I am a bonsai newbie with a variegated Ficus benjamina about 10' and a 2 1/4" trunk diameter. I think it will make a nice bonsai subject. If I want it to be about 18" to 20" tall should I cut the trunk at about 6" above the soil line? I have had this tree for about 20 years and I don't want to lose it. I'm a bit nervous about chopping 9' off this tree, leaving only a trunk - no branches, no leaves!

I can see why you would be nervous...that is a long term investment in a tree. It would be nice to see a picture of the tree from a couple of different angles. I am sure there are those here who can help you with the decision. I hope they reply.

I'm working on the photos, but having trouble uploading from my phone. That's a subject for a different forum :S

The trunk has a nice root flare.





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The trunk is fairly straight and has almost no taper, so a trunk chop would make sense. It's your choice, obviously.

I think you have your measurements wrong, or my understanding of you sizes are wrong. It would be great if you could use international standards for sizes. It looks like you say the trunk is 6cm thick? Looks a lot thinner.

If you repot in a larger pot, and in spring trim back to a few leaves, you may get budding lower down. I have a benjamina which we redus by a metre every year and this one pops a few buds on the trunk afterwards. Ours is in a 40liter container though. The small pot is the reason it stayed so modestly sized.

We Americans cling stubbornly to our "English" units of measure, which even England wisely gave up long ago. The circumference (distance around) just above the root flare is 14.8 cm, which means the diameter (thickness) of the trunk is about 4.7 cm, or about 2 inches.
I decided to chop off the top half of the tree if only to make it easier to move into the house. I expect some leaf drop. The tree already had a bare spot about half way up, so this made sense as a first step. My goal is to make it bonsai as I described earlier. I'm still uncertain when and where to make the next pruning cuts.

wait till spring, then cut it off about, three or four inches up ( 8 or 10 cm for those that have only one measuring system)from the bottom.
Until then look up "trunking", and study bonsai technique, A good rule, is you have an inch of thickness at the base of the trunk, to every foot in height, the really nice ones are thicker though.

Samantha wrote: three or four inches up ( 8 or 10 cm for those that have only one measuring system)

Technically, the standard is metres. This can be impractical, however, (0.08-0.1m) so mm should be used for smaller objects, seeing as cm shouldn't exist except when measuring volume (1cm^3 = 1ml) as the x10^3 scale goes mm, m, km, Mm, etc.
We generally ask you to convert inches to centimetres or millimetres because it makes it easier for people outside of the USA, Myanmar and Liberia to understand.

Surely it isn't too hard to do?

Samantha wrote: A good rule, is you have an inch of thickness at the base of the trunk, to every foot in height, the really nice ones are thicker though.

For most bonsai, a ratio of 1:6 or at most 1:12 between trunk thickness and tree height is used. So for every cm of trunk thickness, the tree would have 6-12 cm of height, from soil level to heighest point. That would typically give the best result.

If you are sure it is almost 5 cm, the picture is deceiving.
Do keep in mind that Ficus does not really heal wounds well, except for in tropical environments. I have figs with cuts made years ago that only show a mm or two of growth over the cuts.