Hi, I grow a rootop garden. Full sun all day, close enough to the lake that there is at least a constant breeze and sometimes it’s quite windy. I have an ornamental garden in containers. Last year we added two lilac bushes in 18” containers purchased from a garden Center. The bushes grew to be 3.5’ high last summer but did not flower. They survived the winter well and have come back to flower this year. But they have only flowered moderately. There are 5-6 bunches of flowers per plant. I like to grow organically and stay away from synthetics. Is there a supplement that I can give these beauties to increase the number of flower bunches for next year. Thank-you!
Thank you for contacting the Toronto Master Gardeners. Your rooftop container garden sounds lovely. It can be challenging to grow lilacs in containers because they have an extensive root structure, which often grows up to one and a half times their width. This is also the case with compact varieties which are the only ones to consider for growing in a container. The container needs to be really big, at least 24” wide and I suggest as deep as half of the height of your plants. It is generally not necessary to fertilize lilacs. They could be lightly fertilized with an organic fertilizer in the early spring, however over fertilizing will result in more vegetative growth rather than the blooms you are looking for. Many lilacs need to mature for a few years before they reach their flowering peak. The number one reason for lilacs failure to bloom or bloom fully is improper pruning. Lilacs must be pruned yearly immediately after blooming to promote vigorous growth that enhances flowering. Lilacs bloom on the previous years growth. For the compact varieties little pruning other than deadheading is required which should also be done right after the flowers fade, and thinning out the oldest stems every few years.
This information from Landscape Ontario might be helpful :
https://landscapeontario.com/qa-why-are-there-no-blooms-on-my-lilac
Best of luck with your lilacs !