I’m new here. My partner and I are getting married and we are relandscaping our garden as our wedding gift. He loves Coast Live Oaks (Quercus agrifolia) and it’ll mean a lot to us if we can plant one or two. That area of the garden is about 30 feet wide. Way to small for a tree that grows up to 85 feet high and 35 feet wide. Can we plant a Coast Live Oak in a big container, wether above or under ground, to keep the roots and the tree about 20 feet high and 15 feet wide? Thanks in advance for your answers.
I'm afraid not, not even in the medium term. The tree is unlikely to reach the size you mention contained in a pot because of root restriction; whilst it may remain healthy for a year or three if grown from seed or a small sapling, it will not do well contained in such a manner. This is one of those trees that produces a large and deep tap root, as well as large lateral roots, and growth of the taproot won't be possible in a container; this will compromise the tree's growth early in its life. If you bury the container, the only way the tree will grow on well is if it manages to punch through the bottom of the container and access the soil around and about. It is likely to do that, and when it does, it might as well be growing directly in the ground, in terms of height and spread as it grows. Burying the container confers no advantage in terms of restricting its growth otherwise.