Defoliation 101

markyscott

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It’s that time of year. Leaves are coming out all over our trees and it’s time to start answering questions about whether or not to cut them all off. So let’s dive in to one of the most misunderstood techniques in bonsai. What does defoliation do and what does it not do? When should we do it? When should we not do it? How do we tell? I’ll share my thoughts about defoliation, at least how I practice the technique in my garden and why I do it. I’m sure that folks will have opinions to share as well. So let’s have a thoughtful discussion - perhaps we’ll all learn something.
 

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What do these things do?

Observation #1
Shoot pruning cuts triggers dormant lateral buds to grow. If the tree is in growth mode, buds will be triggered to activate regardless of whether or not we defoliate. Here is a spring shoot on a Crape Myrtle. It has been pruned to two nodes, but the leaves have not been removed. You can see that the buds the shoot has been cut back to have been activated despite the fact that the leaves are left on.
7C4EDA56-7210-49CE-996B-EA8A11CE7CAA.jpeg
 

markyscott

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Observation #2
Defoliation (without shoot pruning) does not trigger dormant lateral buds to grow. When you leave the growing tip on a shoot, the growing tip will continue to extend, producing younger leaves. Defoliation alone will not trigger growth - it has to be accompanied by shoot pruning if that is the objective. In this picture, you can see a shoot I defoliated several weeks ago, but I left the growing tip. The shoot went right on merrilly extending without activating any of the dormant lateral buds. The leaves the shoot put out immediately after defoliation were smaller, and the younger leaves got bigger and bigger as the shoot regained strength.,
E0888087-B774-47EC-BE6D-3A3627D7EB99.jpeg
 

markyscott

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Having established those definitions and observations, let’s dive in. First, I’ve always viewed the choice to defoliate as based entirely on the species and my objectives. What is my goal and can the tree in front of me handle it? Those are my first thoughts when I’m deciding what technique to employ. I’ll be straight up from the beginning - I almost NEVER completely defoliate my tree. I can think of only one or two reasons that one might make that choice and I’ve only exercised that option a couple of times.

So, I’ll organize try and organize the discussion based on objectives - that’s the way I think. For the purposes of this discussion, let’s assume we’re working with a maple, elm, ficus or crape myrtle - something that can readily tolerate defoliation and will generate a new flush of growth.
 

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I have a a tree that has grown long and leggy. It lacks interior growth. I need to get some back budding in order to have young shoots to cut back to In order to make the tree more compact.

I would not defoliate, leaf thin or shoot prune. I would let the growing tips extend and I would let the branch get strong and healthy. Once the branch is very strong, you can get backbudding. Sometimes it happens without my intervention at all as a healthy branch will just generate back buds. Sometimes you can get back budding when you cut back a strongly growing branch. But I’d let the branch grow until either I had backbudding or until I felt as though it would be highly likely to make them when I cut back.,
 

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I have a tree with a dense outer canopy that has weak interior shoots that I want to keep alive and strengthen.

On such a tree I would either leaf thin or I would defoliate the outer canopy, but I would leave the leaves on the interior growth. Why? Removing leaves from the outer canopy weakens the strongly growing shoots. Leaving them on the weak inner shoots makes them stronger because, with leaves removed from the outer canopy, more light and air can get to the interior. The point is energy balance - weaken the strong and strengthen the weak. Here are some weak interior shoots. I left the leaves on those when I defoliated and cut back the outer canopy. Once they started getting more light, they started growing more strongly. As you can see, this shoot had pretty much stalled. Now that it’s getting more light, it’s triggered new growth on this shoot. This is a good thing - a great thing in fact as you need interior growth to build branch structure. Note that these backbuds appeared on the branch as it was strongly extending - they did not appear BECAUSE I defoliated. I’m defoliating to keep them healthy. Without light they will die.
D22CF6A0-06DB-479E-B784-B54688A59AB3.jpeg
 

markyscott

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I have a mature tree that’s out of balance. I has some stronger areas and weaker areas.

In this case, I’d definitely use defoliation, leaf thinning and shoot pruning to try and balance the energy on the tree. I’d remove leaves from the strong areas and leave them on the weaker areas. Removing foliar mass from the stronger areas will weaken them. Leaving foliar mass on the weaker areas will strengthen them. So on broadleaf trees defoliation is an important technique that you can employ to bring your tree into energy balance.
 

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These last two points are very important. When you remove all of the leaves from all over the tree, you’ve weakened everything uniformly. The strong areas will get weaker. The weaker areas will get weaker. And the interior shoots will get weaker. Some of the weaker areas or interior shoots might die. You’ve just missed an important moment to improve your tree and you may have set it back significantly. Your next opportunity to employ this technique toward these objectives might not be until next year. That’s a waste of time.
 

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I have a tree in development and the leaves are too big. I want them to be smaller.

When I hear this argument, especially for a tree in early development, it always makes me wonder - why do you care? Big leaves mean strong growth and healthy branches. Strong healthy branches are better for developing your tree. For trees in development, I think the desire for small leaves is putting the cart before the horse. Get good branches first. Then worry about small leaves.
 

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I have a refined tree that I’m getting ready for a show. I want to show the tree without leaves or with fresh new leaves for the display.

For this I defoliate as long as the tree is healthy and it’s the right time of season. Definitely. This is one of the exceptions to never completely defoliating your tree.
 

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My tree has some kind of fungal problems and the leaves look diseased.

Never ever defoliate a weak tree. Defoliating a weak tree makes it weaker. If your tree has a disease, treat the problem and let it grow.
 

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My tree has some sort of insect infestation. I need to remove the leaves to get rid of it.

I’d say almost never is this a good reason for defoliation, with one exception. I grow a fair amount of ficus trees. Some of them are extremely large and have thousands of leaves. On one occasion one of them had a very bad scale infestation, but the tree was otherwise strong and healthy. I would normally grab a qtip and scrape them off with rubbing alcohol to get rid of them. But in this case, that exercise would have taken days. Since the tree was a ficus and it was otherwise healthy in my judgement, I removed all the leaves. I didn’t want to, but I felt the risk was small and the time saving was huge. So the leaves went away. But in general, I don’t defoliate trees because of insect damage.
 

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OK - those are the cases I can think of. So you can see, I almost never defoliate entirely unless I’m getting ready for a show (or if I have a giant ficus tree with billions of scale bugs on it). I think it is almost always counter productive, But partial defoliation is REALLY important and a great technique for directing growth and balancing energy on our broadleaf trees. So what have I missed - are there other cases you can think of?
 

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Observation #2
Defoliation (without shoot pruning) does not trigger dormant lateral buds to grow. When you leave the growing tip on a shoot, the growing tip will continue to extend, producing younger leaves. Defoliation alone will not trigger growth - it has to be accompanied by shoot pruning if that is the objective. In this picture, you can see a shoot I defoliated several weeks ago, but I left the growing tip. The shoot went right on merrilly extending without activating any of the dormant lateral buds. The leaves the shoot put out immediately after defoliation were smaller, and the younger leaves got bigger and bigger as the shoot regained strength.,
View attachment 304446
Thank you for this thread Mark as you always come strong with common sense and practical information for all levels of bonsai enthusiasts. I have a question in regards to this post in particular. Very interesting that latent buds will not grow if inner leaves are cut and outer leaves are left. But what is your experience with the opposite? Removing leaves on the outside of a branch but leaving the leaves on the base of the branch? Bjorn Bjorholm posted a recent Bonsai U video where he applied this technique to a Japanese flowering apricot to produce great ramification. He called it Partial Outer Canopy Defoliation. I was curious though if this approach would be useful for other species that are more readily available ie. Elms, maples, crapes etc? Have you used a technique like this on any of your trees and what was the result if you have?
 

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Combination cases where there might be more than one objective!
For example lateral shoot development lower down while continuing apical leader growth!
Some species it is important to develop lower branching prior to continuing with taper and apical growth.
 

markyscott

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Thank you for this thread Mark as you always come strong with common sense and practical information for all levels of bonsai enthusiasts. I have a question in regards to this post in particular. Very interesting that latent buds will not grow if inner leaves are cut and outer leaves are left. But what is your experience with the opposite? Removing leaves on the outside of a branch but leaving the leaves on the base of the branch? Bjorn Bjorholm posted a recent Bonsai U video where he applied this technique to a Japanese flowering apricot to produce great ramification. He called it Partial Outer Canopy Defoliation. I was curious though if this approach would be useful for other species that are more readily available ie. Elms, maples, crapes etc? Have you used a technique like this on any of your trees and what was the result if you have?
Hello Hartinez - I probably described that photo badly. When I defoliated that shoot, I removed ALL the leaves on it, but left just the growing tip. The leaves that are on there now have emerged as the shoot has extended. So all of the leaves have appeared AFTER defoliation. As you can see, no backbudding and none of the older dormant buds on the new shoot were activated. To do that you cut back the shoot. As long as the tree is in growth mode, you’ll get new growth from the spring shoot. This is important - this may or may not produce backbudding. That generally happens on it’s own on strongly extending branches, or when you let a branch extend and get strong and healthy, and then cut it back.

In terms of partial canopy defoliation, yes I’ve heard of it. I think the first time I heard of the phrase was from Bjorn’s teacher Keiji Fujikawa - I took a workshop with him maybe 6 or 7 years ago. But the practice is common and yes I‘ve used it many times this season already. It is what I was trying to describe in post #9. When you have a dense outer canopy with weak interior growth, your objective should be to try and get light and air into the interior. Defoliating the outer canopy while leaving the weak interior growth alone is a great way to do this. That’s what Bjorn refers to as “partial outer-canopy defoliation”. Like any technique in bonsai, it is not blindly applied - there is a reason for it and a goal to be achieved. That’s why I wanted to organize this discussion by goal rather than by technique names.

- Scott
 
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