A Driving Instructor's Blog

We’re coming close the end of the season now. It’s not too late to start dealing with premature leaf drop – you can boost your tree’s health before the Autumn ready for next year.

Our Silver Birch in June 2017I originally wrote this article back in 2014. At that time, our tree began to produce a lot of yellow leaves in mid-June, and after a lot of research I managed to figure out the cause and remedy, which was what this article was about.

However, 2018 was the hottest year on record. The heat lasted for months, and it introduced another problem (which may or may not have been a factor back in 2014 without me realising it) that affected pretty much every tree in the country. Heat stress.

The article becomes popular each year, but things kicked off much earlier in 2019, and I suspect that this was down to a combination of the after-effects of the 2018 drought and a relatively dry and mild Winter. As a result, people began to see problems much earlier because their trees were already on the back foot.

We also had a lot of green fly in 2019, which might also have been caused by the mild winter. In Spring and early Summer they were merrily chowing down on the new leaves on my tree. I bought some Ladybird larvae and released them into the canopy, and they seemed to do the trick.

But to summarise the subject of premature yellowing and leaf drop in Silver Birches over the last few years, it can occur for the following reasons:

  • nutrient deficiency
  • iron deficiency
  • lack of water and heat stress
  • manganese deficiency

It’s easy to get all of them at once.

When I first experienced yellowing back in 2014, I was worried. I thought my tree was dying. Googling for an answer was next to useless, since most of the technical advice is North American and focuses on the Bronze Birch Borer (a beetle that feeds on white birches), or of the perils of trying to grow trees in either deserts or swamps (neither of which Birches are particularly fond of). And that was only from the experts. The general tree-growing public was quite prepared to believe that it was an alien conspiracy, and was more than prepared to defend that view.

Our tree had none of the beetle infestation symptoms other than leaf drop (the Bronze Birch Borer isn’t known outside of North America, so don’t worry about it), and although the British never shut up about the bloody weather, we were not growing ours in anything other than normally-drained British garden soil, where it had been happily and vigorously growing for the previous 15 years.

I discovered that yellowing/leaf drop is usually caused by deficiencies of nutrients and/or iron in the soil. I also discovered, in 2018, that lack of moisture and prolonged high air temperatures can lead to heat stress, which birches are highly susceptible to (I wrote a separate article about it in August 2018).

Note that none of these problems are confined to Silver Birches. All trees and plants can be affected by nutrient deficiencies or heat stress. And you simply have to deal with the problem using the appropriate, easily purchased treatments.

The original 2014 problem was characterised by bright, canary yellow leaves – just like in the Autumn. Not as widespread throughout the canopy, but still enough for the tree to start shedding them. Somewhere in amongst the interminable forum pages and obscure “ask the expert” sites rattling on about the bloody Birch Borer I came across a single one-line comment that made absolute sense, and which could be implemented without calling in David Attenborough and Rentokil.

Birches favour a slightly acidic soil – they are sometimes referred to as ericaceous (lime-hating). Therefore, you can replace nutrients and fix any issued with nitrogen deficiency using ericaceous fertiliser. For several years, I used the Miracle-Gro solid version, which is available from garden centres and online (including eBay and Amazon (above), where I used to get mine). It’s not expensive (you often get multi-pack deals), and there’s enough in one box to manage a small-medium sized tree for at least half a season. You can also get liquid varieties, such as the one manufactured by Doff (above), which I’ve switched to since 2018 because of my new irrigator toy; and slow-release granules, which work for up to three months, and which are great for treating small areas (I use them in my containers of Blueberries, which are also ericaceous).

Normal fertiliser is no good for birches – it has to be the ericaceous stuff – and you just mix it with water and spread it around the tree. The slow-release granules are sprinkled on the ground and watered in (I just wish they’d make the damned things in camouflage green, instead of the “hey, look at me all over the lawn” multicoloured mix they actually are).

Leaves suffering from chlorosisIron deficiency causes leaves to look like those in the images here. It’s known as chlorosis.

Leaves are usually green because they contain chlorophyll. Simplifying the subject, chlorophyll is what allows plants to convert light energy into sugars that they can use as food through the process called photosynthesis. Chlorophyll is green, and it contains iron. So if there isn’t enough iron in the soil the tree can’t make chlorophyll, and you get yellow leaves – and the tree compensates for being hungry by going into shutdown and shedding them.

Chlorosis is resolved using sequestered (or chelated) iron, such as Maxicrop (above). It’s a seaweed extract, so perfectly natural, and it comes as a liquid. You can mix it in with your fertiliser and water it in at the same time. I buy it in commercial 10L containers, which is more cost-effective than buying it in 1L bottles. It stains like hell, so be careful not to drop any on pathways and decking (it’s OK when it’s diluted, though).

Close-up of leaves suffering from chlorosisAnother symptom of soil nutrient deficiency is that new leaves may be small and misshapen, instead of the classic Birch leaf shape. Some of ours were like that in that first season.

A single application of fertiliser stopped the leaf drop almost immediately once the already-dead leaves had fallen. The tree even threw out some large catkins, which had been absent up until then. Since 2014, I start feeding every few weeks from the beginning of March with both fertiliser and iron. We had no more problems until 2018.

Close-up of our tree's leaves in June 2017To get iron into the soil, you can also water-in iron (ferrous) sulphate periodically. It also has the advantage of gradually  acidifying the soil, which might be useful if yours is a bit too alkaline. Iron sulphate is also a superb moss killer and grass greener – my lawn loves it.

This brand – all of them are by TradeFarm NI – is free flowing and stable as long as it is kept dry. Be wary buying cheaper brands which are ‘damp’ crystals, because they go off very quickly.

Why does nutrient deficiency occur? Well, bear in mind that when trees and plants die back in winter in the wild, the leaves they shed decompose and return nutrients to the soil as they do so. In urban gardens leaves are usually swept up and taken to the tip to keep the garden looking tidy. That means the soil becomes depleted of those nutrients over time and you get problems like this. I used to think that all you did was plant a tree and watch it grow, but I know now that you have to look after them like any other plant in your garden.

You have to keep these treatments going at least once a month between March and September, and you have to follow the same routine each year, or at least over alternate years. If you don’t, the problem will come back at some point.

Now we come to the extremely hot summer of 2018. Around the end of June that year, I once again noticed a few sprays of yellow appearing. I briefly wondered what was going on, but I guessed right away it might be linked to the prolonged high temperatures and low rainfall we’d experienced up until then. After Googling it I concluded my tree was, indeed, suffering from heat stress. The solution to this is to get water down to the roots – it’s called deep watering – but that’s easier said than done.

One way of doing it is to use deep watering spikes. These are tapered tubes that are hammered into the ground around the tree, and into which water is fed slowly so that it gets to the roots deep down. I didn’t have time for that (with the ground as dry as it was, it’d have been like trying to hammer a nail into plate steel), so I went for the longer-term sprinkler method. Every night, I set the sprinkler going and watered for a couple of hours in each of several zones to ensure even saturation. We didn’t have any hosepipe restrictions, and I wouldn’t have continued if we had. This allowed water to seep down deep into the hard soil, and it fixed the problem in less than a week. It also turned a completely brown lawn into a lush green carpet.

With hindsight, all trees in 2018 had much thinner canopies than usual. The leaves on my own trees were smaller than they were in previous years, but about a week after starting deep watering the birch produced some new shoots and the leaves that appeared were much larger. And some very fat catkins also appeared.

As a result of the heat stress problem, and still needing to keep the other treatments going, I have now invested in a combined watering/fertilising system, which I have written about separately. I can highly recommend that device – the Access Irrigation Static Dilutor.

Can you rescue leaves which have turned yellow?

No, probably not. I suppose that chlorosis could be reversed if you caught it early enough, but if the leaf is dead and the tree has triggered its shedding mechanism, you’re going to lose them.

The important thing is that by feeding and watering the tree you can stop any further yellowing – and believe me, the first time you do it the effects will be quite noticeable within a short time.

Do you have to keep treating the trees?

Yes. If you don’t, the problem just comes back once the tree has used up what you’ve fed it, especially if you bin the leaves again the following autumn. Huge trees will suck up all the nutrients and water, and if you’re raking up and binning the leaves each year (or if the soil is dry and there are no prolonged periods of rain) nothing gets returned to the soil.

How often should you feed?

Treat them once or twice a month from March until September. And water regularly.

Can heat and drought cause them to lose leaves?

Yes. If they are stressed you may get them dropping leaves. In extreme cases the leaves can go brown and the tree can even die. It’s a good idea to water them deeply during hot, dry periods. Once or twice a week should be enough, though more frequently won’t hurt if the dry period is prolonged.

Remember that after a period of drought (or prolonged dry weather) it needs an extended period of rain to wet the soil again, especially deep down. A few heavy downpours won’t do it, and you will still need to help things along.

Will a Birch recover from drought?

It depends on whether the drought killed it or not. A reader wrote to me in 2018, mentioning that his tree had lost its leaves, and I advised that the only thing he could do right then was to feed and water – and hope for the best. He wrote to me in 2019 to tell me the tree had started to rock in the wind, and that a tree surgeon had subsequently declared it dead, and had had to remove it. Apparently, the roots were rotten.

There’s no way of knowing if it was just the drought that did the damage. The tree may have been weakened by not feeding and watering over previous years, and the drought was just the final nail in the coffin. But the 2018 heatwave certainly caused problems.

Is there any other way to deal with the problem?

You have to get nutrients and iron back into the soil. And you need water in order for the roots to be able to access those nutrients. 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?

In the Autumn! In the UK, the onset varies up and down the country, but it usually starts here from the end of September into October. It often seems triggered by a noticeable drop in night time temperatures. The leaves will begin to fall from that point – very lightly at first, then increasing as the yellowing spreads.

Why do birch trees drop leaves so early?

They don’t. They drop them in Autumn, like all other trees which shed their leaves each year. If yours is turning early, you may have a problem.

How do you apply these treatments?

You make up the required solution as directed on the pack, then water it into the area specified. I use a combined watering/fertilising system, which I have written about separately. However, you can use a watering can and hosepipe/sprinkler as necessary. Note that if the ground is dry, a watering can won’t get the nutrients down to the roots, so a heavy watering is essential.

Why are fallen leaves sticky?

You’ve probably got greenfly! Specifically, the birch aphid, Euceraphis betulae. They feed on the European Birch, Betula pendula, and they increase in number during warm and dry weather – which is what we have right now (and did have for most of the winter). Aphids secrete honeydew as they feed, and that’s the sticky stuff you’re seeing. Apparently, you can get different species of greenfly that feed on specific trees.

You can kill them with a soap/water mixture, though no one has ever been able to tell me precisely how you apply that to a 20 metre high tree. And the same goes for any chemical method relying on direct contact. An alternative solution is to introduce predatory insects – something that eats aphids. The best one is the Ladybird larva, and you can buy them online. There are other predatory insects you can buy, too.

My tree is losing branches and twigs

If the tree is weak then it is understandable that twigs and small branches might fall off. Once they’re stronger this will stop. In any case, if it is windy, a few dead twigs are bound to fall off. It’s just nature – and birches also have a fungus which can cause small twigs to die and fall.

Early in the year, another likely problem is crows (the winged variety). From March (February in 2019) they will be nest-building, and they are very, very selective in their choice of twigs for the purpose. We get them nesting near us, and 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.

Why do Silver Birches drop so many twigs?

As I said above, crows (and similar birds). As of February/March they are usually actively nest building.

When do birches start to show leaves?

In spring, obviously, but the precise date varies depending on both the tree and the weather. In 2019, they were about a month earlier than 2018 in the UK. Mine is usually showing leaves sometime during April each year.

I’ve got catkins but no leaves

Someone found the site in April 2018 with that query. You’ll probably find that in a couple of weeks you’ll have lots of leaves. As I have said in this article, I start feeding mine from March onwards. Leaves start sprouting a week or two earlier than my neighbours’ trees, and the foliage on mine is usually much denser. The catkins often come before the leaves.

Are the leaves changing early this year?

This was a generic search term used to find the blog in mid-July 2017. The short answer is no, they are not – not in July, anyway. They change towards the end of September in the UK.

Do Weeping Silver Birches lose their leaves in Autumn?

Yes.

When do Silver Birch leaves go all brown?

They don’t. The leaves should go yellow and fall off in the autumn.

I had quite a few visitors from this search term in 2018, and when I looked it up it seems that extreme cases of chlorosis and heat stress can result in leaves turning brown (see this supplementary article). It could also be a disease or infestation which you could treat, but the tree itself might also be dead – especially if it has been having any of the problems I mentioned above over previous years. Best to call in the experts.

Does this advice only apply to Silver Birch trees?

No. Chlorosis can affect many plants, and lack of nutrients is a universal issue. You might need a different fertiliser to address any nutrient problem, but iron will likely fix chlorosis. Lack of water can kill virtually any plant.

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