We are in Zone 6A, Oakville, Ontario. We have clay in our yard. We have had purple flowering raspberry bushes for quite a few years and only ever had one bloom. They were planted under a silver maple, so they received part sun. The silver maple had to be removed so last year they were in full sun for about 4 hours per day or more. They spread, but don’t bloom. They are in a raised area where we mulched around the base of the tree. We also planted Cranesbill (wild geranium) in the same garden and they are fine.
How frustrating to have no blossoms on your purple flowering raspberry (Rubus odoratus) so loved as a habitat for wildlife. This North American native shrub, hardy to zone 3 has fragrant, rose-like 2 inch wide flowers from June to September and licorice-scented palmate leaves. There are no serious disease or insect problems associated with this shrub.
Its cultural requirements are average, medium moisture, well drained soil, in full sun to part shade: all conditions which you appear to provide. The following suggestions may help solve the problem.
Rubus odoratus blooms on one-year old wood. If this is pruned off, flower buds are lost for the current year. A late hard frost also may cause the loss of flower buds. Over fertilizing can cause excessive growth that then requires pruning away along with the flower buds. It is recommended for flowering shrubs to prune immediately after flowering to allow the new growth to develop flower buds for the following year, and to remove one-third of the oldest stems each year in order to maintain vigorous newer productive growth.
The removal of the Silver Maple tree may have caused root injury to the shrubs. Nitrogen-rich lawn fertilizer next to the shrub may encourage lush foliage at the expense of flowers, and finally, if the shrub bed is in close proximity to the road, road salt may stress the shrubs and hinder flowering.
Good luck. I wish you abundant blossoms.