Hibiscus is not often the first plant people would think about growing in Scotland, especially with our long, cold winters, and unpredictable summers. As our climate changes though, so does what we plant, and having seen hibiscus thriving in the sunny borders at RHS Wisley, I started thinking about how it might cope here.
Rose of Sharon
The hibiscus most commonly grown in Scotland is Hibiscus rosa-sinensis, a tropical species native to Vanuatu, and sometimes referred to as Rose of Sharon.
It is an evergreen shrub that produces glossy, ovate, bright green leaves, and extravagant blooms that last only a day or two but with the right warm, sunny conditions, will flower over an extended period. It is often grown under glass, or on a warm windowsill as a houseplant, due to it being too tender for our climate. The flowers come in a mix of rich crimson reds, whites, yellows, and oranges with an array of cultivars to choose from.
If like me though, you want to grow hibiscus outdoors then we have to turn to the hardy species: Hibiscus moscheutos, and Hibiscus syriacus. Both are cold hardy between -5 and -15 degrees. They require cold winters and hot summers, and do best in a bright, sunny location. At Wisley they are planted at the front of borders or grown against a wall where they can take advantage of the stored heat through colder spells.
Hibiscus moscheutos is a cold, hardy marsh plant, commonly known as Rose Mallow, that is native to south-east Canada, central and eastern USA, and north-east Mexico. It is a woody perennial that needs to be pruned to ground level in autumn to return the following spring, and requires a hot summer and sheltered location to flower. If however, the conditions are just right you will be rewarded with gigantic, outrageous blooms, up to 20cm across, throughout late summer and early autumn.
It can be slow to start in spring but quickly puts on a lot of growth as the weather warms up. Make sure to give it enough space otherwise it can crowd out other plants reaching up to 2m tall and 1m (78 x 40in) wide.
Soil wise it is not so fussy and will grow well in sand or clay, but a free draining loam is always ideal, just make sure it does not dry out as it requires constant moisture to do well. A good mulch in winter will protect the root system, help to lock in the moisture, and reduce watering.
The straight species is a fantastic plant to grow but there are several cultivars also worth considering for the variety of colour they offer. Hibiscus moscheutos ‘Pink Elephant’ is one that you just cannot ignore. It has the largest flowers of any hardy hibiscus and produces bright, pink blooms with a deep, red eye that can reach up to 30cm (12in) across creating a truly dramatic display.
Hibiscus syriacus is the hardier of the two. It is also a woody perennial with an upright habit of deeply lobed, dark green leaves which produces attractive, showy blooms in shades of blue, white, red, and pink. It is native to south central and south east China and Taiwan and can grow in part shade but prefers a full sun location where it will flower profusely throughout the season.
It can be grown as a specimen shrub but its dense habit means it can also be grown as a hedge or screen. Just remember that in the autumn the stems need to be cut back by at least half or more, to prevent wind damage and encourage root growth.
The cultivar Hibiscus syriacus ‘Red Heart’ is a true garden classic that will never go out of fashion. It can reach up to 2.5m (98in) tall and wide so give it plenty of space to show off its pure, white flowers with prominent, deep red centres that appear in abundance from July to October.
If, however, you want something slightly more muted, Hibiscus sinosyriacus ‘Lilac Queen’ is an exceptional cultivar for its mauve-purple flowers that have a paler throat and dark, red centre that perfectly contrasts against its glaucous, grey-green foliage.
Hibiscus paramutabilis is a lesser known species with 3-5 lobed, mid-green, palmate leaves and attractive, cupped white flowers with a red centre. It has an open, spreading habit that suits a more relaxed garden and does not carry the same density of its upright cousins. It is said to be hardy down to -20, and will survive cold winters but again, it needs a hot summer to flower and will do best in a bright, sunny location. It does not often appear for sale here but if you do see it, I would highly recommend buying it.
I hope I have inspired you to think about growing hibiscus here in Scotland. Growing exotic plants in our climate is always going to involve some trial and error but I have always believed it is from our mistakes that we learn the best lessons in the garden. So if you have a sunny, sheltered spot and want to try something new then please try growing hibiscus and hopefully you will be rewarded with some beautiful blooms next summer.
Scott Galloway is a Glasgow based horticulturist and plantsman, specialising in kitchen garden production and herbs. He is particularly interested in the cross-cultural relationships people have with plants and how this defines their uses.
Working with the National Trust for Scotland, he created a Scottish naturalised herb garden at Greenbank Garden, and is currently writing about plants for the RHS in his role as Plant Profile Writer. He is also building a National Collection of Bergenia in conjunction with Plant Heritage and hopes to be an authoritative voice on the genus.