Common Potted Acer Tree Problems & How to Prevent Them

Acer or Maple trees come in many cultivars with dramatic foliage in various shades and colours. Some grow very tall, whereas other species are suitable for container gardening, due to their manageable small size. One thing’s for sure, whether it’s a native Field maple tree (Acer campestre), Norway maple (Acer platanoides), Sycamore maple (Acer pseudoplatanus) or gorgeous Japanese maple (Acer palmatum), which can easily thrive in a pot, an acer tree can be always a beautiful focal point in your garden if you show it some TLC.

Acer species are relatively easy to grow, once established, but they do suffer from some problems, due to disease, environmental factors and in that sense, due to one’s lack of knowledge on the specific growing conditions a particular type prefers. On that note, our post will focus on the common problems you may encounter when growing an acer tree in a container, as well as on some general issues that could pose a challenge to your woody plant.

Potted acer tree problems

In Britain, one of the popular acer tree variety that can be grown in a pot is Japanese maple. It comes in many subspecies, which boast distinct-shaped leaves in various hues. Its delicate foliage commonly tends to suffer from weather-related problems or wrong growing conditions.

So, if you notice one day that the leaves have suddenly started to brown, curl up or shrivel and the lower branches look as if dying, your potted acer tree has a problem, especially if the variety has overly dissected leaves. Experts refer to the condition as leaf scorch. The cause is most likely excessive exposure to dry winds, direct sunlight in hot summer or frost in the winter.

Also, excessive watering doesn’t do any favours to your potted Japanese maple tree, either, as this type of acer plant doesn’t thrive in waterlogged soils. To be fair, it neither flourishes in heavy shade.

Additional acer tree problems, diseases and pests

Now, let’s look at some other issues that may affect your acer tree, be it a potted Japanese maple or another variety that you’ve planted in the ground.

• Honey Fungus

Honey fungus (Armillaria) affects woody and perennial plants, including acer trees. Species, which are specially grown as hedges, like Boxelder maple (Acer negundo) or Field maple, are usually affected. The main symptoms are decaying roots, fungal growth between the bark and the wood, small and pale leaves, dying branches, clumps of toadstools on the stumps, etc. The infection spreads underground and is not easy to detect, as the signs resemble those of other types of diseases. We wrote a blog post about the honey fungus symptoms and treatment options in more detail, which you can check here.

• Abiotic and biotic leaf scorch

The evidence of the (acer) leaf scorch presents itself as drying leaves. In mild cases, you may notice the edges and veins of the leave going brown. The most common reasons for leaf scorch are unfavourable weather conditions, such as drought, high temperatures, hot dry winds and excessive sun exposure. However, frost and waterlogged soil may also cause the condition, also referred to as abiotic leaf scorch. Japanese maple, Sugar maple and Norway maple are the acer trees that are the most prone to get affected. Here, we should note that the condition is not always “provoked” by environmental factors. There is also bacterial leaf scorch (BLS ), referred to as biotic type of scorch, which affects shade tree species, such as Red maple and Boxelder maple. The culprit is Xylella fastidiosa – pathogenic bacteria that can infect the plant. Scorching is of an irregular shape and not very well defined, with red or yellow bands between green and brown patches on the leaf.

• Powdery mildew

Japanese maple and Norway maple are rather susceptible to this fungal disease, which thrives in humid moderate climates. The surface of the leaves is affected, usually, without any lasting damage. You can try and brush off the whitish powder from the plant, as well as resort to natural or chemical fungicides to resolve the problem.

• Verticillium wilt

Caused by soil-bound fungi, which may stay dormant underground until triggered, the disease is especially common to acer plants. The symptoms are often localised to one side or a specific part of the maple tree and include the loss of leaves, stunted growth, yellow and wilting leaves. The disease usually presents itself in late summer after a particularly hot and dry period. If the problem is not addressed in time, the tree may eventually die.

• Root rot

This is a lethal fungal condition that can infect any woody plant that grows in a wet climate. There are different forms of root rot, such as ganoderma, laetiporus and phytophthora, with the latter being the most common type in the UK. The signs of the disease include cankers or mushroom-looking growth, protruding out of the stump, as well as branch dieback and wilting sparse foliage. The widespread recommendation is to destroy an infected tree, as there aren’t really any control or remedial methods, available for counteracting the problem.

• Insect parasites

There are various types of insects that can invade your acer tree, including aphids, mites (maple bladder, velvet and spindle galls or red spider mites), scale insect or pear thrips. Aphids and scale insects both secrete a sticky substance, which can cause a sooty mould infection of the leaves.

Galls (mites) can affect various acer plants, including Sugar maple, Red maple, Boxelder maple and Norway maple. Depending on the type of galls, the leaves of the plant can be covered in velvet patches, pimples or worm-like protrusions.

Pear thrips are winged insects, which lay their eggs on the plant. In spring, the larvae will feed on the leaves, causing their discolouration. Sugar maple is prone to a pear thrips infestation.

• Rabbits and squirrels

Young potted acer trees are best protected against rabbits with a wire netting around the container. The keen rodent will happily nibble on the bark especially in the winter time. Squirrels also cause problems to more mature acer plants, again in an effort to get to their dose of nutrients. The tree may be able to recover over time if the damage on branches and the outer bark is not severe.

How to prevent the most common potted acer tree problems

With basic acer tree care you can fix and prevent the most common issues with your potted maple plant, which are not related to a specific fungal infection or an insect pest infestation.

Those are best treated after a professional inspection by a specialist, as the solution is always individual and depends on the type and the severity of the condition.

The prevention and care tips below will help you fight environmental causes for leaf scorch and generally improve the well-being of your Acer palmatum tree, especially if you’ve decided to grow it in a container.

• Improve soil condition

Japanese maples thrive in slightly acidic, sandy, water-retentive soil with good drainage. Choose a good-sized pot that has a sufficient number of drainage holes to avoid waterlogging of the soil. Note that loam-based compost for acers is the best growing medium for your beautiful maple tree (ex: John Innes No 2). Furthermore, it’s recommended that you repot your Acer palmatum approximately every 4 years between February and March. As the leaves emerge, you can give your plant a balanced feed, as well. If you’ve never grown a Japanese maple in a container, follow these easy steps on how to plant an acer in a pot.

• Cover with mulch

Mulching around the trunk of your Japanese maple with organic material, such a bark mulch, or with gravel slates will help the soil retain its moisture, especially in hot weather, when you are advised to water the plant a couple of times a day. Make sure that there is some space between the tree and the mulch, however. Again, protecting your acer tree from losing water and drying out is not only a preventative measure against weather-related leaf scorch, but can also reverse leaf damage if the problem is not yet too far gone. In addition, any steps towards enhanced water-retention of the soil will prevent it from case-hardening around the roots, by keeping the growing medium crumbly, but moist.

• Move to a sheltered spot

Finding the right position for your potted Japanese maple is important to avoid the onset of leaf scorch. This means that your plant should not be placed under direct sunlight in hot weather. A semi-shaded spot in your yard, which is sheltered from high dry winds, is the ideal environment for a healthy and flourishing acer tree. You can also resort to using a temporary windbreak around your potted acer plant during unexpected stormy weather conditions.

In winter, to prevent frost damage, cover your plant in a protective insulating material – a large box, placed carefully over a young plant, overnight, will do. Note that experts do not advise to bring the container indoors, as the difference in temperature when you move the plant in and out, can do more damage than good. Defoliation is a common outcome when contrasting temperatures are at play.

• Prune if necessary

Pruning your Japanese maple is only necessary to achieve an aesthetically balanced shape of your plant, as new shoots usually grow in one direction. The task is best completed in late summer or early autumn. Remember that pruning in the wrong time of the year causes bleeding. Still, if your potted acer tree suffers from mild dieback and browning leaves and branches, pruning those will improve the overall look and state of the plant. Naturally, however, you will need to find out the underlying cause for the poor health of your maple tree.

A more specific pruning procedure that requires some knowledge and skills is root pruning, that can be done when it’s time to replace the soil or sometimes, when you replant the acer tree in a larger container. You should prune the roots only lightly, while cleaning them off any dry and hardened-up old soil.

Takeaways

Potted maple tree problems are often preventable by applying vigilance and the right care:

• Make sure your acer plant is potted in a container that is double the roots’ size to provide them with sufficient space to develop and this way, avoid stunted growth.
• Infectious diseases and pest infestations are usually manageable when treated at the first signs of the problem.
• If you’re unsure about how to plant your maple tree and what conditions it will best grow in, you can always seek the professional planting expertise of the Fantastic pros and stay on the safe side.
• Prevention is always the best course of action, especially when it comes to dealing with acer leaf scorch and other environmental factors that can affect the well-being of your potted maple tree.

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Have you had any experience with growing potted acer trees? Let us know in the comments below!

Header image source: Shutterstock: By Vahan Abrahamyan 

• Last update: June 28, 2019

Posted in Garden Advice, Plants in the UK