Question
I have a Swamp rose mallow (Hibiscus moscheutos) that grows 2.4 meters (8 feet) tall and will be in its second winter, but it has never flowered. I don’t know if I should prune it in spring or now or not at all… or should I shelter it for the winter? What should I do in the spring to get it to bloom?
Answer
The Swamp rose mallow is a zone 6 (USDA zones 5–9) perennial that we like to grow in zone 4… with very unremarkable results. In fact, this large perennial from the southern United States is completely baffled by our climate. Under a good layer of snow, it manages to survive, but it’s in such a state of shock in spring that it often doesn’t start growing until July. After that, it’s a race against time: will it or won’t it flower? The hotter the summer, the greater the chance of flowers, but everything comes to a halt when the weather cools in autumn. Under the right conditions, you should be able to get several flowers in early September. Most flower buds, however, fail to open in our climate.
Putting the Odds in Your Favor
For best results, plant Hibiscus moscheutos in full sun, in a location sheltered from the wind and in soil that warms up quickly in spring, but is moist in summer, as this plant prefers high humidity during its growing season. It can even be grown in a water garden if the water is heated.
You can prune the branches in autumn, but there’s a danger that there won’t be enough snow left to protect the crown from the cold. The ideal solution is to leave them upright in winter to catch as much snow as possible, then prune them back to the ground in spring. There’s no point in leaving them permanently upright, as they’ve been killed by frost. For winter protection, a layer of shredded leaves can help shield the crown from the cold. There’s no point in wrapping Swamp rose mallow for winter: its branches are already dead; it’s the crown, the base of the plant, that’s alive and well and deserves your attention.
An Annual Perennial
In colder climates, you should treat Hibiscus moscheutos as an annual. Buy plants that have been forced in the greenhouse and are therefore already in flower or bud in spring. This way, you’ll have flowers all summer long. Now replace the plants the following spring with new greenhouse-forced plants, as this “perennial” hibiscus actually makes a poor perennial in our climate!
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 23, 2005.
Oh, it is not just hibiscus. I get similar confusion with other genera that I bring back from Southern California. Well, also, there is similar but opposite confusion with some genera from the Pacific Northwest, which need a particular chill that they lack here.
Finally some one who is not afraid to speak the truth. I have told several people who complained about the plant not coming back etc. that it is a tender perennial that does not do well in our climate. Its a baby sitting plant if you want it to survive. I wish you wrote a list of perennials that do not do well in our climate. That may survive several winters if conditions are favorable. One that I can list is butterfly bush, ( do not like the non native invasive in warm climate plant) I have told several people how it does die back here and can disappear they tell me how some people have had it for years. I never argue with them I just wait and watch.
Person in the high North use back plastic landscape cloth to warm up garden & watermelon beds. This should work for the flower also.
You have a sun room or greenhouse, root a stem and grow it inside all winter.