Crape Myrtle, Thin Trunk
Last year around April,I bought two red dynamite crape myrtles. Tucson get VERY warm shortly after April, so they went through some heat stress but I just wanted them, slowly each night switching between the two (expensive - in Tucson.) in one of them, because our Arizona soils are extremely alkaline and saline), I applied gypsum to break up the soil and ammonium sulfate to lower the pH. On one, admittedly, I over did it with the sulfate and it stopped leading out completely around late-August and didn't lead out again until October. They've been dormant since.
In tucson, crape myrtles come out of dormancy in early to mid-march. They are still dormant, one (a pink velour that I cut down severely so it would grow as a bush) is already started to have swelling nodes that are turning red.
My question is, I got the trees at lowes. They were $20, about 7" tall and they've grown heidth-wise since then. The issue is the trunk is so thin, as it has been. It's not on the lowes stake. I bought a green, tall, garden steak and garden tape. I move the tape around often to ensure its not being strangled.
I did notice that the mulch from last year had decomposed and turned to soil, so the crown of the tree was about 4" below the dirt. About a week ago, I uprooted both of them without damaging the roots (the roots are spreading into surrounding soil) and lifted them so the crown can dry, rebark, and contribute to trunk/limb growth.
If the stakes they're on fail (I used a limb as a laundry line during a stiff wind today) they bend all the way to the ground. They don't break, but they bend.
What can I add to encourage stringing trunk/limb growth? I would forego an early bloom this year for green growth and thicker would. Any suggestions? Iron? Nitrogen (only) fertilizer?