It might surprise you to learn that there are two incarnations of hardy hibiscus; a shrubby form known as Rose of Sharon or Hibiscus syriacus and an herbaceous perennial called Swamp Rose-Mallow or Hibiscus moscheutos. Both love the heat, blooming from mid to late summer with very showy flowers but as you will see, there is a substantial difference in bloom size, as well as growth habits.
Rose of Sharon
A lot of people confuse the evergreen Chinese hibiscus (H. rosa-sinensis) with the deciduous Rose of Sharon, probably because they have seen it growing in landscapes while vacationing in Florida or Hawaii. While the former can be brought outdoors for summer or be grown as a houseplant, at USDA zone 9 it will never survive a Canadian winter; while Hibiscus syriacus is USDA zone 5 rated, meaning that it is hardy throughout the Fraser Valley and into the Okanagan. One quirk about this shrub is that it is one of the last ones to produce leaves in spring, often just budding when rhododendrons are in full bloom, so some gardeners assume it has suffered winter damage and remove it prematurely. Also, while it is quite hardy when planted in the ground, it fares very poorly in containers as the roots do not like the temperature fluctuations; the only exception here would be extremely large pots.
Rose of Sharon prefers a fun sun exposure (but will tolerate part sun) with well-drained fertile soil. Since it is quite a tall shrub (8-10’) it can be pruned (or grafted) into a standard tree form. Pruning should occur in spring, usually April, and since it blooms on new wood you are always guaranteed flowers by late summer. They can be notorious self-seeders, so if you want to avoid this just deadhead the seed pods in late fall but leave the major pruning until spring. The bloom period generally runs from late July through to October with single or anemone (ruffled centre) flowers averaging 3-4” across and double blossoms (which much resemble a carnation) about 2-3” across. There are several forms including the columnar ‘Purple Pillar’ and ‘White Pillar’ which only grow 2-3’ wide, while the compact ‘Lil’ Kim’ (white, red blotch) and the hybrid ‘Pollypetite’ (pale lavender) only reach heights of about 4’ tall. Tree forms or standards are also available but the grafting work makes them a little more expensive.
The colour range of this species is quite impressive and includes blue (‘Marina’ or ‘Blue Satin’, ‘Bluebird’ or ‘Oiseau Bleu’), pink (‘Woodbridge’, ‘Minerva’, ‘Rose Satin’, ‘Hamabo’), white (‘Red Heart’) and purple (‘Violet Satin’, ‘Purple Satin’), all of which are contrasted by a deep red central blotch. For pure white there is ‘Diana’ while the carnation-like double-flowered forms include ‘Ardens’ (violet), ‘Blushing Bride’ (pale pink) and ‘Collie Mullens (deep pink). The Chiffon Series provides a range of large anemone-form flowers and include ‘Lavender Chiffon’, ‘Blue Chiffon’, ‘White Chiffon’ and ‘Pink Chiffon’. So one can’t complain about a range of flower form or colour with Rose of Sharon.
Hibiscus syriacus 'Woodbridge', Hibiscus syriacus 'Rose Satin', Hibiscus syriacus 'Red Heart', Hibsicus syriacus 'Lavender Chiffon', Hibiscus syriacus 'Ardens', Hibiscus syriacus 'Marina'
Despite its massive size (3-7’ tall) and enormous flowers (up to 10-12” wide), Hibiscus moscheutos is actually an herbaceous perennial. Although quite exotic in appearance this species is native to Ontario, growing in shoreline marshes of the Carolinian Forests around the Great Lakes. Swamp Rose-Mallow prefers a full sun exposure with evenly moist, rich organic soil and can even tolerate boggy sites. Like the shrubby form, it is late to break dormancy as it requires warm weather to enact growth, so here on the ‘cool’ coast it can take up to June for them to begin producing buds.
A top-dressing of compost in spring is usually enough to sustain the emerging shoots, but I know some gardeners who like to push the foliage a little with regular applications of a balanced liquid fertilizer (or once with some 14-14-14 slow release) when in growth. As the name implies, they do not like water stress and will quickly scorch if allowed to dry out, so keep the irrigation consistent. The dormant stems can be cut to 6” above the ground in spring before the new growth emerges and gardeners in northern regions will often cover the base of the stems with coarse bark mulch for winter. Although generally hardy to USDA zone 5, it is really important to site them where the sun can warm the soil in spring.
Bloom time is from July to September (later here on the coast) and while the individual flowers only last 1-2 days, the sheer size and glossy satin finish make them real showstoppers in the garden. They are not a common find here in coastal BC, but over the years I have managed to be able to bring in several cultivars (some of which are complex hybrids) including ‘Kopper King’ (pale pink, red center and bronze foliage), ‘Robert Fleming’ (compact red), ‘Fantasia’ (rosy-pink), ‘Lord Baltimore’ (crimson) and the compact (2-3’ tall) Luna Series which comes in red, pink, rose and white. All of the blooms have a satiny-finish that shimmers in the sunlight and make for wonderful features in our often lack-lustre late summer gardens.
Hibiscus x moscheutos 'Kopper King', Hibiscus moscheutos 'Fantasia', Hibiscus moscheutos 'Lord Baltimore'
All Images Copyright 2020 to MK Lascelle