do not prune at all unless it hits the ceiling when the pot is on the floor. In my house that is 9.5 feet.

No. Three reasons. 15 inches is still small and honestly you don't want branches that low anyway. Seed grown trees get a slow enough start as it is and you will make it worse if you remove anything. It will branch on its own when it's ready. It is winter still once you take it outside in the summer it will branch.

Thanks, Steve and pip313. It did not occur to me that the branches would end up that low!

My 3 year key lime (or so I believe) has 2.5m height, with one main vertical branch and 2 small underdeveloped branches at about 50cm from soil.

I will let-it grow to the ceiling, hoping that it will reach maturity first. If no flower until then, I will autograft the tip (with highest node count) on the base, at about 40-50cm from soil. Then it will grow again until ceiling and hopefully it will flower.

eSilviu. I did that with my in ground sweetlee tangerine trees. Unfortunately all the grafts died. Leaf node 110 grafted to base of trees.

3 years from seed key lime should flower real soon. I wouldn't waste time grafting it. Unless you graft it to a different scion.

So... should I let-it bend under the ceiling? :D At the current growth rate, that moment it's only 3-4 months away!

The thing to remember when you snip it or otherwise encourage branching is that no matter how big the tree becomes, that branch will never get higher; it may grow taller, but that branch point will never grow higher.