Step 3: In mid-September (or when vigorous growth has ceased), cut wisteria back again, though not as ruthlessly, to its desired shape for the following spring. This time, leave four or five nodes or buds per branch; these will form next year's flowers and branches. If you see something resembling a witch's broom at the ends of the branches (several short, dense shoots clustered together), don't remove it all—it's full of buds, so just shape carefully and thin out weaker stems and dead tips (about one-third in total). Cut back any split panicles (flower heads) and seed pods so they don't rob energy from flower production.
Step 4: Wisterias usually bloom on the lower 30 centimetres or so of last year's stems. By pruning these back several times a season, a flowering, multi-branched stem—much like an apple spur—can be created. Every three years or so, when these stems have grown out too far from the main structural stem, give the plant a renewal pruning right after flowering, severely cutting back mature wood to within 7.5 centimetres of the main stem. This will open up the plant and allow in light to help stimulate new growth.
Tips from a pro
At Ontario's Royal Botanical Gardens, gardener Bob May is affectionately known as Edward Scissorhands. For more than a decade, Bob has devoted a great deal of time to pruning or espaliering climbers on the massive pergola in the rose garden located in RBG's Hendrie Park. When he took over this job, the pergola plants were in a bit of a state, but Bob has successfully coaxed and bullied recalcitrant cultivars into orderly, free-flowering submission.
This self-taught pruning guru has been fascinated by plants since childhood and showed his vocation early by cutting back pussy willows and shaping specimens in his parents' garden. Over the years, experience has taught him how to prune sympathetically, working with each plant's natural tendencies. Therein lies his artistry (Bob has also taught classes at RBG in bonsai techniques and is happy to answer questions regarding his creations). As Winston Churchill famously said, “Give us the tools and we'll finish the job.” For Bob May, that tool is a Felco #8 pruner, which he likes for its narrower, more pointed blade that enables him to “get right in there.”