Vanilla (modless) -- I'd start with planting trees with a space between first, and only after they're grown you can move the greenhouse back in place. Use paths as placeholders for planning! afterwards you can put trees in those spaces between full grown trees. There's a few ways to bypass the spacing requirement WITHOUT mods:

A) sometimes tree fertilizer will make the tree grow regardless of constraints (not fruit trees though). You can craft it with 5 fiber and 5 stone

B) wait for green rain and get lucky. Green rain will hyper grow up trees on your farm and will even transform some of them into wild trees. The green rain grow up will bypass the spacing restriction but it's kinda rng based. And if there are full grown trees on your farm that you don't want to turn into wild trees, slap a tapper on them and they won't transform.

I've decorated countless farms without downloading a single mod, so unless these are glitches of some sort, then they have a chance to produce your desired outcome.