Back on March 7th I posted a blog on my large hawthorn that needed repotting. You may recall the scarcity of roots on such a large tree four years after collection. Here’s what I had to work with:
Not much in the root zone, eh? Hawthorns are a bit peculiar in that they don’t necessarily root as vigorously as the top growth on the tree might suggest. This was is a classic example of the phenomenon. But regardless of how vigorously your tree roots, it’s always advisable to repot every second to fourth year (I don’t like going beyond three). This is because the soil tends to “wear out” with repeated watering and fertilizing, and it’s good to find out if anything is going on beneath the surface you need to know about.
When I repotted this tree I did something I’ve never done before: I placed a layer of pea gravel in the bottom of the pot to provide better drainage in that lowest strata of the root zone. As you probably already know, drainage in a container that is less deep than it is wide has physics stacked against it. Head pressure, or the force of the water pressing down in the pot, causes it to drain at a certain speed; the more head, the faster the drainage. As the container empties, the speed of drainage slows simply because the amount of water available to press down on what’s below is severely reduced. Drainage slows to a crawl as that last eighth to quarter-inch is all that’s left. What this means for a bonsai is, the roots in the very bottom of the pot tend to stay wet and fail to get enough oxygen. Root death occurs most readily in this zone, for this reason. So, by putting the pea gravel in this area my hope is to reduce the normal holdup you’d expect a standard bonsai soil to provide (which exacerbates the wetness by preventing drainage of that last bit of water). To be sure, I anticipate roots will grow down into the pea gravel layer; what’s unknown at this time is what condition they’ll be in when I pull the tree at its next repotting.
The growth density was fairly consistent between the first (lowest) branch and those in the upper part of the tree. So my goal was to both lighten the density as well as do directional pruning. At this point in the tree’s life as a bonsai, my work is focused on building the secondary and tertiary branch structure. Given that it’s a larger tree, this does take more time since the primary branches need to be proportionately thicker than on a smaller tree in order to make them believable relative to the trunk thickness.
Here’s the result after pruning. The tree will continue to grow, which I’ll allow for another four or five weeks before doing any more trimming. Remember, don’t keep your trees “show ready” all the time, meaning don’t pinch every new shoot that appears and starts to extend. In order to encourage robust health, let your trees grow out unhindered for a time and then prune back relatively hard. Otherwise, the tree can weaken over time and become more susceptible to pests and diseases.
Finally, the lowest left branch is my thread-grafted branch, which I believe can be set on its own next year by cutting the supply branch on the right-hand side of the trunk. A close examination of the collar looked very promising. You may notice that its growth density is not quite up to par with the other branches. This is simply due to the limited moisture and nutrient supply through the sapwood caused by the restriction on the supply branch, still connected, and the fact that the new supply through the left hand side of the trunk into the new layers of sapwood on that side has not yet caught up. So my strategy is simply to let the thread-grafted branch run wild, to build more supply and layers of sapwood.
All comments are welcome. Let me know what you think.