Question
I bought a mock orange (Philadelphus) three years ago and placed it in full sun and out of the wind. It grew, but never flowered. However, I see much smaller specimens in bloom in other gardens. Should I fertilize it? Prune it?
Answer
I’m increasingly convinced that we need to start giving flowering shrubs two hardiness zones. One for the branches and another for the flowers. Philadelphus coronarius survives the winter in our northern regions, but doesn’t always manage to flower. This is because its leaf buds are quite hardy (zone 3b, USDA 3b), while its flower buds are less so (zone 5b, USDA 4b). But if winter is even the slightest bit cold (and we’ve had three colder-than-normal winters in a row), the flower buds freeze.
So, while the plant survives the winter with ease, it doesn’t flower in spring. I’m inclined to think that this is the reason for your lack of flowering. If this winter is mild, you should have flowers in spring without difficulty. If you’ve seen other Philadelphus flowering, they’ve probably benefited from a slightly warmer microclimate (on almost every plot in our region, there are corners that are colder than others).
Hardy Mock Oranges
Some Philadelphus are much hardier than P. coronarius.. P. lewisii ‘Waterton’, for example, was collected in Waterton Lakes National Park in Alberta and survives the winter effortlessly in zone 2b. Even there, its flower buds are less hardy than leaf buds: zone 3b, but ‘Waterton’ then flowers faithfully every year in our region, even in exposed locations.
By the way, adding fertilizer won’t make any difference (on the contrary: over-fertilized plants freeze more!). And pruning at the wrong time can harm rather than stimulate flowering (philadelphus can be pruned after flowering, but pruning in midsummer or autumn will eliminate the following year’s bloom).
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil on October 9, 2005.