Experts and BHG readers answer.
WISTERIA THAT WILL NOT PRODUCE BLOOMS.
Thanks for writing. I hate to be the bearer of bad news, but if you have a Wisteria sinensis type and you live in Zone 4, you may never see blooms on your vine as this plant is not reliably flower-bud hardy in your region.
If you're determined to try to get it bloom, start by figuring out a way to give it better winter protection. For example, if it's growing on your deck railing, try wrapping the railing in burlap and filling it with leaves or another type of insulation to help keep the flower buds from freezing. Or try wrapping the vine in Christmas lights over winter; the extra heat from the lights may protect the vine from the cold temperatures.
You may ultimately wish to replace your vine with one that is reliable for your region. 'Blue Moon' and 'Aunt Dee' are two varieties in particular that have shown excellent hardiness in Zone 4 (neither are a W. sinensis type).
---Justin, Senior Garden Editor, BHG.com