In this garden‑based instructional video, Harry Harrington explains why calcium is essential for bonsai health and shares his personal experiments transitioning from hard tap water to rainwater, supplementing with seaweed, CalMag, and calcium products. He describes how the change to rainwater eliminated limescale and chlorine, initially boosting colour and vigor, but later led to stunted growth in several native species (elm, hawthorn, cedar, etc.). By adding calcium—first to a struggling dogwood (Cornus) and later via surface applications and a liquid silicon CalMag product—he observed rapid shoot extensions and improved overall vigor across many species. He stresses the need to balance calcium with magnesium, warns about pH‑raising calcium products for acid‑loving plants (azaleas, Acer palmatum), and provides a species‑specific recommendation list. The video also showcases an air‑layered large elm taken from a hedge, turned on its side, and planted in a half‑meter‑wide tub, with plans to develop it into a bonsai by 2026 after further growth and branch thickening. Throughout, Harry combines practical observations, product usage, and horticultural reasoning to guide bonsai enthusiasts on calcium management.
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Hello, my name is Harry Harrington, and today I'm speaking to you from my garden here in Aylesbury. Now, this video is all about calcium, um, why we need it, why we need to make sure our bonsai receive it on a regular basis, and what happened with me when I started switching from our hards water, tap water that we have around here. I moved over to rainwater, which is a fantastic step forward because you don't have limescale, all the chemicals that come from tap water. But I started to notice that my trees weren't growing in exactly the way that I wanted them to. And the reason I'm standing in front of this Amazing big elm in the garden is just to is to make a video about calcite just a little more interesting than it might be. So this really large elm was was actually taken from high up on a hedge by apprentice Sean Stolpe. I think he has to. home ladders to get up there, put the air layer in place. In fact, he would have layered it in this area here. And then if I, uh, show you an image of the tree when I first received it from Sean, you can see that it was actually at quite a different angle. And I realized that the best way of using the material would be to, to turn it on its side and turn it into a raft. As you can see in the following images.
So the tree was turned anti-clockwise onto its side and the root ball combed out so the tree could be planted really nice and flat on top of a—
in a very big pot.
As you can see, it really looked very, very impressive, and just put my hand in there for scale, it's a big trunk.
The tree has now been here growing in the garden at the foot of the very large willow tree that we have here, and it's settled into a large half-meter-wide tub. I want to allow plenty of extensions to pick up the initial shoots and create some, some branches of an appropriate size for Such a big trunk. So going back to calcium, when I stopped using tap water, I also started using a lot of seaweed, which is very useful because it contains magnesium, which is something that helps build cell walls within plants and makes a more robust cell wall structure. And I found really within half a season that everything greened up. We didn't have the chlorine, I didn't have the hards, I didn't have the limescale that hard water produces. And the seaweed, as I say, produced beautiful colors in all my trees. And there were a lot of positive effects. But as time went on over the next 2 or 3 years, I noticed there were a few species that just weren't growing in the way that they used to. They would, um, not extend. And this particularly was happening with my natives, my elm, my hawthorn, and there were a few other species like cedar that just weren't happy about something, and I couldn't put my finger on it. So late last year, noticing that one of my dogwoods, my Cornus species, were getting really, really slow and actually starting to die back. And realizing that this Cornus as a species really enjoys a chalky, high calcium environment or soil, I added some calcium to the soil surface, and literally within 2 weeks, the dogwood responded with huge extensions, something that hadn't happened for literally 3 or 4 years, and I realized that, you know, this was— this something was happening here, and it was a lack of calcium. This year I applied calcium to the soil surface, and I started using CalMag, which is a liquid silicon product, essentially giving calcium to the soil of all of my K-natives, just to start to balance the calcium and magnesium levels in each of the trees. And growth rates, particularly in hawthorn, elm, um, prunus, particularly the mahaleb cherries, cedar, have all vastly improved. And I'm getting some really good extensions. The extensions are kept in check, and they're really well distributed thanks to the, the Dan-O products that I'm also using, the bacteria that I'm using. But the calcium has just given a lot of my trees a really, really good boost. Now, my understanding is calcium and its role is slowly building up, and I understand that calcium actually also contributes hugely to cell wall structure along with magnesium. And we need that balance of calcium and magnesium in our trees, in our fertilizers, to get the very, very best growth habits and health in our trees. Now, one should be wary of using some calcium products that can raise the pH and be wary of applying them to azalea species such as Azalea that are acid-loving. Ace palmatum like to have more of a neutral pH and don't like too much calcium. So you need to do a little research, but certainly those UK natives will really, really benefit well. And I'll put across a list of all the species that I would recommend adding calcium to at the bottom. Bottom of this video. If you look at the growth in the first couple of years from these old images, it's really quite insipid. It's not strong and coarse as I need and I would like. I want growth on this tree this year, having applied the calcium and used CalMag through the season, essentially just keeping the levels of calcium up. Along with the magnesium and fertilizer, I'm just fantastic, fantastic extensions of nearly six foot, of nearly a meter in many directions. I have pruned this tree back as well, just to redirect some of the energy. But I'm getting some some fantastic growth. Hopefully after next season's growth and thickening of the branches, the tree should be able to go quite comfortably into a large bonsai pot in 2026, and I'll update you then.
