Answer: Trumpet vine flowers occur on new growth (as opposed to flowering on old wood). I would suspect that the plants are busy building up reserves in their root systems to enable flowering. Also, your gardening region is the northern edge of their range and they may not have a long enough growing season to put out flowers, especially if they dieback to the ground each year. Pamper them through the winter, especially before you get your famous snows, and they should flower as the plant ages and matures. Wait until frost kills the tops, then cut everything down to just above soil level and mulch over the top with several inches of straw, compost, shredded leaves, etc. to keep the roots from freezing. Best wishes with your trumpet vines.
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