Root cuttings will often grow no matter what you do to them. you'll see that if you make the mistake of digging near a Chinese elm tree or other species prone to root suckering. I've even had some grow after planting the roots upside down but generally best results from planting the roots vertical with around 1/2" to 1" of the top (part which was closest to the tree) above soil level.
Some pics of Chinese elm root cuttings:
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Even quite thick roots will strike if you have them and that's a good start on your next Chinese elm trunk!
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Root cuttings planted. These are thicker so I have a little more than usual above soil level.
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12 months later:
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The 2 long shoots have grown from the top of the root cutting and roots from the bottom. What was once a piece of root is not the first part of the new trunk.
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look close to see the join between old root and new shoot (quite low this time). The difference in thickness gives a good start to trunk taper.
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A couple of years later:
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And after a few more years:
This one is English elm but similar response to Chinese elm. You can probably pick out the change in diameter from the original root to the new part of the trunk. This one has also grown a new, thinner branch part way down the original root - bonus.
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A couple more really small Chinese elms grown from root cuttings:
Start with a straight piece of root and keep several of the new shoots for broom style
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Use a long, bendy root for a literati style shohin Chinese elm
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