Repotting season is here.

It is still officially winter here at Shibui Bonsai but a few warmer nights has stimulated some buds to start growing so I’ve been stimulated to get on with the repotting.

Yesterday I root pruned and repotted all the small shohin sized bonsai. I’ve found that these little ones do better if they are repotted every year. The roots grow fast enough to completely fill a tiny pot in just one summer so the following year it becomes difficult for water and air to penetrate into the root zone and the trees start to suffer.

Look how tight the new roots are on this one

After pruning the roots there is now room for more roots to grow this summer.

Here’s another shohin sized trident maple. Again, the pot is crammed with roots so I need to remove some to make room for the new roots this summer.

I root prune these little ones just like the larger trees. First cut around the edge of the root ball to remove all the matted new roots that are circling the pot.

Then slice off the mat of roots underneath.

Followed by a rake out of the remaining root ball if needed and maybe sip off any more longer roots. then the tree can go back into the original pot with some fresh potting mix to fill the spaces and give somewhere for new roots to grow.

Developing better nebari

Starting seedlings through holes in metal plates is not the only way to grow good nebari. I also grow many trees just by regular root pruning to control and direct root growth. Here are some photos to show that method.

Seedling before

Most seedlings develop a strong vertical root to ensure the little tree has access to deeper damp soil so it can survive a dry summer. In bonsai we don’t want deep roots we prefer to see well spread strong lateral surface roots. First step is to cut the vertical root. I usually look for a spot where there are several lateral roots around the trunk and cut just below.

Often I need to also remove any single higher roots. If I leave those to grow they tend to grow faster and create lopsided root system. The remaining lateral roots don’t have to be right opposite. In fact it can be better if roots on one side are higher so the seedling can be planted at an angle with remaining roots horizontal. Initial leaning trunk tends to yield a better informal upright tree.

Another seedling before
and after pruning roots

I also cut the trunk shorter to help push multiple leaders the following summer. Multiple leaders give me better options for bends in the trunk and better taper.

These trimmed seedlings are then potted into pots for a year’s growth. After a year I should end up with something like this small trident. Note that this is not the same tree as above but it is representative of tridents after initial root pruning as shown above.

In the second winter the roots are trimmed again. Note how well the tree has responded to the previous radical root pruning with a mass of lateral roots around the area I cut last winter.

roots after trimming a second time
Again, down growing roots are eliminated

At this stage the tree can go back into a grow pot for controlled development or into the ground for faster growth. The following tree was started using this root pruning technique followed by 2 years in the grow beds. Tridents in the grow beds are dug and root pruned every winter at Shibui Bonsai

More field grown trident maples

In the last post I showed a couple of the trident maples I’m currently digging and pruning.

You may have noticed the very horizontal root spread.

These tridents have been developed using the ‘plate’ method.

Start with a piece of metal sheet with a hole drilled through the middle.

Mine have been cut from old aluminium roadsigns I bought at the scrap metal yard but any sheet metal will do. Some growers use wall or floor tiles but they can be difficult to drill and I found that growing trident roots exert enormous pressure and broke the tiles. CDs are also crushed by growing roots but I’ve used these pieces over and over for nearly 10 years now with excellent results.

I start with seedlings. Thread a seedling through the hole.

Don’t go too far through. if any roots are above the metal one usually grows far quicker and gives an unbalanced root structure.

Note that I’ve placed the trunk on an angle through the sheet. If you want a vertical trunk stand yours upright but make the choice now because next year it will be too late to make changes.

Plant the tree with its metal collar about 1cm under the soil. Too deep an you’re likely to get more roots than you need and they may not be evenly spread.

As the trunks grow fatter the metal starts to cut off circulation in the tree. The tree responds  by producing callus and then new roots just above the restricted area. The only option for those new roots is to grow out over the metal in a flat plane. Eventually circulation to the roots under the metal is completely cut off and those roots will die off leaving your tree with a new, flat root system.

I find it far better to keep these trees in pots in the nursery for the first year. In the ground, new roots are forming just as the heat of summer hits and if they don’t make it over the plate and down into deeper soil there is a risk that they may dry out on hot days. Daily watering ensures they will grow and survive until, after a year, the tree looks something like this

Here’s a closer shot of the new roots above the metal plate.

Now those new roots above the metal are pruned quite short.

Then the tree can be planted back into a grow pot or into the ground to thicken up. remember not to plant too deep. Tridents love to grow new roots right near the surface so if these roots are buried deep the tree will grow new ones that may not be as good.

Another year of growth and I now have a thickened trident with developing horizontal nebari. A side benefit is that the base of the trunks tends to swell and buttress far more than with other techniques. You can see that there’s no real advantage in using larger plates. To get good root ramification the roots need to be pruned quite short each year and larger sheets just make it harder for the new roots to reach moisture deeper in the soil.

Another year or 2 and I have the sort of tree shown at the start of this post.

I find that threading trident seedlings is an easy way to produce superior root spread on trident maples but I have also used the same technique on Japanese maples and even on Japanese black pines. It should work with most tree species and I’d be interested to hear from growers who have used it on other trees.

Field grown trident maples

At Shibui Bonsai I have finished digging the new root over rock trees so it is time to get on with the rest of the trident maples.

here’s one of the tridents I dug up this week.

It seems that I use a few different techniques when growing trees in the ground for bonsai

Let’s focus on the roots for a start. I dig most deciduous trees every winter and prune the roots. I know that many other growers claim this slows down development and maybe it does but the little I lose, if any, in size is more than made up for in nebari.

Just so it is easier to see, I’ve washed the roots in water and done a rough trim.

Last year the roots were cut very short – about 2cm from the trunk. You can now see the result. Lots of new, white roots have grown from the cut ends. These new roots will thicken and gradually merge together to form the characteristic plate like nebari in coming years. Those new roots will now be cut short again, probably 1/2 cm this time and, of course, each one will grow a number of new roots again. Just as branch ramification is valued above ground, Root ramification below ground should be the aim of good bonsai growers.

Now both roots and branches have been shortened ready to go back into the ground or into a pot for further development.

Here’s another trident maple after trimming both roots and branches.

You may note that I’ve left a few extra leaders on both these trees. I remove any that I think are growing in the wrong direction or in poor positions but I generally leave several possible new possible leaders as insurance. When the tree is growing well next summer I can make final decisions about which trunk line to follow and the others will be pruned then.