Five years ago I planted a large trumpet vine (‘Madame Galen’). It has grown into a beautiful shrub, but has never flowered. It gets full sun, is protected by and planted on the south side of our shed. I have tried different fertilizers, raised the pH, lowered the pH, pruned it, and not pruned it. A few neighbors have trumpet vines; they put little effort into them, yet they flower.
Getting a trumpet vine to flower is simiar to getting a wisteria to flower; it sometimes takes patience. First, make sure the location you choose has lots of sun. Plants in shade rarely flower. Second, avoid high nitrogen fertilizers. If you have a lawn service, don’t let them fertilize near the trumpet vine. Nitrogen fertilizers promote vigorous stem and leaf growth which will take away the plant’s energy from producing flowers. Third, prune the plant back each spring to only a few buds per stem. This way the plant is expending less energy on vegetative growth (stems and leaves).
If these methods fail to produce flowers, you can try fertilizing with a high phosphate fertilizer such as superphosphate or try root pruning. Root pruning is cutting a circular slit in the ground that is centered around the stem of the plant. The circle should be 2 feet in diameter for every 1 inch of trunk stem diameter. Use a sharp, pointed shovel to make the slit by digging into the ground as deep as possible and rocking the handle back and forth to create a V-shaped cut. You can apply the superphosphate directly to this cut if you like. Root pruning often produces flowers the following year. Once the plant begins to flower, removing the seed pods will encourage more flowers.