I have a nice phlox plant that looks heathy but it never flowers, despite watering, fertilizing. It is in full full most of the day. What can I do to finally see it bloom ?
Thank you for contacting the Toronto Master Gardeners.
Garden Phlox, Phlox paniculata was once a staple of summer gardens providing dazzling mid season colour. This perennial prefers to grow in areas that receive full, 6 hours or more of sun, in moist, fertile, well-drained soil enriched with compost or other organic matter.
There could be a number of reasons why your phlox did not flower: You mention that your phlox receives full sun. Does it receive 6 hours of continuus sun? Looking at your photo it could be that the phlox is being shaded from the afternnon sun by your fence. Phlox will tolerate light shade, however shading decreases both bloom quantity and quality, while it increases disease problems. Phlox should not be planted near trees and shrubs that compete for both moisture and nutrients.
As mentioned above, phlox benefits from the yearly addition of organic compost. Not only will this add much needed nutrients to your garden it will also improve the texture (porosity) and quality of your soil. Adding oroganic compost is different from fertilizing your perennials with synthetic fertilizer. You mentioned that you have been fertililzing. It could be that you are adding too much nitrogen to your soil which results in excessive vegatative growth.
Did you prune your plant? Pruning the plant in early to mid-summer could delay flowering.
Since your plants appear to be well watered my inclination is to look at your soil. Some plants require different nutrients than others and sometimes we need to add thing like blood/fish/bonemeal where others need a bit of lime, others are happy to just get some compost. We assume when perennials are planted in a mixed border that if some plants are happy, then they should all be happy, however this is not always the case.
If after the addition of some compost in the soring or fall your phlox still will not flower then this is telling you that the conditions are still not right. You might also wish to consider moving your phlox to a different location. Sometimes, certain plants are just not happy where we think that they would look good.
This link contains additional information on growing phlox.