A Driving Instructor's Blog

Our Silver Birch in June 2017This is getting a lot of hits this year (2017).

Back in 2014, our silver birch tree 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 yellowing can be caused by two different things. You can’t really do much harm if you just apply both remedies, though.

The type we had was where the leaves really do turn a bright, canary yellow – just like they do in Autumn – and begin to fall off the tree. It started in 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 two ideas that made absolute sense, and which can be implemented without calling in David Attenborough and Rentokil.Fertiliser and Iron Treatments

Summer leaf-drop and leaf yellowing is caused either by a deficiency of nitrogen in the soil, or a deficiency of iron (or a combination of the two). You can remedy nitrogen deficiency 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 and slow-release varieties).

Bear in mind that 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. The slow-release granules of the same fertiliser are just sprinkled on the ground and watered in, and they apparently work for up to 3 months.

Leaves suffering from chlorosisLeaves that look like the images here are probably suffering from iron deficiency – known as chlorosis. This is easily dealt with by buying some sequestered (or chelated) 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, especially when lit from behind.

Close-up of leaves suffering from chlorosisNote also that you may see new leaves to be small and misshapen, instead of the classic Birch leaf shape, if you have a soil nutrient deficiency.

In our case, after a single application of fertiliser treatment, leaf drop stopped immediately in that 2014 season (after the already-dead leaves dropped). The tree even threw out some catkins, which had been absent up until then. In 2015, I started feeding every few weeks from March with both fertiliser and iron and we had no leaf drop at all. In 2016, it was the same, with very fat catkins hanging from the branches, along with quite significant new growth. As of May 2017 our tree is putting out a lot of new leaves and has lots of catkins. The photograph at the top is our tree as it looks in June 2017. And here’s a close-up of the leaves.

Close-up of our tree's leaves in June 2017Does that look healthy, or what? And we had a serious case of summer leaf drop in 2014!

You can also treat the area around 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 also good for your lawn.

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.

Let me stress: you have to keep the treatment going about once a month between March and September. And as a final footnote, be aware that in very hot weather or during drought trees can become stressed and a few leaves my yellow and drop. This isn’t too much of a problem, and a weekly deep-watering is all that’s required (hosepipe bans notwithstanding).

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. However, if not treated then the leaves do die and fall off easily once they are predominantly yellow.

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 dependent 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 (and water regularly in hot weather anyway, as they do need moisture).

Can heat and drought cause them to lose leaves?

Yes. If they are stressed you may get them dropping a few leaves. It’s a good idea to water them deeply during hot, dry periods. Once a week should be enough.

Is there any other way to deal with the problem?

You have to get nutrients and iron back into the soil. Yes, you could use your own mulch or bought compost, but obviously this is not so attractive in a normal garden (removing it is what got you here in the first place). It would also take longer to have an effect. But it would still work, given time.

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.

In 2016 (almost overnight during the first week or so into October), ours produced a lot of yellow on the inside, whereas the outer canopy remained green – it looked rather nice. The neighbours’ trees had much sparser canopies than ours and they had 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.

What I do is make up a concentrate in a 5L drum, then use 1L of that in a 15L watering can topped up with water. It’s quicker this way. I put 14 large spoonfuls of Miracle-gro and about 250mls of Maxicrop in the drum (marked with 1L divisions) and make up to 5L with tepid water. Once dissolved, I just pour out 1L into my watering can, top up from the garden hose, and evenly spread it over about 10-20 square metres, making sure I include flower beds as well as the lawn.

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.

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