AusBonsai

At Shibui bonsai I've finished digging and preparing the Root over Rock trees and they're heeled in together waiting potting.
Next on the list are the other tridents.
This is what I like to see when I dig trident maples lots of fine roots right round the trunk.

I don't normally wash the roots but it makes it a bit easier for you to see what is happening down there.
Last year the roots were cut quite short - around 2cm from the trunk. You can now see the result - lots of new white roots growing from the cut end of every root (look for the change in colour) Just as branches above ramify when they are pruned so do roots. Good maple nebari is not made from just a few roots sticking out from the trunk. These dividing and tapering roots will all thicken and merge into that characteristic plate like nebari flowing over the surface of a bonsai pot.

Here's the tree after final pruning both roots and branches. At this stage the tree can go back to the grow bed for further bulk or into a pot to start developing branches and ramification.
And another one from the same row. I've left extra leaders on both of these as usual. Those are insurance policies in case one or another does not continue to grow in spring. after the tree is growing well next summer or the following winter I'll select one with good direction and strength and remove the others.

Some of you will have noted that I do a few things a little differently from many other growers.
1. Quick growing trees like trident maples are dug and root pruned every winter. While general consensus is that yearly digging slows growth I'm not convinced I loose much and, if I do, it is more than made up for in improved nebari. Trees are certainly easier to dig as well because the shovel just has to cut through those finer new roots.
2. Trunks are also pruned regularly. Unpruned trunks grow like telephone poles - thick but with no taper. When they are eventually cut the growers are left with a stump also with no taper and a very large cut to heal over. Yearly pruning gives me lots of leaders which I believe add up to a similar amount of trunk thickening but as the growing points are now located on different sections of the trunk I get more growth low and less up higher leading to far better trunk taper. When pruning I can also select which leaders to cut and which to leave for both taper and trunk movement. I end up with more pruning cuts but each is much smaller and tend to heal far quicker as growth continues.

Apologies for some pictures showing sideways. Ausbonsai seems to be reorienting photos to their original camera position and I haven't found a way to stop that.

Amazing pre-bonsai stock as usual Neil.

Thanks for the info Neil I hope those seedlings I got from you are as good as these in years to come

Thanks Neil, a great snap shot of your methodology for field grown development of trunk and nebari. Clear,concise and the pictures support the method perfectly.

What a great post!!! A million thanks for providing it

Some of you will have noted the horizontal root spread on these trees. Shallow roots are needed to fit a tree into a shallow bonsai pot and a good, spreading nebari is prized in bonsai, especially maples because that's how older trees appear in the landscape.
There are a number of ways of achieving this radial root spread.

Both the trees above have been grown through metal plates to force radial roots. I've read about lots of different variations on this technique and tried most of them. Holes drilled in wall tiles - many tiles broke while drilling and the incredible pressure of growing roots smashed the rest in the ground. Trident roots just crush Cds as well and they are just a bit large for my needs. I've even seen people drilling holes or planting trees over dinner plates
I suspect that some of the odd ideas have come from mistranslations from Japanese or different meanings of English words. Plate means different things to an engineer and a chef.
My 'plates' or 'tiles' are aluminium sheet. Most of them are cut from old roadsigns that I purchased at the scrap yard and cut into suitable sized squares. I also have some smaller stainless steel discs which are also good but its far easier to cut and drill aluminium. I've used these same pieces over and over for the last 15 years or so. Here's step by step for my version this method;

Thread a suitable seedling through the hole. Note that the tree is at an angle. I find that this makes better informal upright bonsai but if you want an upright trunk plant the seedling straight up through the plate. Plant the tree so the metal is a few cm below soil level. Depth is important with tridents because they love to grow new roots just under the surface. Plant too deep and you will get new roots higher above the plate and probably not evenly round the trunk. Also don't push the seedling too far through. If any existing roots are above the plate they will grow stronger and faster than any new roots and spoil the even root spread.
I now plant these threaded trees in pots in the nursery for the first year. As the trunk thickens circulation gets cut off forcing the tree to grow callus and then new emergency roots just above the plate. That usually happens in mid summer. If the soil in the grow beds gets a little dry on a hot day before those new roots grow long enough to reach damp soil below they will die and the tree fails. In the pot the tree gets watered every day to help those new roots get out over the plate and down into deeper soil.

After a year I expect to see something like this: I now have a full radial root system on one level without any tedious pruning. As a bonus the base of the trunk tends to swell giving even better taper to the trunk.

Now roots are cut back to force more root ramification. Prune these roots quite short because we want the roots to start ramifying close to the trunk.
These trees will now be planted in grow pots or in the ground for another year to thicken even more.

Plates like these can also be used to grow multi trunk trees and clumps. Just drill the required number of holes and thread a seedling through each hole. As the trunks grow and thicken they will fuse together at the base. Fusion is faster because of the callus forming when the trunks are constricted by the metal. Remember that multi trunk trees are far more difficult to grow. Each trunk needs to be in harmony with the others and be different thickness and height. When multi trunks are started excess trunks can later be removed. Each trunk will have added size to the base of the final trunk

Do you hold some of the trunks back more than others by pruning hard over the growing season?

Very astute Mel.
It does help to monitor growth of multi trunk trees and intervene to give better size trunks. The ones on the outside tend to have better roots so better access to water and nutrients so they grow faster and thicker while centre trunks stay smaller - exactly the opposite of what a good clump should be. Selective pruning can manage growth to prevent excess thickening in the wrong places. Pruning can also direct growth so the outside trunks grow out, away from the larger centre tree.
All that is much easier when the tree is in a pot. Lying on the ground while checking trunk size and pruning can be uncomfortable.
Often I just let them grow and take the loss for the ones that cannot be saved.

Cheers Neil, that makes sense. I appreciate the effort you go to, both in the field and here on the forum.

Threading trees through some sort of plate is not the only way to develop good nebari.
I also use plain old root pruning to grow good roots on trident maples and many other species.
Here are some photos to illustrate this process:

Start with a typical 1 yo seedling Then cut the roots really short I try to find a spot where there are already a few roots around the trunk at an appropriate level. Cutting the roots really short will encourage your tree to grow more lateral roots near the cut.
Remember that the trunk does not need to be vertical so if roots on one side are a bit lower that's even better.
Cutting the trunk will cause more leaders to grow next year and that will help with both bends in the trunk and taper later. Here's another example. Note the trunk is at an angle so that roots are horizontal. This one is another year on and has developed much better radial roots Again the lateral roots are cut short to encourage further root ramification and any vertical roots are removed completely so the tree will concentrate on the lateral surface roots to develop better nebari. At this stage the tree can go back into a pot for slower, controlled development or into a grow pot or into the ground for faster growth.

Both of these trunks have never had the plates mentioned above. Both developed purely with regular root pruning

Neil , thanks once again for sharing your techniques.