Toronto, zone 6, mature diamantina clematis , 5 years old, mature with lots of green leaves, about 7 ft. tall , sunny area all day, not blooming since 2019, should bloom mid July.
Need help with pruning and fertilizing info.
I am sorry to hear that your lovely Diamantina Clematis is not blooming for you. It is certainly a fantastic sight when it is in full bloom. Many times when we hear of Clematis failing to bloom it is the result of improper fertilization, that is too much fertilizer which would account for your description of “lots of green leaves”. Is your clematis near a heavily fertilized lawn? If this is the case your clematis is receiving too much nitrogen which may produce lush foliage. As a general rule, clematis benefits from a handful of 5-10-10 fertilizer in the spring, along with a layer of compost. Apply a water-soluble fertilizer once or twice during spring and summer.
Clematis ‘Diamantina’ is a Group 2 type meaning that it blooms on the previous year’s growth. For that reason it will benefit from two trims; one in early spring to remove weak growth and then again after the first flush of flowers in order to promote a second blooming. If it is the case that your 5 year old vine has never been pruned and since you have not seen any sign of flowers this summer you may have to resort to a harder pruning to renew the growth of your mature vine. This would entail pruning hard, leaving only about 3’ of the vine in early spring. This will encourage new and more vigorous growth.
Before you take this drastic step, I would revisit the proper growing conditions for clematis, and the link below may be of help to you.