How to save a dying hemlock tree (diagnosis guide, pictures)


western hemlock tsuga heterophylla needles
Western hemlock near me

There are around 10 hemlock species, but there are two that people ask me about time and time again – western and eastern hemlock. Both are native to North America (west to the west coast, east to the east coast) and are widely planted in Europe and the UK, particularly the graceful western hemlock as an ornamental.

They’re a temperamental tree at times – not the most resilient. How do we save a hemlock that looks sick?

Hemlocks can decline due to environmental factors, such as poorly-drained soil and winter injury, or due to pest infestation, typically either by woolly adelgid or elongate hemlock scale. Trees can be revived by correct watering practices, protection from desiccation and treatment of infestation.

I’d love to bring you all I’ve learned about hemlocks through my years of passion for trees – both hands-on and through personal study (I’ve always enjoyed studying!).

Which hemlocks am I writing about? This article will cover all the common species that I work with, including the eastern hemlock (Tsuga canadensis, also known as common or Canadian hemlock), the western hemlock (Tsuga heterophylla), Carolina hemlock (Tsuga caroliniana), mountain hemlock (Tsuga mertensiana), Chinese hemlock (Tsuga chinensis) and Japanese hemlock (Tsuga sieboldii).

By the way, tsuga trees are sometimes called hemlock-spruces.

How to tell if a hemlock tree is dying

OK, first we need to know if there’s a real problem or not.

Hemlocks are evergreen – but that doesn’t mean their leaves (which you might call needles, although they’re typically fairly flat) won’t ever go brown, or fall off. All evergreens including hemlocks shed their leaves eventually.

In hemlocks, the leaves stay on for longer than most other evergreens – up to 8 years in fact. The innermost needles are also the oldest – they’re shaded by the upper foliage, particularly around the tree’s base. They’ll go brown and eventually shed in autumn.

So what are the signs that a hemlock tree’s sick?

  1. The tree isn’t growing, or growing very slowly (most hemlocks will put on 2 feet per year)
  2. Dropping more foliage that it should in autumn (only the innermost leaves drop)
  3. Dropping leaves at other times of year, such as early spring
  4. Asymmetrical browing or leaf drop (top more than bottom, or one side more than the other)
  5. Browning of new leaves i.e. those near the branch tips
  6. Signs of disease – white fuzzy lumps at needle bases, or flat brown bumps on the needles themselves
  7. The tree looks sick to you – brittle, a poor colour – yes, your instincts will usually be right!

WHY is my hemlock tree dying?

When diagnosing hemlock problems, we need to find out if the tree is

  1. Stressed – that it, due to it’s environmental surroundings, or
  2. Infected by disease, or infested with pests.

First, let’s talk about stressed hemlock trees.

Causes of stressed hemlock trees

Soil problems

What’s your soil type like? They can tolerate most soil types, except the chalky variety. They also grow a lot better on soil that’s a bit acidic (pH 5-7); if your soil’s only slightly alkaline it might survive but grow more slowly, while if it’s significantly alkaline, the tree might not survive for long after planting.

It’s easy to test your soil pH to rule this out. I use the Rapitest home test kit, which is pretty straightforward.

Water – too little, or too much

I find gardeners regard evergreens as tough old trees that they don’t need to water, and focus on their flower and fruit trees! It’s often the evergreens that need the attention.

Young hemlocks usually need to be watered weekly during the warmer months, and monthly in the cooler months (more on that later – see ‘winter injury’ section).

When I say water, I’m usually talking about a good 30-minute soak with a hose around the drip line (under the canopy’s edge) – not a 30-second blast at the trunk!

While they like regular rainfall, hemlocks have difficulty getting established in boggy or poorly-drained ground, especially younger trees. Rather than overwatering, this is usually down to where the tree’s sited – hemlocks planted at the bottom of a yard or property often struggle due to this problem, especially younger ones. You’ll find they don’t grow well, and are losing a large amount of their inner foliage in autumn.

It’s not easy to improve drainage on a boggy part of your garden. If the tree’s not establishing itself, consider starting again in another part of your property – and consider mixing in some grit with the planting soil this time, to help with drainage.

Hemlocks and shade

Unlike some other evergreens, hemlocks will generally tolerate either full sun, or full shade.

However, it does depend on your local climate a bit. In warmer regions like southern US states, hemlocks will grow better in at least partial shade. In cooler regions – such as northern Europe or Canada, they’re more likely to thrive in a slightly sunnier spot, even in full sun.

Hemlocks in warm regions that are exposed to full sun may dry out too quickly – resulting in a sickly-looking tree with off-green, brittle, scorched-looking foliage. The easiest remedy it to increase the watering frequency. If in doubt, dig an inch or two down into the soil under your tree. If the soil here doesn’t feel damp or stick to your finger, the tree needs water.

Winter injury

western hemlock losing foliage
This Tsuga heterophylla has lost more foliage on the wind-exposed aspect

Also known as winter desiccation, this is a major cause of tree stress that usually becomes apparent in late winter or early spring.

Hemlocks being evergreen trees, they keep their foliage through the winter, meaning right through the cold and windy season moisture is constantly evaporating from the leaves (unlike deciduous trees which have long since shed their foliage). This can happen rapidly in winter, particularly in strong winds.

The moisture has to be replaced via the roots. If the roots can get enough moisture, the foliage will dry out and turn brown. This is even worse if the ground freezes and the roots can’t take up any water at all.

If you suspect winter injury, look for:

a) Brown needles at the branch tips in late winter/early spring, or needle drop at this time

b) One side of the tree is affected more than the other – often the wind-exposed side

If your hemlock’s near a road or driveway that is salted to prevent ice forming, winter injury is particularly likely – salt spray dramatically increases the risk of desiccation.

Preventing winter injury in hemlocks

  1. Consider some basic wind protection – this might be fencing, burlap, polythene sheeting stretched between poles – it’s a simple way to reduce the drying effect
  2. Apply a 3-4 inch layer of mulch around the base of your tree. Be generous! A circle of 4-5 foot diameter around your hemlock will allow water to permeate into the soil and insulate the roots from temperature changes
  3. Water your hemlock once a month during the cold months – as long as the temperature’s above freezing
  4. Consider an anti-desiccant spray (for small hemlocks) – this puts a seal over the leaves to prevent more water loss.
  5. Limit application of road salt near your tree (or erect a barrier that will prevent it getting on the foliage).

Usually, a winter-injured hemlock will make it through – it’s rarely lethal. But if it’s happening year after year – especially if it keeps getting sprayed with salt – you’ll end up with a lopsided, poorly grown hemlock on your property.

Transplant shock

Consider transplant shock if your hemlock’s been planted or moved in the last 3 years – but particularly if it was in the past 12 months.

A tendency to transplant shock is a phenomenon that hemlock shares with many other evergreens, such as spruce, pine and cedar. When plants are moved, the roots are suddenly thrown into a new environment and can fail to absorb water properly. Young trees can lose most of their foliage through transplant shock, and if they do, that means a large reduction in photosynthesis and a tree that won’t survive for long.

In a mildly transplant-shocked hemlock, you’ll see browning and leaf loss at branch tips in the months after planting. This is likely to be worse if the tree was planted in summer, when the ground’s hot and dry.

Hemlocks usually make it through transplant shock. The antidote is regular watering. This is particularly important in the days after planting, but I’d just adhere to the rules above – weekly when it’s warm, monthly when it’s cold – and that should be enough to see it through to the next growing season.

Did you apply the appropriate mulch to the ground at the base of the hemlock after it was planted? If not, get out there with some organic bark chippings and spread a 3-4 inch layer that at least extends as far out as the longest branches. This will help rainwater get through and insulate the hemlock’s roots.

Hemlock pests and diseases

The vast majority of discussion on hemlock diseases is on woolly adelgid, and for good reason. But it’s

a) only a major problem for certain types of hemlocks and

b) not the only common hemlock pest!

Here’s a breakdown for everyday hands-on gardeners:

Woolly adelgid

Hemlock wooly adelgid
Hemlock wooly adelgid

Woolly adelgid is a tiny insect, similar to an aphid, that sucks on sap. It’s spread from tree to tree by wind, birds, and even human touch and equipment like chain saws. It originates in Japan but has been found widely in the US since the 1950s, and now has spread to other parts of the world, including the UK and Europe.

It’s easy to diagnose – you’ll see what looks like white cotton balls at the base of leaves (that’s where the ‘woolly’ part comes from), mostly towards the underside – these are the adelgid egg sacs. These become visible in early spring and can be seen over the summer months.

Woolly adelgid’s important because it can kill a hemlock tree within 4 years. However, it depends what type of hemlock you have, as some species are naturally resistant. When you read about woolly adelgid devastating hemlock populations, it’s typically referring to either Eastern hemlock or Carolina hemlock.

Hemlock species common nameLatin nameWoolly adelgid susceptibility
Eastern/common/Canadian hemlockTsuga canadensis  Highly susceptible
Western hemlockTsuga heterophylla  Resistant
Carolina hemlock  Tsuga caroliniana  Moderately susceptible
Mountain hemlock  Tsuga mertensiana  Resistant
Chinese hemlock  Tsuga chinensisResistant
Japanese hemlockTsuga sieboldiiResistant (though acts as host for adelgid)
Susceptibility of common hemlock species to disease caused by woolly adelgid

However, even resistant hemlocks can succumb to disease caused by woolly adelgid if they’re stressed – for example, after drought or winter injury – see the section above.

Infested trees will drop more of their needles than they normally do in autumn time and look thin, with off-colour, greyish needles and dieback starting in small branches.

How to treat woolly adelgid in hemlocks

Not all infested hemlocks will die – so some will opt for a ‘watch and wait’ approach. But this bug can cause only some needle-loss and crown thinning for a few years and then end the tree’s life – so it’s risky business.

This can be treated by a homeowner if the tree’s small enough to spray yourself. If it’s too big, you’ll need to call in an arborist. They may use injections into the soil or trunk with insecticides such as imidacloprid.

Horticultural oil or soap sprays, applied in spring and in late summer, is an effective, environmentally friendly and easy method of controlling this pest. It typically doesn’t work as prevention – it’s only useful when the tree’s already infested.

Keep an eye out for any white fuzzy balls at the bases of needles, and keep spraying the tree regularly if you do.

If you do any pruning, dispose of tree parts in the trash, so they don’t spread the infection to other trees – and sterilise your pruners or your saw afterwards using rubbing alcohol or alcoholic hand sanitizer.

Prevent infestation from occurring in the first place by preventing your trees from becoming stressed! That means:

  1. Regular watering on a well-drained site
  2. Plenty of mulch around the base
  3. Some protection against winter injury (see above).

Here’s a useful and succinct video on hemlock woolly adelgid from University of Georgia Extension. It has some great images.

Hemlock scale

Elongate hemlock scale caused by Fiorinia externa
Elongate hemlock scale caused by Fiorinia externa

Hemlock scale (also known as elongate hemlock scale or Fiorinia scale) isn’t nearly as big a problem overall as woolly adelgid itself, but it can significantly stress – particularly when a tree’s got both hemlock scale AND woolly adelgid infestations.

Hemlock scale’s an armored insect that sucks from needles, causing premature needle drop, browning and a generally sick-looking tree. It can cause tree death, but it’s not so common.

Again, it’s quite easy to diagnose – just inspect the needles for flattish brown scales. This video from New River Gorge National Park and Preserve explains the infestation well and has some great images:

Elongate hemlock scale

Unfortunately, it’s Eastern hemlock and Carolina hemlock again that are most susceptible, just like with woolly adelgid.

It can be managed in a similar way to woolly adelgid if the tree’s small enough – horticultural oil or soap sprays in spring and summer – but it might need a few sprays during the summer, rather than just the two sprays per year.

Remember to consult a trained arborist if you want to make sure your tree makes it – and they may be able to use ‘systemic’ insecticides – for example via root or trunk injections – and moreover, these can last longer.

Phytophthora root rot

This fungal disease has worldwide prevalence – but it often isn’t identified as a cause of a declining hemlock until it’s too late, predominantly because it’s difficult to diagnose from the signs above ground – you need to look at the roots, which turn red-brown – but you can’t do that without damaging them!

Phytophora literally means ‘plant destroyer’ in Greek (phytón, ‘plant’ and phthorá ‘destruction’) – because that’s what this mold (fungus-like organism) does. Trees that don’t like boggy or waterlogged soil – including hemlock and yew (read more about phytophora in that tree here) are particularly susceptible, This also includes white pine, boxwood and dogwood.

Hemlock’s at a low point of a property where the ground gathers more moisture are more likely to be affected. They decline slowly – so slowly that it can be difficult to notice. Sometimes trees are infected for years before signs become obvious.

Look for reduced growth, thinning of the tree’s grown, excessive needle loss and browning. A badly afflicted tree will eventually die as the infection takes over the root system.

Hemlocks that have died back significantly as a result of this root rot will be hard to restore. I’d more likely plant a new tree in a location with better drainage (make sure you remove and dispose of all parts of the old tree, so they don’t infect other plants on your property). Trees that have mild chronic infection can be managed with regular fungicide application, but I’d consider calling in an arborist to consult on that one.

Fertilizing hemlock trees

Will fertilizer help?

Yes! Particularly, except that high-nitrogen fertilizers can sometimes stimulate excessive new growth, making your tree a target for a flourishing woolly adelgid infestation! This is going to be more of a consideration if you have an Eastern or Carolina hemlock though. Check the needles first for any sign of infestation.

I’d recommend a balanced slow-release fertilizer that contains nitrogen, phosphorus and potassium, such as Jobe’s Evergreen Fertilizer Spikes or those by Miracle-Gro Tree and Shrub plant food spikes. Always follow the manufacturer’s instructions – I’ve seen too many tree owners overdo it and harm their tree. Too much of a good thing!

How to revive a dying hemlock tree – a summary

  1. Keep your hemlock’s soil moist, but never waterlogged. Water through the winter.
  2. Maintain a layer of mulch under your tree at all times
  3. Protect your tree from high winds in winter, and excessive sun in hot summers.
  4. If you’ve got white fuzzy lumps at the base of foliage, or brown flat ‘scales’ on the needles, you’ve got an infestation that needs treated
  5. Consider a balanced slow-release evergreen fertilizer.

One last thing! A very frequently asked question…

Are hemlock trees poisonous?

Hemlock trees are not poisonous. The name ‘hemlock’ was derived from the notorious poisonous plant of the same name, because the tree’s foliage, when crushed, is believed to have a similar scent – even though Tsuga species are biologically not closely related.

References

  1. Pontius JA, Hallett RA, Jenkins JC. Foliar chemistry linked to infestation and susceptibility to hemlock woolly adelgid (Homoptera: Adelgidae). Environmental Entomology. 2006 Feb 1;35(1):112-20.
  2. McClure MS. Diapterobates humeralis (Oribatida: Ceratozetidae): an effective control agent of hemlock woolly adelgid (Homoptera: Adelgidae) in Japan. Environmental Entomology. 1995 Oct 1;24(5):1207-15.
  3. McClure MS. Effects of implanted and injected pesticides and fertilizers on the survival of Adelges tsugae (Homoptera: Adelgidae) and on the growth of Tsuga canadensis. Journal of Economic Entomology. 1992 Apr 1;85(2):468-72.
  4. P McCarty E, Addesso KM. Hemlock woolly adelgid (Hemiptera: Adelgidae) management in forest, landscape, and nursery production. Journal of Insect Science. 2019 Mar;19(2):iez031.

Image attributions:

Michael Montgomery, USDA Forest Service / © Bugwood.org

Eric R. Day, Virginia Polytechnic Institute and State University / © Bugwood.org

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