I would not pile soil around the trunk. Instead, make a 1 meter high ring around the trunk with whatever you're using for the terraces so that there is space around the trunk without soil.

It is very likely to rot. It's not recommended that you add more than three INCHES of soil, let alone three feet. That is an extreme form of root disturbance, believe me.

I would plant a new hibiscus in that vicinity after the new soil has settled for about a year or so. By then, the established tree will be in serious decline.

It is not the trunk you should be worried about, but the ROOT SYSYEM. Of course, the trunk will be negatively affected by three feet of soil but if the entire root system is buried, you can kiss your tree bye bye.

Tree well construction needs to be done carefully, with the actual root system in mind, not the trunk! I think that tree wells look funny, odd, out of place, and never encourage their use.

This post was edited by rhizo_1 on Sun, Jan 11, 15 at 6:09

i think of it this way ...

roots are made to go in the ground and cope with moisture ... trunks are not .... though they may get wet.. they dry rather quickly ... ergo.. as noted.. they rot ..

second.. roots need air as much as soil ... burying them an extra 3 feet deep .... defeats such ...

remove it .... and start with a fresh plant ...

ken

laurietennant: clearly you want to keep the existing hibiscus, so I think the tree well is worth a try. How it looks is a matter of personal taste. You do need to allow for sufficient space around the trunk. It would also help if the depth you're planning to add were reduced. Do you really need a full meter or would half of that or even less be enough for what you would like to plant in the terraced areas? (That's assuming the terraces will be for plants. If instead this is more a landscaping project with a full meter necessary to do what you want, much changes). A half meter or less over some of the hibiscus roots will not drown them or smother them if you use coarse fast draining material like bark or bark compost mixture with or without grit or sand. Do not use soil. It is too dense and not porous enough to allow oxygen to reach the root zone.

can you give us a latin ID on the plant ...

if we can actually ID the plant.. there may be alternatives ...

are you talking about the common rose of sharon???

where are you???

there is no plant.. that cant be cut back..and lifted to the proper height... and then pruned back into shape .... perhaps we can step out of your box of a 9 foot tall plant ... and offer other options ....

of course.. we dont dig things like this up.. in the middle of the heat of summer .....

ken

The work required to keep the tree alive (a dry tree well) seems like a lot more than the work required to replant. And, even with a dry tree well, it's not a guarantee that it would survive. For me, the ONLY option would be to remove the hibiscus and replant it at its new level. It's just a no-brainer, IMO.

That's just about what I was going to write. The effort digging up the Hibiscus and replanting it is way less work than trying to accommodate it in a new terrace without moving it.
Why do you want to terrace it in the first place?
I do understand that some people like their landscape flat and level and are not comfortable unless it is. Usually no valid reason given.
Mike

laurie,
Brandon is right.
Dig it up in spring, or if you can do it now where you live, and replant after the beds are formed.
I have never dug up a hibiscus and replanted one that large, but if you are careful, and mulch well, and water it in good, it should be fine.
Then it will look better too, in the terraced beds, and if it dies, which I doubt, you could always replace it.
Is it a tropical?
Go buy some Great Start by Miracle Grow at any home center store, and use that after replanting.
That stuff is wonderful for replanting all kinds of plants and trees and shrubs.