Back in 2014, our silver birch trees began to yellow and drop leaves in mid-June! We were worried, and Googling for an answer was next to useless.
Most of the technical advice was North American, and focused either on the Bronze Birch Borer (a beetle that feeds on white birches), or trying to grow trees in arid and/or swampy regions. Our trees had none of the beetle infestation symptoms other than leaf drop, and although the British never shut up about the bloody weather, we were not growing ours in either a desert or a mangrove swamp.
Since I first wrote this article, I have discovered that there are two distinct types of yellowing. You can’t really do much harm if you just apply both remedies, though.
The first type is where the leaves really do turn a bright, canary yellow – just like they do in Autumn – and begin to fall off the tree. This is what we had (it was mid-June), and although the yellowing/leaf drop wasn’t as widespread throughout the tree as it is during Autumn proper, it was worrying all the same. I can’t honestly remember where I found this now, but somewhere in the hundreds and hundreds of forum pages and obscure “ask the expert” sites rattling on about the bloody Birch Borer I came across an idea that made absolute sense, and which can be implemented without calling in David Attenborough and Rentokil.
The gist of the solution is that summer leaf-drop is caused either by a deficiency of nitrogen in the soil, or a deficiency of iron. You can remedy the first using ericaceous fertiliser (for lime-hating plants, which is what birch trees are). It is available from various manufacturers (I use Miracle-Gro, as shown above), and can be bought from most decent garden centres and from many online retailers (including eBay and Amazon, where I get mine). It only costs about £6 a box, and there’s enough to manage a handful of trees for at least a year (you can also get liquid varieties).
Normal fertiliser is no good – it has to be ericaceous – and you just dissolve it and water it in around the tree. Remember that the roots extend outwards quite a long way and you’ll need to cover a wide area. Slow-release granules of the same fertiliser can also be bought which you sprinkle on the ground and water in, and they apparently work for up to 3 months.
The second cause is also dealt with by simply buying some sequestered iron (I use Maxicrop, as shown above). It comes as a liquid, and you can mix it with your fertiliser and water it in all in one go. Plants need iron to produce chlorophyll, and since chlorophyll is why the leaves are green, not being able to produce it means the leaves become less green and take on a yellowish hue.
In our case, after a single application leaf drop stopped immediately in that 2014 season (once the already-dead leaves dropped). The trees even threw out some catkins, which had been absent up until then. In 2015, I started feeding every few weeks from March and we had no leaf drop at all. In 2016, it has been the same so far, and we have very fat catkins hanging from the branches, along with quite significant new growth.
You can also treat your trees with iron sulphate. This replaces iron in the soil, too, but it also acidifies the ground over time, which is good for ericaceous plants.
It’s funny looking back, but I used to think that if you planted a tree you just forgot about it and let it grow. In fact, it turns out you need to look after them almost as much as you would a tomato plant or an ornamental cactus! And judging by the number of hits I’m getting this year, a lot of other people are just discovering the same.
Can you rescue leaves which have turned yellow?
No, probably not. Not the ones which are canary yellow, anyway, since they’re already dead and will have to fall off the tree. How long that takes will vary, and a little wind can speed things up. The important thing is that by feeding the tree you can stop any further yellowing – and believe me, the first time you do it the effects will be quite noticeable.
I would imagine that chlorosis could be reversed if it is caught early, since the yellowing is not due to leaves dying – they’re just iron-deficient. In that case, you might be able to save some yellowed leaves by applying the chelated iron treatment.
Do you have to keep treating the trees?
Yes. If you don’t, the problem just comes back depending on how long the previous treatment lasts for, and that is dependant on how bad the deficiency is, how big your trees are, what else is growing there, and how your soil drains when it rains. Huge trees will suck up all the nutrients, and if you’re raking up and binning the leaves each year nothing gets returned to the soil. It stands to reason, really, but I was as blind to it as anyone else until I encountered the problem.
Treat your trees from March until September. Feed at least once a month.
When do Birch trees normally start to shed their leaves?
The short – and very obvious – answer is: in the Autumn. It can vary a little up and down the country (just as Spring tends to start earlier the further south you are), but in the Midlands they usually start to show sprays of yellow from early October. The leaves will begin to fall from that point – very lightly at first, then increasing as the yellowing spreads.
This year (almost overnight during the first week or so into October, 2016), ours produced a lot of yellow on the inside, whereas the outer canopy is still green – it looks rather nice. The neighbours’ trees have much sparser canopies than ours and they have clumps of yellow all over.
Autumn officially begins in mid- to late-September and you probably can’t do much to fix your trees after August if you’ve got the early yellowing problem. I’d still recommend a good feed or two, but not beyond the end of September.
But be ready to start feeding from March.
How do you apply these treatments?
You make up the required solution as directed on the pack, then water it into the area specified. A single watering can is usually spread over 10 square metres (a medium sized tree probably requires watering over as much as 100 square metres). You can also buy mixer units which have a small tank and connect to your garden hose. You put the concentrate into the tank and the device mixes it with water as you spray the ground under your trees.
Although I haven’t used it yet, I have built an irrigation system which mixes the fertiliser/concentrate and feeds it to a garden sprinkler system. It consists of a venturi mixer and 5L container, and can treat around 30-40 square metres in a single go without any human intervention. I’ll post more next year when I start using it.
My tree is losing branches and twigs
If the tree is weak then it is understandable that twigs and branches might fall off. Once they’re stronger this will stop.
Another likely problem, though, is crows. Yes, the winged variety. From March they will be nest-building, and they are very, very selective in their choice of twigs for the purpose. We get great amusement watching a pair that have nested near us for the last 10 years or so. They will tear off a hundred twigs and drop them until they get the one they want. It’s nature, so we don’t worry.
Mind you, I’ve never seen a crow carry off a branch – they stick to the smaller stuff – so if your tree is dropping large branches you might need to get a tree surgeon in to have a look at it.