see this: http://www.socalplumeriasociety.com/plumeria-care

If you have heavy clay, you should dig up the plumeria and put it in a pot with well draining container soil. Wet clay in the winter during the plumeria's dormancy won't be good.

You have to either deal with the soil you have or amend the entire planting area down to about two feet. You just dug out a large bathtub by making a hole 5x the size of the pot but it's still a bathtub. Research plants that like clay soil.

I just noticed that you said you amended 5x depth as well. That's a mistake. You should never dig the hole deeper than the root ball of the plant. This is because as the soil naturally compacts back down the soil level will sink. then you have a depression that will collect water around the crown or root flare which is bad. That depression will eventually fill up with soil burying the crown or root flare which can lead to rot and girdling roots.

Hi nil, thanks again for your advice. I was on the fence about amending the soil, having read a massive swath of conflicting advice.

"You just dug out a large bathtub by making a hole 5x the size of the pot but it's

still a bathtub."

Would leaving the soil unamended have been any better? There are so many blooming plumerias in my area (San Fernando Valley) and at least half a dozen my neighborhood alone, growing in the same heavy clay soil that from the looks of it doesn't look like it was amended with anything

Frankly I didn't use much amendment, maybe 20% + 80% native soil at worst. The amended soil isn't much different than the surrounding soil, the amendment was mostly small wood flakes/dust and gypsum. My probably fallacious conjecture was that maybe the roots of the plumeria would have an easier time pushing through this slightly lighter soil. I do not plan to water the plumeria very much, since it's a pretty low-water plant. Maybe I'm stubborn, but I think I'll stick with it. If I see signs of stress, I'll put it in a container, but I don't like container gardening very much.

"Research plants that like clay soil."

Yep, I have, the majority of my garden is roses and canna lilies, like so many in my area.

Clay tolerant plant list

Usually when I see plumerias in clay down here, they are under the eaves of houses where they stay relatively dry in the winter.