Hi there!
my Rose of Sharon shrub is starting to yellow and is dying . I had Tru Green come out to protect my shrubs and plants with an application. I noticed it was green and lush a few days before and we have had extreme heat and humidity, Is their anything I can do to save it?
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.
The first thing to do is to make sure that your Rose of Sharon is situated in the right place for optimal growth. In general, Rose of Sharon prefers full sun (for maximum bloom), soil that is rich and moist (but not soggy), and is hardy to USDA zone 5 (i.e. Canadian zone 6). Since it is borderline hardy in Toronto, it is best to give it a sheltered location to protect from winter kill. Giving it a good 2-3 inch layer of organic mulch around the root zone in the Fall will also help (make sure to keep the mulch a good 2-3″ away from the trunk of the shrub to prevent rotting). In late Spring, after it has leafed out, prune out the winter damaged branches and apply a balanced 10-10-10 fertilizer to give it a head start. A mid-summer application of a low nitrogen 0-10-10 fertilizer will promote root health and flower production. Do not apply fertilizer after August as this will encourage tender new growth that will not overwinter. Water deeply but not too frequently, and take care not to let in sit in wet soil or the roots will drown. Amend the soil regularly to keep it humus rich and healthy.
You mention that the shrub was given an application. I am assuming that it was treated with a fatty acid type spray, such as insecticidal soap. Did you notice a decline in the shrub before the application? You do not mention what damage you are currently seeing. Are the leaves yellow? Are they wilting? It could be that your shrub was stressed due to the extreme heat. Just from your description it is difficult to make an accurate diagnosis.
Rose of Sharon is susceptible to several problems, not least of which are bacterial blight, root or stem rot and verticillium wilt. There are also a number of insects which attack Rose of Sharon. Aphids (the application of fatty acids suffocates these pests), japanese beetles (which can skeleonize the leaves) and root knot nematodes (damage may appear as wilting or drying up of the plant).
My suggestion would be to make sure that your plant is watered well, especially during the extreme heat that we had and check your shrub for the above mentioned diseases and pests.
This website lists the common issues of this plant along with photos for comparison.
Good Luck with your Rose of Sharon.