If possible, don't mess with a bougainvillea's roots. They're very sensitive to root disturbance!

Try using a fertilizer with more nitrogen, but use it lightly. Seaweed extract may also be helpful, especially in repairing damaged roots. Long term, use a fertilizer with moderately high phosphate content--such as Miracle-Gro for Roses, or Grow-More Magnum Rose Food.

Good luck!
Kevin : )

Key considerations with Bougies:

1.Leave roots alone, prune very minimally as the roots from this tree are very delicate.

2. Let the soil dry out completely between waterings. Very important for survival.

3.A heavy eater, therefore needs adequate fertilizer, but ensure roots are well watered BEFORE applying fertilizer.

4. Flowering is reported to take place when the tree is stressed either by lack of water and/or being root-bound in a pot.

RM

My dad had lots of Bougenainvilleas when I was young, and they always looked amazing, constantly overflowing with flowers. He told me he used to apply a very small amount of weed killer to stress the roots, and it sure worked well. I wouldn't reccomend this for your bonsai, obviously, but it works great in the right circumstances.

Its also important to remember that when developing bonsai (i.e. prebonsai or bonsai in development) the production of flowers will likely stress the tree of energy, that would normally be put into developing the tree growth (i.e. trunk elongation and thickening, nebari enlargement, etc.). It have therefore read that young pre-bonsai should have their flowers removed so that the tree can focus on growth vs. flowering.