a red currant shrub

A plant that loses some but not all of its leaves may eventually recover, though its growth will likely be stunted.

However, in many cases with non-life cycle defoliation, if all the leaves are lost, the plant is unlikely to be able to make a comeback and unfortunately, will often die. [source]

Most plants need leaves to survive.

foliage of a bush eaten away at by caterpillars

The leaves are the ‘factories’ where photosynthesis occurs, where they use sunlight and carbon dioxide to create the energy reserves they need. [source]

Whether or not a plant can survive defoliation will depend on how many leaves have been lost, the type of plant, and when exactly the defoliation has occurred.

Evergreen plants store a substantial part of their energy reserves in their leaves, so defoliation can often kill them because they not only lose their ‘factories’ but also their starch reserves. [source]

base view of a teak tree that has lost many of its leaves

Deciduous plants, on the other hand, store reserves elsewhere, in stems, trunks and roots, so they can often withstand defoliation more easily.

Deciduous trees and shrubs can tolerate the loss of up to half their leaves and will often survive.

Even those which lose more than half of their leaves may make it through.

However, if the defoliation occurs over multiple years, the trees will be weakened and will not survive further leaf loss.

magnified view of a caterpillar on the foliage of a tree

Sometimes, when defoliation happens on deciduous plants earlier in the growing season, plants have time to put out a second flush of growth to replenish energy supplies before winter.

In the middle of the season, deciduous plants may not have time to recover.

If defoliation has occurred not long before natural leaf drop, this may not be as much of an issue.

Identifying The Cause

grasshopper defoliation damage on a Abelmoschus esculentus plant in heavily mulched ground

The first thing to determine is whether environmental issues have led to defoliation.

1) Temperatures

Extremely high and unexpectedly low temperatures can both cause this problem.

So think about the temperatures that a given plant can be expected to survive, and look at whether the temperatures in your area have jumped outside of that range.

grasshopper sat in the damaged leaf of a Abelmoschus esculentus plant

2) Water

When it comes to environmental issues, water is another important thing to look at.

Has there been extremely high rainfall, or a period of drought?

Might you have watered your plant too much or too little?

Too much water and too little can both cause defoliation in their own right – but note that they can also increase the likelihood of defoliation due to disease.

fiddle leaf fig houseplant by a window with a visible yellow leaf caused by overwatering

Often, it can be difficult to determine whether water issues of disease are the actual cause of the defoliation – but in either case, the issue can often be traced back to water.

If you have ruled out an environmental issue with temperature or water, think about whether the leaves fell off, or were eaten!

3) Pests

Deer and rabbits can quickly eat a large number of leaves, and may make you think that these leaves simply dropped off if you do not catch them in the act.

rabbit eating the leaves of a hosta plant

If you look out, you may well be able to see the foragers to blame in your area, and identify how and where they entered your garden.

Leaves may also have been eaten by a wide range of insect pests, such as sawflies, grubs, caterpillars etc.

There are a huge, huge range of pests that can quickly eat a lot of leaves, defoliating your plants.

Narrowing down the list of potential culprits will involve, first of all, looking at the list of common pests for a particular plant species.

magnified view of a gooseberry sawfly insect on the leaf of a currant plant

The culprits themselves may still be around if you look closely at your plants.

4) Plant Diseases

Diseases are also a common cause of defoliation.

Unfortunately, trying to work out which particular disease plants may have is not always easy.

Again, you can narrow down the likely diseases by looking at the most common issues for a particular plant.

Finding A Solution

wilted cucumber leaves

If sudden temperature changes or weather extremes were to blame then unfortunately there is not likely to be much that you can do, other than simply waiting to see if your plant will recover.

Mulch well around the base of your plant with organic matter to provide slow-release fertility to give it the best possible chance.

If too much water was the problem, or waterlogged conditions have arisen, taking steps to improve drainage through adding organic matter or even moving the plant to a free-draining location could help improve the chances of recovery.

neglected cherry tomato plant with leaf damage caused by a lack of water

If there has been a drought, obviously providing water will be a top priority.

Look at changing the location of plants or improving the soil if you are seeing repeated issues of these sorts.

If leaves have been eaten by deer or rabbits or other foraging creatures, take steps to protect your plants.

a hedgerow used as a border

Make boundary hedgerows or protective rings with plants which provide them with food, or which they will not eat, to distract them or encourage them to browse elsewhere, for example.

You could even use physical barriers or tree guards where necessary.

Insect pest numbers can be kept down in a number of ways in an organic garden.

Pick off pests you do find on your plant by hand and take steps to apply integrated pest management in your garden.

hand holding many gooseberry sawfly insects with defoliated leaves

This means attracting wildlife for natural predation, boosting biodiversity and taking other steps to maintain balance in the ecosystem in an organic way.

Some diseases can be managed with organic control, and mitigated with garden hygiene practices (like removing all infected leaves from the plants and from the ground).

Others unfortunately are much more challenging to manage – treatment will very much depend on the specific species, and on the plant and disease in question.

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