Orchard People

Classic Episode Vertical Mulching
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Mulch Beats Fertilizer
Susan: What's the best way to feed a fruit tree? If you're a home grower or small-scale orchardist, the answer may surprise you.
The best way to feed fruit trees naturally is not by using fertilizers from the garden center. It's actually by using something so simple: compost or wood chips or another type of natural and organic mulch.
These products, these mulches are so good for the soil. They feed the soil. They support beneficial organisms.
They help to keep moisture in the soil, and over the long run, they'll help to keep your trees really healthy.
But what if your soil is so rock solid and so hard and so compressed? Can mulch still help your fruit trees then?
In today's episode, we're going to talk about something called vertical mulching. Just imagine drilling holes into your soil
and then inserting into the holes good quality compost and [00:01:00] worms, and then letting nature do the work.
That's what we're gonna talk about in this classic Orchard People episode today. It was originally recorded in 2020, and my guest was Dr. Glynn Percival, senior arboricultural research manager at Bartlett Tree Experts in the UK. He explains how vertical mulching can help boost tree health and why you might wanna try it with your fruit trees.
Why Compacted Soil Hurts
Susan: I started off by asking Glynn how you know right away that your soil is actually compressed,
Glynn: If you don't like digging it, trees don't like growing in it. So just go out there with your spade or fork, and if you're having issues turning the soil over, then that's a really good indication that the soil's compact.
Susan: What about worms? If you're growing vegetables, you do expect to see worms in your soil. What about, in a general site where you're thinking of planting a tree? Should you have worms there?
Glynn: We have to remember one important thing. Soil is a living, breathing [00:02:00] organism. And as you say, it is, in essence, alive.
So a real quick way of determining how fertile the soil is the general rule is for every spadeful of soil, you should have between four to six worms. If you're turning the soil over and there's no worms in it, it is a really good indicator that there is something wrong with the soil, i.e. it could possibly be contaminated, very low fertility.
It could be compact.
Susan: You've planted your tree in the soil that may be less than perfect. How will it actually affect the tree? Is it simply a problem with the roots not being able to push through?
Glynn: There are two major problems with a soil that is compacted. One is what we would term physical impedance to root growth. The roots simply cannot push through the soil. You can't get that nice, big spread of root system we want.
And the other issue is, because the soil is so compact, you've [00:03:00] squeezed all the oxygen out of it, or most of the oxygen. And obviously, roots are alive, and they need oxygen to breathe. And, without that oxygen, or with low levels of oxygen, it's gonna influence how they grow. Just as it would us. The oxygen levels go down, and we struggle quite badly. So does any living organism. So those are the two main issues that we face.
Spotting Tree Stress Signs
Susan: What will a tree look like if it's struggling and it doesn't have enough oxygen? It doesn't have enough nutrients? The soil?
Glynn: You will have a sparse canopy. You'll see it influenced by premature leaf drop. The leaves will be paler, maybe slightly necrotic, as in they start to go brown around the peripheral edges.
One of the techniques I always like to use is most healthy trees, irrespective of species, should grow at least 10 centimeters or four inches a year. So I [00:04:00] always measure the new growth the tree has put on at the end of the growing season. I know it will differ between species, but really, if you're having less than four inches, then it really is indicating the tree's not very happy.
Fruit Trees Need TLC
Susan: So most of the people who are listening to the show today, they grow fruit trees. And fruit trees are expected to grow way more than four inches a year. There is a big expectation that it be minimum 12 inches, maybe 24 inches, depending if it's a cherry tree or a plum tree.
How are fruit trees different in terms of what they need in terms of soil?
Glynn: They tend to really need potentially more TLC. They need
the wood chip layer of mulch and the biochar.
These are excellent soil amendments we can use to really improve fertility. They maybe need higher inputs of types of fertilizers, whether they're [00:05:00] organic or natural, compared more to like trees grown in urban landscapes.
They are designed, as I say, to produce fruit, so sometimes you tend to find maybe their stress tolerance, their disease resilience, isn't quite as great as maybe some of our more hardier trees that would see growing in like urban landscapes, such as streets and car parks.
Why Vertical Mulching
Susan: We know that compressed soil is not a good thing. Tell me what happened with you. How did you start your study?
Glynn: Compressed soil in urban landscapes has always been a major issue.
We have pedestrians, we have traffic, we have lots and lots of infrastructure. And it's probably one of the major, I would say, potentially killers of trees in urban landscape. I've seen people planting trees and they're using pickaxes and crowbars to try and get a hole in the ground.
And I would say, if you're [00:06:00] planting trees into that level of compaction, they're really not going to survive, irrespective of species. And, it was really just constantly seeing, go into these new building developments, go into construction sites, and even a lot of supermarket car parks have lots of trees, and they all look awful.
They're all struggling, and it's just this issue of compaction. So that's what really brought me into that area. And then we started to look at potential technologies that we could use to really overcome compaction. What works, what doesn't work. Because sometimes you can get some systems that really do work, but they're incredibly expensive.
And of course, time and money then become factors, so it's a case of we'd love to use this technology, but it's too expensive, so maybe we can start looking at other ones. Which is really where the vertical mulching came into [00:07:00] play because that is actually a relatively cheap and inexpensive way of decompacting soils.
Vertical Mulching Basics
Susan: We talk about mulching our fruit trees. We do circles around the fruit trees, expand the circles out to the edge of the canopy to make sure that we can feed the roots. So there's no grass around our fruit trees up until the edge of the canopy.
We put down beautiful compost. We might put wood chips on top. And that's mulching from the top. So maybe that's horizontal mulching. We'll call that horizontal mulching. But what is vertical mulching, and how is it different?
Glynn: All we do is we take a core of soil out of the ground before any large scale planting. As I always say, I teach a lot of arborists, and the first thing they do, is look up. They always look into the canopy. And I'm like, "No, you don't." 90% of all tree-related [00:08:00] problems are caused by problems below ground, i.e. the soil.
You get the soil right, and the trees will do wonderfully well. So really, given the fact that in many cases, the vertical mulching is where we simply would do a soil analysis. We would work out whether we do have a compact soil. We would send it away for analysis for its nutrient composition. We'd maybe look at its drainage.
And if the soil really is very poor, we simply take out cores of soil. Just take it away, and then we backfill.
We buy soil in the UK called John Innes. It's a very high quality topsoil. We would mix it with a really good kind of tree compost.
We would put some organic matter in there. Really something that the roots are going to flourish and thrive in. And that's really about it. There's many [00:09:00] types of formulation you can use once you've taken the bad soil away. But we just find a good quality topsoil mixed in with compost and some organic matter.
If, for example, the soil is very alkaline, if the pH is too high, that would be a good time to add some of these pH amendments such as like iron sulfate, something like that, just to lower the pH. So really, that's it in a nutshell, just taking cores out at fixed distances under the canopy and either replacing it with a really good quality soil. Or if the soil you have is reasonable, you could then amend that soil.
And a great additive you could use is biochar that you mentioned earlier, and I'm a huge fan of biochar. I think it's a great soil amendment, and we've had some really excellent results using it. So I always look at addition of biochar into soils through vertical [00:10:00] mulching to either the existing soil or to a fresh soil we've made up.
Tools and Root Concerns
Susan: You talk about taking these core samples. So describe what that process looks like. Do you need a fancy machine? Obviously with a shovel you couldn't really make a long, skinny hole without damaging the roots of your tree. So describe what are these core samples?
Glynn: There's two pieces of equipment you can use. One, if you do have the money, you can buy what we call a mechanical auger. It's in essence like a very large drill. It makes holes. You only need to go three inches wide, and we normally make them down to something like maybe 12 to 15 inches below ground.
You can either use a mechanical auger, or there is another implement you can buy.
It's called a Dutch auger. I don't know if you've ever heard of it. In essence, it really is like a [00:11:00] giant corkscrew, and you can just twist it round and round. It is slower, but at the same time, it's far more inexpensive. So if you have a small area, it makes it economically feasible.
And the Dutch auger will last for years. So in some instance, we will just come in with a Dutch auger and start taking cores out, doing it that way.
At the same time, there has been a lot of criticism of the vertical mulching because people will say, it damages the root system.
That little bit of short-term damage is more than compensated by the long-term gains because if you don't alleviate the compaction, the tree will not magically get better. It will just go into a slow and steady decline and it will eventually die, and you'll end up removing it.
So that short-term damage, I'm sure [00:12:00] there'll be people out there who will disagree, but I would always say, to me, it's worth it. Alleviate the compaction, and that little bit of damage to the root system is more than justified.
Susan: You were saying you would do a three-inch hole, a number of them, under the canopy of your fruit tree or your native tree, in periodical little spots.
Glynn: Yeah.
Susan: And you would put good soil and compost, a mix of good quality ingredients in there, and that hole would be 12 inches deep.
Glynn: There's really no hard and fast rules here. It's just that we know the bulk of tree root systems are in the top 60 centimeters of soil. So by going down 12 to 15 inches, you are going to get a good portion of the root systems. But any, even if you were only going down 2 to 3 inches, would be beneficial.
Stockley Park Trial
Susan: Let's talk about Stockley Park. That's where you did your experiment, and vertical mulching has been around for [00:13:00] a while, I understand.
But what you did was something different. So tell us the story.
Glynn: Stockley Park is a business park, and it's located on the peripheral edges of Heathrow Airport.
It's this huge, beautifully, heavily manicured landscape with thousands of trees and hundreds of thousands of shrubs and fountains, lots of pedestrians and traffic. But the dilemma is they have huge issues with compaction. So all these thousands of trees are, what we say, they are surviving, not thriving.
And I mentioned growth rates. So on average, they were growing maybe two centimeters a year. So in 10 years, they will grow 20 centimeters, which is about 4 or 5 inches. It's really not much.
So the problem was that there was such a vast expanse of area that decompacting everything [00:14:00] was just impossible. We couldn't do it. So we sat down and we thought, can we have an alternative approach? And what happens if we have something like, say, a 500 square meter area with trees and landscaping? It's where people come out and they sit under the trees for the shade.
We can't decompact it all, but we can't afford to replace our trees. So I said, "Why don't we take pockets, like islands, within that area, say 20 square meters, and why don't we decompact it in such a way that, in theory, once we've decompacted it, we can add things to the soil that will slowly move out and decompact the surrounding area?"
And, of course, in nature, what decompacts soil in nature? It's worms. So we started to think about, why don't we start looking at adding worms? Put them into these 20 [00:15:00] square meter pockets, these little islands, make sure the soil is really well decompacted, we've got lots of organic matter in there.
And then in theory, once the worms establish, they're gonna start to push out. They're gonna move from that decompacted area. And the hope was that with time, these little islands, the worms would push out and actually meet up. So that's exactly what we did. And it was a great trial site because it allowed us to look at a whole range of soil amendments, such as biochar, such as slow-release fertilizers, such as organic fertilizers.
And another thing we did, which I think you mentioned earlier, was it also allowed us to do what we call a green cover crop, where we literally decompacted the soil and then sowed like clover. And then of course, later on in winter when the clover dies, it's fixed all the nitrogen. [00:16:00] So again, even then we were trying to look at ways and means of really letting nature do the work. Decompact the soil and let nature do the work. And the reason why we went with the vertical mulching is because- it's actually very quick. There are other technologies to decompact, and one of them's called air spading, where you use compressed air. But that is ... It does a better job, but it's far more time-consuming.
And what we found with vertical mulching is, we could do 8 to 10 square meters of vertical mulching in the time it took to do one using an air spade. So I'm not saying don't use an air spade, 'cause it really is a great tool. This was just a niche situation where, you literally had thousands of square meters to decompact.
So that was the premise behind it. And what we found over time is those worms are [00:17:00] doing exactly what we wanted them to do. They are pushing out, and that heavily compacted soil is slowly, over time, this experiment's been running for three years, we want it to go for at least another two, but they are definitely pushing out. The trees are just looking so much better.
As you mentioned, you referred to my PowerPoint, where we have some really great photographs of trees that are in that decompacted, worm-amended soil versus trees that are still in the compacted soil.
Worms and Hole Spacing
Susan: I wanna clarify, so however many square meters in that space, 5% of the square meters will be the holes that you're making.
Glynn: Yes.
Susan: 5%, let's say. Now, you're making the holes, and you will fill the holes with a mix of soil and compost and whatever appropriate amendments.
Are you actually putting the worms into those [00:18:00] holes?
Glynn: In some cases, we do. We actually add the worms. In other cases, and I know it sounds absolutely incredible, but the worms we actually use, they were native British worms. I couldn't believe they existed, but we actually bought them off the internet, and I didn't realize you could buy worms.
But you can 'cause they use worms for composting food waste. They use tiger worms. But these are specifically designed to decompact soil. So really we just bought these boxes of worms, and some we added directly to the hole, and in other cases, we made a bigger hole, amended the soil, and just simply opened the box, gently put the box in the hole, and then just covered it up.
Honestly, there's no real rocket science here. I sometimes feel a bit fraudulent as a scientist saying it's really that easy. We are just drilling holes and taking away the bad soil and [00:19:00] replenishing with a good soil and adding worms. Now, obviously, the worms in Canada, I don't know what the native worms are in Canada, but they will be different from the UK.
But in my situation, we just simply, as I say, literally got them off the internet and did exactly what you've just said.
Susan: Okay, so you've got a site. A large site. You've got your box of worms. Are you expecting each worm to move for miles and miles? Or is your big hole quite close to the small holes, the vertical mulching holes?
Because they've got terrible soil to be working through. How are they gonna do it? Even if you've got 100 of them in a hole, they might just end up squirming around and hanging around each other and not going anywhere.
Glynn: Yeah. No, that's a great point. That's why I was really keen to emphasize the holes really have to be maximum 50 centimeters apart, which is half [00:20:00] a meter, one and a half feet.
If you make the holes too far apart, then potentially the worms will exhaust all those resources. And, if you've got too many worms in too small a hole or too great a distance, and they're not going to make it. So spacing is critical. So 30 to 50 centimeters apart, 3 inches deep. Go down, as I was saying, 12 to 15 inches and then add a few worms.
And that's what gave us really good results. Obviously, over time, we may play around with the spacings, but we know the system we have works really well, so we're staying with that. And as I say, we'll tweak it over time.
Measuring the Results
Susan: So Glynn, I wonder how exactly you were able to measure the results at Stockley Park.
How did you know? Was it just by looking at the trees and you saw, okay, one tree looks a little happier than the other, [00:21:00] or was there something a little more, in terms of record keeping that you were able to record?
Glynn: Soil is a living, breathing organism.
So one of the things you can do is literally measure how much oxygen and/or carbon dioxide the soil gives off. So we literally looked at the decompacted soil and the amount of carbon dioxide it was producing, and then we looked at the compacted soil. And then over time we could start to take these values.
So we really looked at below ground and then we knew things were happening 'cause we were starting to see worm casts coming above ground, so we knew that was good. And then the soil that we hadn't treated, we would send it away to look for a nutrient analysis. So this is all the science-y stuff we did below ground.
And then also the tree itself, we could measure leaf color. We can [00:22:00] measure how green. There are various instruments like a leaf chlorophyll content meter, which sounds very geeky and science-y, but you really just clip a leaf and press a button, and it tells you how green, how much chlorophyll there is. So we could measure how much chlorophyll it was putting on.
We did more things like look at how much photosynthesis, et cetera.
And then what we also did, because there's a lot of interest in trees for their functional value, as in how much pollution are they absorbing? How much shade are they producing to protect from ultraviolet light?
So we did a lot more of these measurements as well 'cause importantly, we have to remember that, sadly, there's obviously money needs to be behind this and the people at Stockley Park wanted to see a benefit. So we were looking at all these other hidden benefits and saying, "Oh, by the way, you now have, your canopy has increased by 20 to 30%."
And they're like, [00:23:00] "so?" And I'm saying, that now means you are now, basically, neutralizing three or four cars. Or you're neutralizing a bus with all the pollution it's giving off.
So we did lots of these types of measurements. We really wanted to show how much carbon the tree was sequestering.
So again, that's important at the moment in terms with climate change, and trees are being looked at for carbon sequestration. So we could say, "Look, our treatment is giving you all these extra functional values." So that's really what we did. We did lots of the basics, the size increase, the canopy increase, the girth increase, and then how much more pollution, carbon dioxide, how much more dust it was filtering, how much ultraviolet light protection the tree was giving.
Susan: It's interesting because in your slide, there's so much detail. And there's one picture that's [00:24:00] really striking, and where you show four trees in a row, and one says control, and it's a small tree. It looks small. And the treated one is huge. So it looks to me like the treated trees actually get bigger and grow more.
Glynn: We were seeing like 30 to 40% increases in canopy size. We were expecting some benefits, but we were really like, wow. Because when we first set this project up, we just really had no idea it was going to work. So every so often, and you go along the lines with an experiment, and a lot of experiments I do tend to be what we call busts.
They don't really show that much. But this was great. We really saw some excellent benefits from it and visually are quite striking.
Royalty Tree Tale
Susan: Now, we have just a few more minutes left, and listeners to the show, I don't know if you guys realize this, Glynn is actually a very special person, and he [00:25:00] has met royalty in his work. So Glynn, can you tell me how it is that you hobnob with royalty and what kind of interactions you've had?
Glynn: I'm very lucky. I do look after the trees, or I help manage and advise on the trees at some of the royal households, and it's quite an honor and a privilege. Do you want me to quickly tell the story?
Susan: Yes. Yes, please.
Glynn: Okay. When I first met the Queen, and I was in the palace. And there are two rules when you're in the palace, is no mobile phones and no walking on the ornamental lawns.
My mobile phone went off, it was the health and safety officer. So I walk right into the middle of the lawn with the mobile phone. Out of nowhere, these two rather feral corgis came and were ripping away at the bottom of my chainsaw trousers.
And, we have a saying in the UK. I was on the verge of sticking the boot in, and I was just about to kick the...
Which I didn't, because then [00:26:00] I heard this voice and it said, "Come here. Come here. Come here, you."
And there was the Queen of England walking straight towards me, and I'm in her ornamental lawn on the mobile phone, and she's getting closer and closer, and I could not turn the mobile phone off. It wouldn't turn, and I'm trying to say to the health and safety officer, "I've got to go. I've got the Queen of England walking towards me."
And he's saying, "Yes, of course you have." I said, "No, I have."
And she's six feet away from me when I managed to turn the phone off, and she just stopped and she smiled. And just smiled so sweetly and just said, "There was no need to hang up on my account. It might have been somebody important," which I thought was truly wonderful.
Susan: Oh, that's so adorable. That is very sweet. Aw. That is such a great story. Glynn, thank you so much for being with me on the show today. I love having you on the show. You're gonna have to do some more research on something else amazing because we gotta get you back.
Glynn: I would love to come back on the show. Yeah. It's, been an absolute pleasure, and I [00:27:00] will carry on doing lots more research.
Wrap Up and Resources
Susan: I hope you enjoyed this episode with Dr. Glyn Percival. And if you found this conversation interesting, go back and listen to a couple of earlier episodes on mulching fruit trees. The first is episode 36 from 2018 called Willow Mulch for Fruit Trees with Glynn. He explains why willow mulch is so powerful when it comes to boosting tree health.
Then check out episode 31 from 2018 called Mulching Fruit Trees: Which Mulches Are Best? with Professor David Granetstein. Together, we explore the pros and cons of different mulch materials and how to choose the right one for your fruit trees.
In the meantime, if you wanna learn more about caring for your fruit trees naturally, you can also check out my book,
Grow Fruit Trees Fast. You'll find it on Amazon, and it's a quick read, and it'll teach you the essentials of fruit tree care in about an hour.
Thanks so [00:28:00] much for tuning in, and I'll see you next time.
Happy growing!