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Thuja hedging (otherwise known as Western Red Cedar and Arborvitae) is considered to be fairly resistant to most garden pests and common plant afflictions, but now and then one will start turning brown and dying for seemingly no reason. We want to equip gardeners with the ability to spot the warning signs for afflictions – big or small – that could affect their Thuja hedging. See below a list of the most common Thuja hedge diseases, pests and other problems, and how to treat them.
A fungal disease that attacks the leaves and shoots of Thuja species, particularly Thuja Plicata (Western Red Cedar). This can be fatal to seedlings and young plants.
Symptoms of this Thuja Hedge disease can include the leaves turning yellow, then brown, and large black fungal fruiting bodies developing in the leaves. These will split open when mature and release spores, which can overwinter and germinate the following spring, allowing the disease to come back year after year. You may notice you have Thuja Blight by spotting cavities in the leaves, where fruiting bodies were before releasing their spores.
Much like many of the other Thuja Hedge diseases, Thuja Blight is cultivated in wet, humid conditions. Ensuring you’ve planted your Thuja hedging somewhere with good air and light circulation is a great preventative measure, but if the disease has already taken hold, then pruning is your friend. Prune away as much of the diseased foliage as possible. The beauty of Thuja is it will re-grow new growth even when cut back hard.
If the foliage is particularly dense, carefully prune away some of the inner branches too. This will allow for sunlight to reach the inner canopy of the Thuja hedging, improving air circulation within the plant and helping to keep it dry. Make sure you do all this with clean tools, sanitising before and after, and that you collect and dispose of all the prunings so the disease can’t spread further.
Pestalotiopsis is another fungus that can affect your Thuja hedging in humid conditions. Similarly to Thuja blight, it causes brown patches and creates black fruiting bodies all over the diseased foliage, but it can also sometimes cause severe dieback. Like before, prune away the affected branches with sanitised tools, and ensure you have as much light and air circulation for your plant as possible.
This is a common and disfiguring fungal disease that is best known for affecting Cypress and other Conifer Hedges, but can very occasionally cause your Thuja hedge problems too, particularly Thuja Plicata (Western Red Cedar).
You may notice wilted twigs with yellow and brown needles, but the disease normally manifests as sunken lesions or “cankers” on the larger branches. Coryneum Canker, unfortunately, is not treatable with chemicals and can be incurable – you need to remove diseased branches as soon as you see them, and if you can see cankers on the main trunk, it may be too late to save your Thuja hedging. In this worst case scenario you may be best removing the infected tree and replacing with a new one.
If the foliage of your Thuja hedging is rapidly browning and the plant is at risk of dying, a root disease could be the problem. Honey Fungus, for instance, is a type of fungus that attacks and rots the roots of woody plants, including Thuja hedging.
If you notice white fungal growth between the bark and wood of your Thuja hedges, paler leaves, and honey-coloured mushrooms appearing around the roots in autumn, you have Honey Fungus. This is a difficult disease to fight – your best bet is to bury a plastic sheet under the soil but above the roots, which will prevent the fungus from reaching them. Learn more about Controlling Honey Fungus.
Lastly, if you’ve had an infestation of Scale insects, you might notice Sooty Mould on your Thuja hedging. Scale insects leave “honeydew” excretions on the twigs and needles, and Sooty Mould (a black, powdery mould) may grow on top. This is a, mostly harmless, fungal pathogen that is unlikely to affect the health of your Thuja hedge.
Many different types of aphids could affect your garden, but Cypress Aphids are a particularly common pest for coniferous hedges like Thuja Occidentalis – in fact, research by the RHS and East Malling Research indicated that over half of the cases of brown patches appearing in conifers are caused by Cypress Aphids. If the foliage of your Thuja hedging is yellowing and turning brown, and if you notice cast Aphid skins and sooty mould on the leaves (particularly around the base of the plant), you may well have/had a Cypress Aphid infestation.
Frustratingly, Cypress Aphids are normally gone by the time you start to notice your Thuja hedges turning brown – be vigilant of any browning appearing to try and catch them before too much damage is done. If you then use an insecticide on the plant and build up its nutrients again with feeding and mulching, the brown patches may start to come back to life. However, if in doubt you can cut off any browned/dead branches as they will re-grow with healthy new foliage. If you would like more information, we have a dedicated guide to Controlling Cypress Aphid outbreaks.
Similar to Cypress Aphids, Scale Insects are an invasive species that feast on the sap in the foliage of Thuja hedges. They are considered fairly harmless – with only large infestations considered to cause dieback for Thuja hedging – but if you want to ensure they can’t do too much damage, there are several commercial solutions. You can find insecticides that will stop an infestation quickly, or for a gentler organic solution, consider spraying or dabbing the plants with a horticultural oil like neem oil to get rid of them.
Cypress Tip Miners are the larvae of Cypress Tip Moths. Similarly to the Arborvitae Leafminer or Thuja Mining Moth native to the US, mature Cypress Tip Moths lay their eggs on species like Thuja hedging. The larvae that hatch then nestle into the foliage.
If your Thuja hedges are turning yellow in winter and brown in spring, you might be seeing the Cypress Tip reproductive cycle in action. Shaking the branches and seeing if the white and beige-speckled moths fly out is the best indicator. The solution here is simple – prune away and dispose of the affected branches and hope the moths lay their eggs somewhere else next time!
Speaking of larvae, Bagworms are another Thuja hedge pest. If you notice cones of conifer needles woven together with silk hanging from the branches of your Thuja hedging, you could be seeing the cocoons of Bagworm larvae. These hatch in summer and the emerging Bagworms start to eat the plant’s foliage.
These have to be taken care of by hand – cut the bags off the Thuja hedging and destroy them, either by squishing them or submerging them in buckets of soapy water. Bagworms can affect deciduous hedging species too, so make sure you’ve definitely got all the egg sacks.
So far, we have discussed a number of Thuja hedge pests and diseases that can cause problems such as die back and leaves turning brown. There are also several environmental reasons why this could occur, including:
The last common Thuja Hedge problem to consider is the fact that they will take a little time to regrow from old wood. Pruning too liberally can create gaps in the canopy that may appear unsightly. To learn more, read our guide on When to Trim a Thuja Hedge.
If you want to know more about the different types of Thuja hedges and their needs, take a look at our other Knowledge Base Pages for in-depth solutions to an array of gardening problems. Here at Hopes Grove Nurseries, we have more than three decades of experience caring for hedging plants and are eager to share our expertise.
Explore our range of hedging plants today and, if you have any questions about common Thuja Hedge pests, diseases and other problems, contact our team for more information!
