Answer: Campsis radicans blooms on the new growth of the season, so for best flowering you would prune in early spring. (Summer pruning will reduce flowering.) This plant will scramble upward on a trellis to the top of your pergola and then grow across the top. You could prune it back to the top of the post each spring, this will encourage branching at that point and help it create a thicker canopy across the top. This vine grows somewhat slowly the first couple of years while it develops its root system. After that it is a rampant grower reaching about 30 feet so it should have no trouble covering your pergola. I hope this helps.
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