Saturday, August 21, 2010

Plant Care Profile: Swamp Rose Mallow (Hibiscus moscheutos)

When my family was still living in Germany and I was in elementary school, someone in our street, whose garden I passed twice everyday on my way to and from school, acquired a specimen of swamp rose mallow (Hibiscus moscheutos) with enormous, deep red flowers. I, of course, was utterly fascinated. Not only were the flowers stunningly large and colorful but the plant producing them was actually a hardy perennial! For years I tried to get a hold of a specimen of this amazing plant but none of the local nurseries and garden centers ever seemed to stock it. Imagine my delight when we moved to Michigan and I found that here in the US, Hibiscus moscheutos is widely available in a number of different varieties!

A cultivar of swamp rose mallow in my family's previous Zone 6a garden in southeastern Michigan

Origin: Hibiscus moscheutos is native to wetlands throughout the eastern United States and even occurs as far north as southern Ontario.
USDA Hardiness Zone: This plant is reported to be hardy at least to Zone 5a.

A variety of Hibiscus moscheutos with solid red flowers and finely divided leaves in my family's former Zone 6a garden

Size: Depending on the variety, Hibiscus moscheutos specimens can range in height from 2 1/2' (ca. 75cm) to 6' (ca. 1.8m) and old plants can reach a width of up to 3' (ca. 90cm).
Flowering Time: Depending on local climate conditions, an established swamp rose mallow will usually begin flowering in mid or late summer and will continue to be in bloom for several weeks.
Light Requirements: Hibiscus moscheutos may tolerate light shade for part of the day but it really prefers full sun.

A large specimen of swamp rose mallow in the Katz'sche Garten in Gernsbach, Germany (Zone 8a)

Soil Requirements: Despite its lush and rapid growth and massive flowers, Hibiscus moscheutos can make do with average garden soil. That being said, the plants do benefit from a fertile soil with lots of organic matter and respond positively to yearly top dressings of compost as well as an organic mulch.
Siting in the Garden: Swamp rose mallow should be planted in a sunny spot with moderately moist to wet soil. While it grows mainly in swampy areas in the wild and sometimes even occurs in standing water, it can nevertheless put up with a bit of drought and does not necessarily need more water than most other garden flowers - in my family's first Michigan garden I had a beautiful specimen that flourished in a spot with sandy soil and full sun with minimal irrigation. Now about eight years old, it was still there and covered in blooms when I drove by our old house a few days ago. Apart from sun and soil requirements, it is also important to keep in mind that Hibiscus moscheutos can get quite large and to plant it in a spot where it will have sufficient space to develop without crowding other plants. Finally, since this species begins growth rather late in spring, it is a good idea to site it away from plants that leaf out or spread rapidly in spring or early summer since these might easily overgrow it.
Care: Hibiscus moscheutos should be planted between late spring and early summer so that the plant has enough time to get properly established before the winter. Bought plants might be further along than they would normally be if planted outside, so if you buy a flowering plant in June be aware that it might not flower until July or even August in subsequent years. Add plenty of organic material such as compost into the soil at the time of planting, as well as a dose of organic fertilizer such as bone or blood meal. Apply a good layer of mulch around the plant and keep it well watered throughout its first season. Remove spent flowers regularly to keep the plant from wasting its energy on the production of seeds. In later years this is not really necessary and if you want to propagate the plant you can let the seed pods ripen and collect them when they have turned brown and papery and begin to split open. In the fall, wait until the stems and foliage have turned brown - in most of the plant's range this will be after the first couple of frosts - before cutting the plant down to abut 6" (ca. 15cm). Hibiscus moscheutos resprouts each spring from the lower portions of the old stems rather than from the roots proper, so cutting the old canes too close to the ground might be detrimental to next year's growth. In addition, the stiff stumps of the old canes somewhat protect the young shoots as they emerge in late spring and help to prevent them from being overlooked or damaged. After cutting down the canes, it is a good idea - though be no means necessary - to protect the crown of the plant with some leaf mulch and a few pine boughs. These ought to be removed in mid-spring and once new growth begins to emerge the plant should be given a new top dressing of compost and some new mulch, all the while taking care not to cover the crown of the plant itself. While the plant is growing, one or two doses of liquid fertilizer might be advisable, especially if the plant is not growing in particularly fertile soil. The only pest that I have encountered as a problem with this plant is the Japanese beetle (Popillia japonica), which likes to snack on the flowers and sometimes also the leaves. Collecting the beetles daily - or several times a day if the infestation is particularly bad - is the most effective way of dealing with the problem in my experience, even if it is a bit tedious.

Flower of a white, pink-flushed variety of Hibiscus moscheutos which I planted this spring in my parents' new front yard in suburban Michigan (Zone 6a)

Propagation: Hibiscus moscheutos may be propagated fairly easily from seed, though plants might take two or three seasons until they first flowers and might differ in characteristics from the parent plant. The seeds can be started indoors in spring and the young plants set out in the garden after all danger of frost has passed or they can be sown directly in the garden once temperatures have warmed up sufficiently. In either case, the young plants should receive a good layer of leaf mulch for winter protection during their first year. Stem cuttings are supposedly possible as well, though presumably a bit trickier than propagation through seeds. Finally, division is also reportedly a possible means of propagation. However, since the plant quickly develops a woody base I would imagine it to be difficult to do without considerable damage to the plant.
Use in the Garden: Swamp rose mallow, available in shades ranging from white over various hues of pink to dark red, makes an excellent addition to perennial borders and adds color in mid to late summer when many other perennials and flowering shrubs are past their prime. Most varieties get rather tall and should therefore be planted in the middle or the back of the border but there are also some dwarf cultivars that work well in the front of flower beds. Due to their tidy habit and massive flowers, the larger varieties also make stunning solitary clumps surrounded by lawn as well as great center pieces for island beds.


  1. A desirable plant to have. Thanks for all the information.

  2. A wonderful native plant! Still trying to find the best way to preserve the seeds. Last year, mice carried off all the seedpods I saved. This year I husked the pods, leaving only the seeds, but found small bugs coming out of the pile of seeds. The bugs seem to be gone, but I am wondering how to best store the seeds. Can I freeze them (to kill off any other pests). I've found starting them in pots ensures a plant, not mouse lunch.

  3. I am not sure if freezing them is good for their viabillity. Perhaps store them in a closed tin in the fridge, or an unheated garage or something of the sort? I had collected some seed pods of Yucca filamentosa last summer but when I husked them found little beetles and holes in some of the seeds, so I did not want to bring them inside or put them near my other seeds lest those get pest-infected. I left them in a saucer on a shelf in the garage and planted them in the garden in late May, and they produced a good batch of seedlings.

  4. you can take cuttings and re plant in the spring instead. They grow very well like this...why bother with seeds.

    1. Really? I have never had any luck with cuttings of any type of hibiscus; they do not root particularly easily, even though that is of course how the shrubby types are propagated commercially. The seeds germinate very easily, the plants just take a few seasons to reach full size.

  5. my hibiscus moscheuts looked like it got sick when the cold came so now in the spring I cut it all back. Do you think it will come back?

    1. It should resprout from the base of the old stems; the old shoots always die back with the cold.


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