Can you limit tree root growth by trimming regularly?
I have planted two Prunus nigra in my garden. It is a small walled urban garden bounded by other properties on all sides. I had understood that these would only grow to around 6m, but I now understand that it could be 10-12m and they are fairly close to buildings. Is this likely to cause me problems and would it be advisable to dig up before they become to well established? They are already about 10 feet tall. Any assistance would be greatly appreciated.
If you mean can you limit the root growth by trimming the tree, the answer is no. You wouldn't be the first person to have a tree too big for it's surroundings, and to be constantly pruning and cutting back. However, if you can possibly remove the tree, I would recommend replacing it with one of the size you want at maturity. Select a tree that is hardy for your climate zone -- that's what we mean by zone. You can find out about zone here: https://www.gardeningknowhow.com/planting-zones/usda-planting-zone-map.htm
After you select for zone, select for size, available light, and appearance. You can consult with the local Extension Service to help you make such choices; use this site to help you locate one: https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm
This applies only if you live in the USA, of course, but the same information is available through other sources in other countries.
Or you could hire a reputable landscaper.