My Chambeyronia macrocarpa has developed a beautiful 'Watermelon' trunk.

Whoa! How old is it?

Whoa!
That is one gorgeous trunk.

I planted this the year the greenhouse was built as a seedling about 10 inches tall. That was almost 17 years ago. The crown is at about 12 feet now and the fronds are huge and king of lay partially flat on the ceiling of the GH at its highest point (which is almost 20 feet). I am hoping it does;t get too much taller, they are said to normally max out at 20 feet height.

The smaller one I have in the back I think is the 'non-watermelon' form but its still too early to tell.

A while back I learned that the max height of a banana is measured only by the trunk itself. The fronds which can be of equal length are not even considered. How is the eventual height of palms determined?

I think pretty much the same way, from the ground to the point of the crown. I bought Chambeyronia thinking it would be 20 ft max all-inclusive but it seems like it may be a little taller. I had toyed with the idea of trying to make this plan 'lean' like a coconut palm can sometimes be coaxed to do, to grow a little horizontally. But my friends over on the palm forum did not think that would work.

Palmpedia says they max out at 20-25 ft, and the fronds can get 10-12 feet long and I can attest to that the fronds are huge. They are also supposedly cold hardy to 25F but I have had friends say they lost their at higher frost/freeze temps than that

Maybe try christmas lights, the old timey ones that get hot, wrapped around the trunk???

Its way too big to move now. I could move the smaller one. but the one in front, no way. I'm just going to have to go with it and see what happens. I still think I might be able to get it to lean, but then it would run into the cross struts in the attic of the GH.

I bought Chamaedorea plumosa about the same time, it is supposed to stay under 20 feet. Its one of those oddball Chamies that is very tall and very slender. My oldest one is about 15 ft tall on a trunk the size of a hot dog with a little tuft of fronds right up at the top. Like a Dr Seuss tree almost LOL