Answer: In my experience there are two fairly common reasons wisteria does not bloom. One is that it was grown from a seed and is still not yet mature enough to bloom. Occasionally it can take 20 years or so for them to bloom from seed! If you want to be sure of a blooming-capable wisteria you would need to purchase one that is in bloom, usually these are rooted tip cuttings or else grafted from proven blooming vines.
The other fairly common possibility is that it is setting buds but spring frosts are killing them before they can open into flowers. For this, microclimate and the plant's siting are key and with a 12 to 15 year old plant unfortunately transplanting is really not an option as the root system is incredibly massive. You might be able to reach and cover it or a portion of it on cold nights, depending on how you have trained it, if this is what is happening.
Additional reasons for lack of bloom include overfertilizing with nitrogen, poor pruning, and lack of sunlight. Here is some general information that may help you review your care practices and optimize them for getting it to bloom. You may need to cut and paste the complete url into your browser to make it work correctly.
Good luck with that wisteria!
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