Question
I live in a building that has an old and huge wisteria vine growing on the  side of the structure, it's about 30ft. high. It hasn't blossemed in a few years. It was previously a showpiece. What has happened and how can we get it to blossom again? I was told to fertilize w/ super phosphate, is this correct? Thanks, Doug.

Answer
Get the Soil under this specimen tested before you start adding stuff -- too much of 'x' (including Phosphorus) will lock out too much of 'y' and 'z' and then you have a sick Wisteria in addition to one that does not bloom.

There are several possible explanations for a Wisteria's failure to bloom, including depleted Soil.  The most frequent problem however is lack of Sun.  The more floriferous Wisteria sinensis -- Chinese Wisteria -- and W.floribunda -- Japanese wisteria -- can reach 25 feet when mature, but if an Oak or even a building is in the way, it will never be as vigorous as it used to be.  Surrounding Trees can hog all the light; for years they were too small to matter, now they're the only game in town.  You can't tell right now but when Trees leaf out in Spring, study by the hour how bright it is where your Wisteria is growing.  It needs FULL SUN to do well and to bloom.

Remember, Wisterias use the energy of the Sun to grow.  It takes a LOT of energy to make those huge, beautiful flowers.  This plant flowers best when grown with tons of solid Sun -- 6 to 8 HOURS daily.

Severe Winter temps can injure buds, in which case the shrub will fail to bloom.  If so, Winter protection would help.

Next, Wisteria bloom best on horizontal branches.  They also like slightly Acid Soil (which you can evaluate when you get your Soil tested), around 6.5.

Very old Wisterias sometimes re-charge their batteries with a good pruning, either branches or the root system.  This takes a little skill.  You can try pruning it yourself, but I would have a professional do it to avoid permanently damaging the plant.  After that, regular pruning should be done at least once a season to maintain vigor and promote flowering.  Roots can be pruned now.

L.I.G.