How do I give structures a center when spawning?

Hi! So, I'm trying to add palm trees into my mod using structures and then spawning those through features, but it keeps on spawning either floating or buried a bit underground. I assume it's because of the fact that the structure block is the defined part where it'll place the tree and not directly under the trunk. I wanna know how to give a structure a center so that my trees don't float or get buried a bit by sand. It's really annoying me and I can't figure out what to do. Help.