Why are my conifers turning brown? Treating brown patches

As a professional gardener and working on my family’s nursery for over 20 years, one of the most common questions I get asked is, ‘Why is my conifer turning brown?’. 

After further investigation, it’s nearly always usually caused by one of two reasons: by trimming to hard into brown wood or at wrong time of year. The second reason for browning is the Cypress aphid, but more on this further down. There are a handful of other causes, which I’ll cover in more detail below. The first thing worth noting is that it’s usually either Cuprocyparis leylandii (leyland cypress), a very fast growing conifer which is very popular, and the other conifer where I see this issue more is Thuja plicata, also known as western red cedar.

Over-enthusiastic hedge trimming or trimming at the wrong time of year

Firstly, and possibly the most common reason for browning conifers, gardeners who have trimmed their conifers at the wrong time of year. When pruned too late in the season, October or later, I found this is a significant cause of dieback as the new growth doesn’t have enough time to recover. Getting a hard early frost can cause brown spots and dieback to develop, especially on established hedges.

When should you trim conifers to reduce any chances of dieback? 

Personally, I try to trim conifers as part of my garden maintenance business in April, June and August. I have found this significantly reduces the chances of browning conifers. It’s also worth noting that you should trim lightly and never into old wood; most conifers want to produce new growth from old wood. Also, avoid trimming conifers in excessive heat on hot days.

Cypress aphid

Over the years, I’ve also found that brown areas on conifers, usually lower down towards the base, had been caused by Cypress aphid (Cinara cupressivora). The problem is you only typically notice damage once the aphids are gone. However, Cypress aphids are easy to spot as they are large greyish aphids. Even once they have gone, they leave behind honeydew excreted by the aphids that develop into the sooty mould, so you often spot this first in late spring or early summer. To treat this, ideally, you need to spot the aphids first before they do the damage and treat them with a pesticide. 

Other less common problems that cause browning of conifers

For most gardeners, the reasons I’ve already covered above are probably the most likely cause of conifers developing brown spots and dieback. Now, there are a few less likely causes, which I cover in more detail below. These include a few diseases that affect conifers, including needle cast and needle blight. I also talk about fungal diseases, which include Coryneum Canker, as well as root issues such as root rot.

Conifer Diseases

Diseases like fungi, bacteria or viruses can impact coniferous trees (conifers) but are not very common. Many fungi are microscopic, and the only way to determine if you have an infestation or infection is by way of the symptoms. Still, sometimes people overthink it and its really just trimming late in autumn that causes those brown patches.

Diseases can often affect the needles, the roots, the trunk and even the stems of your tree. If you notice brown spots or other symptoms, you can typically trim away any diseased portions, apply a pesticide (if needed) and only if you can see the pests such as aphids or even remove a nearby plant or tree in order to make more room and allow for better airflow which can also help.

Needle Cast

Needle cast is another disease that results in conifers shedding their needles. The symptoms of this disease will first appear on the needles where you will notice that they change colour and produce yellow spots which will eventually turn brown. You might also see small fruiting bodies on the surface of the needles. If this is left untreated, it will kill the entire needle. If you identify needle cast I recommend applying fungicides and removing any diseased needles as soon as you spot them.

Needle Bligh

Needle blight is another disease that attack conifers, not only at the needles but also on the branch tips tips, which results in brown spots. Brown patches on conifers will typically start on the lower branches and if you allow this to continue uninhibited for many years it will kill off the limbs and eventually the tree itself. There are things you can do to prevent this, such as using a copper fungicide spray.

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Conifer Fungal Diseases

Coryneum Canker on coniiferd

Problems from fungal infections can also impact the rest of the conifer, not just the needles. Cankers, dead or blistered areas on the bark itself will result in a waxy discharge. Blisters will form on the branches in the form of a tumour and produce a yellow discharge. Much the same as other diseases, you will start to see them on the lower branches as they make their way up the tree. With this, you want to prune any impacted areas immediately and use a fungicide to treat the conifer. Coryneum canker usually affects the Leylandii cypress and the Western Red Cedar that many hedges are made up of, but it’s most common on large trees and is not often seen on small hedges, so if you have a hedge, it’s very unlikely to be the issue.

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Root diseases that often affect single trees

There are even deeper problems that you might face, such as root diseases. These types of issues will get into your conifer by way of a wound, usually at the lower part of the tree, or by penetrating through the roots directly, and this is why it is often referred to as root rot.

The symptoms will manifest in the form of needles dying off, bark peeling away and branches dropping off. Your tree will become completely unstable as the rot progresses, and in the worst-case scenario, the tree will have to be removed and replaced. If you have individual trees within a hedge that has died, this is usually caused by root rot.

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Winter Browning

If you see your conifer needles turning brown, a very common reason is winter browning. These evergreen trees get their energy from the sunlight the same way as any other plant, and they require water, even in the winter. If the trees do not have a sufficient amount of water to last them through the winter they can dry out, at which point the needles turn brown.

This rate of drying can be impacted by things like rapid temperature fluctuations, cold winter temperatures, or habitual freeze and thaw cycles from inclement weather, which unfortunately we are seeing more off in the UK. The part of the conifer facing the sun will typically brown in a more pronounced fashion.

This winter, browning can be visible in the winter or in early spring. It is recommended that you water your tree in the autumn during dry spells in order to help combat winter browning. You should also add a little extra water during the summer, especially in August because this will give your plant enough water to store for the winter and prevent the stress and subsequent browning of the needles.

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Prevention

My advice is to trim hedges at the right time of year, between April and early August, avoid trimming during times of drought or hot weather, and mulch and feed in the late winter.

If you notice a bad infestation of aphids, spray with a pesticide straight away to help minimise any damage to the conifers.

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Last update on 2024-04-04 / Affiliate links / Images from Amazon Product Advertising API