Non flowering Hydrangea petiolaris

Petiolaris does not need pruning. But if you want to I should think March time as for other hydrangas

Only pruning required on these is to keep the plant close to the wall and to remove any dead or diseased shoots at the base. As Drc says, this is best done in April.If its not flowering, it could be that its too dry for flower initiation - next year, top dress the soil around the roots with sulphate of potash, raked in, and keep it watered in any dry spells, if we have them, during May and June particularly.

i keep mine well watered is potash required if it flowers? Bamboo

Not usually, Drc - its often applied to fruiting plants like raspberries early in the year as a standard procedure - I'm only suggesting it on this occasion because the hydrangea isn't producing flowers at all, so its worth a try. And sulphate of potash does encourage flowering, yes, but I wouldn't use it all the time on everything.