Hi. We live near Binbrook, Ontario. We had a lovely Rose of Sharon tree (photo attached) which bloomed beautifully until 2016 when it got some kind of disease and died. Sunny part of the garden. We replaced it with the same type of tree. Summers of 2017, 2018, it got as far as having some buds on it, but never actually blooming. This summer it again has some buds at the very tips of the top branches. The old one bloomed all through July & August & September… is there anything we can do to encourage this one to bloom?
Thank you for writing to the Toronto Master Gardeners with your question regarding your Rose of Sharon. This lovely shrub, close cousin to tropical Hibiscus makes for a great show in late summer when every other shrub has turned in for the year. Rose of Sharon is relatively easy care, it does need to be in full sun to give you maximum bloom, it also needs rich, moist (but not soggy) soil.
To get to the bottom of your shrub not blooming you might first check if indeed your sunny spot has not become semi shaded by overhanding branches. Is your shrub being over or under watered? Have you enriched the soil by adding compost or a top dressing of manure? Next we should look at a more serious problem; Pests & disease. Rose of Sharon is susceptible to several problems, not least of which are bacterial blight, root or stem rot and verticillium wilt. You mentioned that you lost your previous Rose of Sharon to disease. Did you plant your new one in the same spot? The Pathogens from many diseases especially the ones mentioned above can live on the soil, so we usually recommend replanting the same type of shrub or tree in a different spot. Aphids can also be the culprit, they feed on the vital juices of new buds, causing them to wither and fall off. I would carefully monitor your Rose of Sharon for any signs of pests, disease or general loss of vigor all the time carefully removing any dead or fallen leaves or branches to avoid any potential spread of disease.
If you feel that there are no signs of disease, and that your shrub has all of its light and soil requirements, then you might try pruning it back. The best time to do so would be in early spring, note that this shrub sets its bloom on new growth, so pruning, although reinvigorating your shrub will encourage larger, but fewer flowers! Early spring would also be a good time to apply fertilizer and should help kick start your Rose of Sharon for several years of health.
Attached is a link that may give you more information on the care of Rose of Sharon, including information on fertilizing.