The HBR process was popularized by Boon. Boon Mix soil works very well for bonsai, but transitioning from traditional nursery soil to Boon Mix can lead to problems if not done correctly.
All that BS about the core, and it being the “heartbeat” of the tree is just that, BS. There are usually thick roots there with no feeders. How can that be the “heartbeat”?
Roots serve two purposes for a tree. One, they affix the tree into the soil. “Tap roots” hold the tree in place and support the tree and keep it from being blown over. We don’t need tap roots for bonsai because we tie the trees into the pits using wire. The tree should be tied in very securely, no wiggle at all. A tree that wiggles in the pot will have a difficult time growing new feeder roots as they are extremely tiny and fragile. Wiggle the tree in the soil, and you damage the new feeder roots.
The second purpose of the roots is to take up water and nutrients. This is done by the feeder roots. The mire feeder roots you have, the better the tree can absorb water and nutrients. A solid “shin” with all heavy roots, or compacted soil is just taking up valuable space in the pot that could be filled with feeder roots! You might as well bury a rock in there for all the good it does.
So, an ideal root ball is one that has a radial surface root system which break up into multiple smaller roots which in turn exploded into a mass of fibrous roots with feeders. An open, granular, well draining soil allows for oxygen and water to reach the roots. Every time the tree is watered, as the water passes down thru the soil, air is pulled down into the soil, pulling in oxygen to the roots.
Root rot occurs where anaerobic bacteria and fungus take over. Those live in the absence of oxygen. That means if your soil pulls in air, and oxygen, you won’t get root rot. A solid “shin” that is hard to get fresh water and air into is far more likely to develop root rot because of the lack of oxygen.
So... how do we transition to use a good inorganic bonsai Mix such as Boon Mix?
First off, let’s perform this procedure in the late winter/early spring when most trees will begin emerging from winter dormancy.
Let’s assume we are starting with a typical nursery can. They are usually deeper than a bonsai pot. It’s usually safe to simply saw the bottom half of the existing root ball off entirely. Then reduce the remaining rootball by removing any and all circling roots. All circling roots!
Inspect the bottom of the rootball. Identify the large “tap” roots growing staring down. Use a chop stick to probe around them and dig out the soil around them so you can see if there are any branchlets up higher closer to the trunk. Try to cut the heavy tap roots off shorter if you can.
Using a 3 pronged root fork, scrape across the bottom of the root ball. This should loosen some soil, and help you identify any more heavy, downward growing roots. Cut off the downward growing roots with root pruners or concave pruners. Then trim the remainder of the root ball flat with shears. Try to get it as smooth as possible.
Now set the tree upright on the flat bottom. Using bent tip tweezers and a root hook if necessary, comb the top of the root ball from trunk (nebari) out towards the edge. Scrape in one direction: from the trunk, out radially. Turn the tree around as you do this. You will almost surely find more long circling roots. Cut these off with sharp scissors.
Continue to work around, your goal is to create a slight slope down from the nebari to the edge of the rootball. Trim any upward growing roots off flat with the surface.
Now for the edges: using the bent tip tweezers, remove 3/4 of the soil all around the sides of the rootball. Any roots that extend beyond 3/4 inch, trim off with sharp scissors.
NOW, the HBR begins!
Select one half of the root ball to bare root. If could be any half, one side or the other. Front, back, left or right. I generally choose the weakest half. Using your bent tip tweezers, and root hook, remove as much soil as you can from that half. You will try to leave as many roots intact as possible, just remove the soil. If you need to, you can wash soil off with water, just be careful to leave the other side intact. It will probably require several iterations of washing, picking, washing, picking, washing... try to get all the way under 1/2 of the trunk. You will probably find more heavy tap roots that you can remove as you do this process.
So, when you are done, you have a rootball with soil on one side, with small roots sticking out 3/4 inch. The other side has a skeleton of roots! With lots of thinner roots.
Prepare the new pot, place soil in the bottom, build a mound of new soil, and scrunch the rootball into the mound. Tie it in securely. Now backfill with soil. Use chopsticks to gently push new soil into all the voids between the roots.
On the side that was not bare rooted, remember we left small roots hanging out? These are important. Make sure they get surrounded by the new mix. Use chop sticks to make sure they extend into the new mix.
Fill the pot up to the nebari with new soil. IMPORTANT STEP: with one hand push down on the nebari, with the other hand, tap the side of the pot with the meat of your fist a couple of times. Do all four sides. You will be amazed how this “settles” he soil! You’ll probably have to add more soil on top. Smooth the soil out level. NO mounding! The top of the soil should be about 1/4 inch below the rim of the pot. This assures all the water that hits the soil will stay in the pot and not run off.
Water until the run off water is clear.
Water every day. At first, you may need to water throughly to keep the old side moist as the new soil will drain very quickly. When new feeder roots start to grow, the new soil will still drain well, but not as fast as when brand new. At that time, the difference between the new and old soil is not as great, and you won’t have to worry about keeping the old half watered as much. Also, the tree will start using the HBR side more.
You can HBR the other side the following year, or wait another year if you like.
I have HBR’ed dozens of trees with 100% success.
