Growing in a graceful pyramidal fashion, Douglas firs are popular ornamental and landscaping trees. However, I’ve come across quite a few specimens lately that look sick, have brown parts and are looking bare and thin. The owners often don’t know what to do – can it be saved?
A little close-up inspection can reveal the cause. If you’re seeing your Doug fir declining, I’d love to share my knowledge and experience (and a little bit of science) with you. There’s every chance we can get it looking healthy again.
Douglas firs may turn brown, lose excessive needles or die when subject to severe environmental factors such as drought, winter desiccation or poorly-drained soil. Fungal infections or pest infestations can be diagnosed by careful analysis, such as by the observation of fruiting bodies on needles.
Let’s clear up some things before we start.
Douglas firs, Pseudotsuga, are not ‘true’ firs which have the genus Abies. It was misclassified a number of times; even Pseudotsuga misleadingly implies they’re ‘false hemlock’ (Tsuga). The name is often written ‘Douglasfir’ or ‘Douglas-fir’ to clarify that they don’t strictly belong in the fir family. Think of Douglas firs as their own genus of evergreens.
How to tell if a Douglas fir is healthy
All evergreen trees, including Douglas fir, will see some of their needles go brown and drop off every year. The difference is that in deciduous trees, all foliage drops off – in Douglas fir, it’s just a small proportion each year, so it isn’t so noticeable.
Typically, a healthy Douglas fir will grow symmetrically at a rate of one to two feet per year. Those in forests lose their lower branches gradually (as they don’t get as much light) while those in your yard will generally keep them and assume a Christmas-tree-shaped form (indeed, Douglas fir is one of the biggest names in Christmas trees worldwide.
The darkish green needles on most Pseudotsuga species are 2-3cm long, flat and soft, and should be clear of spots or brown bands when inspected closely. The older needles on the inner parts of the branches, eventually discolor and drop, typically in autumn. This is normally much more evident on the lowest parts of the tree, particularly when shaded out by upper foliage – as they aren’t getting much light and therefore aren’t useful for photosynthesis, and are shed.
If your tree looks like this – it’s happy and doesn’t need much intervention, though it might grow a little faster with some slow-release evergreen fertilizer. It’s important to prevent winter injury and drought and to monitor for infection. Read on!
Why is my Douglas fir turning brown?
Douglas firs typically turn brown due to environmental causes, such as severe drought or winter injury, or infectious causes, which are generally fungal needlecast diseases. Close inspection of the tree can reveal the cause.
Douglas fir water requirements
Could there be a water issue?
In general, Douglas firs can tolerate mild drought, but over hot dry spells of prolonged drought it has been well-recognized that they can go through subsequent die-back – that they can be fragile enough in dry weather ¹. This is of particular concern to the lumbar industry, where certain dry years have had a major impact. It’s not just the dry soil, but the scorching effect of sun that goes with it, which desiccates the needles.
If the tree’s getting thirsty, it tends to shed more of its inner needles than usual as a defense mechanism against further water loss. Usually, the tree will look brown on the inside, and the outer needles will look somewhat brittle – but the outer needles typically will stay green unless the tree’s totally parched (usually a container-grown Douglas fir that’s not been watered).
On the other hand, a Douglas fir specifically won’t thrive when it’s overwatered either – or on boggy, marshy, poorly-drained soil. They’re not as intolerant as some evergreens to this state (such as yew) but excessive browning and needle drop (coupled with poor growth) can also be seen in such Pseudotsuga specimens.
Douglas fir soil requirements
Is your soil the problem?
Douglas firs grow best on acidic soils (pH 5-6.5) but will tolerate neutral soils (pH 7). They’re intolerant of chalky soil, but will otherwise grow well on clay, sand or loam. Do lumps of chalk ever emerge when you’re digging on your property?
A good gardener knows his soil, right? Now’s the time to test the pH of your soil if you haven’t already. If you happen to have alkaline soil, that might explain why your tree’s not doing well.
I use the Rapitest soil testing kit. It’s easy and straightforward. Remember though that some properties are finished with the application of a few inches of topsoil, but this may be a different pH to the native soil underneath. Dig a hole nearby a foot deep and take some soil here for testing.
Frequently, soil on newish properties has been heavily compacted by machinery during the building phase. This compounds the issue of poorly-drained soil, which Douglas firs struggle with. Often gardeners don’t loosen a sufficient border of soil when transplanting a Doug fir from a container, instead just digging a container-shaped hole and plopping it in. This can create an underground basin for water to collect in, making things worse.
Douglas fir sun requirements
Douglas firs grow well in full sun, i.e. on a south-facing aspect if you’re living in the Northern Hemisphere, but they’ll tolerate some shade. 4 hours of sunlight a day will be adequate according to Arbor Day. Pseudotsuga seedlings in particular can struggle if there’s too much shade.
In forest settings, Douglas firs tend to shed their lower needles (and then the lower branches themselves) and put on more height to compete with their neighbors for sunlight – but this won’t do if you’re aiming for a pleasant ornamental Christmas tree-shaped specimen. If I was planting a Douglas fir today, I’d be looking at the sunniest part of my property.
If your tree isn’t getting much sun, it may explain a brown or bare appearance – particularly if you’re seeing these changes on the most shaded parts of the tree – (usually the lowest or innermost aspects, or parts that are shaded by fencing or other trees).
A particularly common cause of sick-looking Douglas firs is winter injury – also known as winter burn or winter desiccation.
This means browning and needle loss from the tree, often asymmetrical and becoming apparent in late winter or early spring, caused by moisture loss from the needles through the winter. It most often affects young trees, which have a less established root system.
As evergreens keep their foliage, they’re susceptible to the drying effects of sunlight and strong winds throughout winter – and the roots have to draw up water to replace it. However, tree owners don’t often water their trees in winter (I get it… it’s freezing out there), and the ground might even be frozen, so the roots struggle to replenish the moisture loss, resulting in needles turning brown and falling off.
The browning is often apparent on the tips of branches (brown needles should normally only be seen much further in and at the very bottom of the tree).
When one side of a Douglas fir tree’s turning brown, this often corresponds to a) the side that’s exposed to the strongest winds or b) the side that gets the strongest sun or c) a side of the tree that’s getting sprayed by road salt – this significantly increases the chance of winter burn.
Inspect any brown or fallen outer needles close up. If you can’t see any spots or lines on them, it’s probably winter injury. Doug firs usually make it through winter injury once they’re past the seedling stage – but repeated injury each year will affect the vigor and appearance of your tree in a more long-lasting way, especially if it’s only affecting one side of the foliage.
Treating and preventing winter injury in Douglas fir
This mostly comes down to protection against the elements and adequate watering through the winter.
As a rule of thumb, Douglas firs normally need to be watered once every 1-2 weeks over the warmer months and every month over the cooler months.
Give it a 30-minute soak around the drip-line (the area on the ground that corresponds to the edge of the canopy above). Avoid blasting the trunk with a hose. Avoid watering when the temperature’s below freezing though – freezing the roots will make things worse.
Of course, it does depend if you’ve had a lot of wet weather recently. It’s easy to test if your tree’s thirsty – use a spade to make a 1-2 inch slit in the ground near the base of your tree, and touch the soil with your finger. If it feels dry (or doesn’t stick to your finger), water it.
An oft-neglected element of watering is mulch application. Practically all trees should have a layer of mulch, such as organic bark chippings, several feet wide around the tree base. This mimics the forest floor, allowing water to permeate and suppressing weeds and grass that compete for moisture.
Mulch also insulates the roots against temperature drops, preventing root freeze that accelerates winter injury.
If you’re seeing browning and needle loss on the wind-exposed side, consider a bit of basic wind protection over winter. This can be done simply enough by stretching burlap or hessian between two stakes on the exposed side.
Anti-desiccant sprays are commercially available and used to spray small trees – coating their needles and preventing as much transpiration (water loss) during the winter. It’s a cheap and straightforward method of limiting winter injury – but only if your tree’s of a small size and you can reach every part that you need to.
The effects of recent planting
Has your Douglas fir been planted or moved in the past 3 years?
‘Transplant shock’ may be a term you’ve heard of. It’s a bit of a dramatic term, but it refers to the process trees can go through when their roots are thrust into a new environment and expected to struggle on as before.
Planting often means a) some of the roots are cut, torn or damaged b) a change in water availability c) a new soil type and pH. Landscaping projects are often undertaken in the summer – even though it’s not the best time to plant a tree unless you’re committed to a very earnest watering regime.
Young Douglas firs undergoing transplant shock can go completely brown and lose all the needles – particularly if planted and summer and the ground’s allowed to dry out. More often you’ll see excessive inner needle loss in autumn, and sometimes browning of needles at the tips of branches or at the top of the tree.
If it’s been planted within a year, this is a really likely cause for your douglas fir looking sick and you should do two things:
- If you haven’t been watering it frequently, start – see the water section above
- Get some mulch applied (about 4 inches thick) around the base of the tree, to help water permeate. Be generous in the spread – ideally, a ring of mulch should be 5 feet wide. Just avoid piling it up against the trunk, where it can cause rot – leave a slight gap to prevent it from touching.
The best time to plant Douglas fir trees is between October and April when the soil is cool and moist. This allows time for root establishment before the summer growing season.
Douglas fir pests and diseases
Like other evergreens, there are many pests and diseases that affect Douglas fir – but which are the ones that you’re actually likely to come across? Which actually cause issues and make your Doug fir sick? I hope to simplify it for you.
Generally, the main diseases that cause browning and excessive defoliation of Douglas fir can be divided into
- Fungal needlecast infections – chiefly Swiss needlecast and Rhabdocline needlecast
- Root rot diseases such as Phytophthora
- Insect pests – particularly Douglas fir beetles
Fungal needlecast infections of Douglas fir
Swiss needlecast and rhabdocline needlecast are both fungal diseases that cause excessive needle drop of inner needles (usually sparing the newest needles at branch tips).
Swiss needlecast is common wherever Douglas fir is found worldwide ². Rhabdocline is most common in the United States, but has spread to Europe ³. Both are primarily a cosmetic concern for ordinary tree owners (but they’re a big issue for Christmas tree farms). Badly infected trees can lose all their needles (especially on the lower branches) apart from the current year’s growth, right at the ends.
Both types of infections display tiny marks on infected needles before they fall. This is best seen with a microscope but you should be able to have a look with a magnifying glass. Rhabdocline needlecast causes yellow or brown spots on needles that appear in the springtime, while Swiss needlecast display rows of black dots – known as ‘fruiting bodies’ along the infected needles in spring and early summer.
These diseases will rarely kill a Douglas fir, but repeated bouts of infection year after year will leave it looking very poorly indeed – and the lack of needles limits the tree’s ability to photosynthesize and put on growth, leaving a stunted tree.
Management of Douglas fir needlecast
Like most fungal infections, cure is not the aim here – rather prevention and control.
- If your trees are crowded, consider thinning them out to improve airflow between the trees, helping the needles to stay dry (fungal spores love to multiply in moist environments).
- Fungicidal sprays can be applied when new foliage has recently opened and at specific intervals thereafter – sometimes just one more application a few weeks later will do, but you’ll need to repeat the treatment in subsequent years. Such methods on a larger scale are used by Christmas tree farmers, but I’d suggest contacting an arborist for advice on this.
Root Rot in Douglas fir
Phytophthora root rot is common in other evergreens as well as Douglas fir and again is a problem for Christmas tree growers as well as the lumber industry. It’s found all over the world.
In particular, Douglas firs that are already struggling in poorly-drained, wet soil are susceptible to this mold-like organism, that often infects a tree for years before displaying symptoms – typically browning of some whole branches before others, before the whole tree becomes brown and begins to shed its needles.
It’s a difficult one to diagnose without actually digging the tree up and looking at the root. However, suspect this if the whole tree’s turning brown branch by branch and it hasn’t been subjected to severe drought or recently transplanted. Fungicides don’t work, so call an arborist. They may recommend removing the tree (and burning it to get rid of the spores). And you’ll need advice if you want to replant anything in the same spot.
Insect pests in Douglas fir
Douglas fir beetles are found in the United States. They’re particularly threatening as they can cause death of large trees. Initially, trees will thin out, particularly at the crown, then go brown and lose their needles.
Douglas fir beetle’s the most common threat. The beetles or their larvae can sometimes be directly identified on the tree – but there are characteristic bark patterns displaying their burrowing habits that are diagnostic.
I had a hunt online and found this to be a good video if you’d like to know more – from the Oregon Department of Forestry. It should equip you to check your tree for bark beetle infestation.
Will fertilizer help?
Yes, some fertilizer’s a good idea if your tree’s not looking healthy. I’d always recommend a slow-release evergreen fertilizer that has a balanced composition of nitrogen, phosphorus and potassium – such as Jobe’s Evergreen fertilizer spikes or Miracle-Gro Tree and Shrub Plant food spikes. Super easy to administer, and difficult to get wrong.
How to revive a Douglas fir tree
I hope you’ve found this article helpful. Let’s draw together some of what we’ve explored and make a short revival guide for ailing Pseudotsuga.
- Use the guide above to diagnose the issue first – if there are brown or dropping needles, which time of year are you observing them and do the needles have spots, dots or lines on them?
- Make sure there’s a good layer of mulch right over the root zone
- Water every one to two weeks and monthly through the winter – but avoid making the ground boggy!
- Consider providing some protection in winter from strong winds, salt spray and consider an anti-desiccant
- Apply a slow-release fertilizer so the tree has the nutrients it needs to regain its vigor.
Thanks for reading – I hope it was helpful! Please have a look at my latest posts on my homepage.
- Sergent AS, Rozenberg P, Bréda N. Douglas-fir is vulnerable to exceptional and recurrent drought episodes and recovers less well on less fertile sites. Annals of Forest Science. 2014 Sep;71(6):697-708.
- Kimberley MO, Hood IA, Knowles RL. Impact of Swiss needle-cast on growth of Douglas-fir. Phytopathology. 2011 May;101(5):583-93.
- Samek M, Novotný P, Modlinger R, Fulín M, Beran F, Roy A, Pešková V. Impact of Rhabdocline pseudotsugae and Phaeocryptopus gaeumannii on the selection of suitable provenances of Douglas fir in Central Europe. Forests. 2019 Feb 26;10(3):204.