Several hundred species belong to the plant genus Hibiscus or marshmallow (Hibiscus) worldwide, which all originate from Asia, but are widespread everywhere in the tropical and subtropical regions of the world. They are part of the large mallow family (Malvaceae). They are both annual and perennial herbaceous plants, semi-shrubs, shrubs and trees. The Chinese marshmallow (Hibiscus rosa-sinensis), also known as the rose marshmallow, is a very popular indoor and pot plant. In Central European latitudes, only the garden or shrub marshmallow (Hibiscus syriacus) and the breeding forms of the hardy perennial hibiscus (Hibiscus x moscheutos) can be considered for the garden.
Appearance and growth
The shrub marshmallow, also called garden hibiscus or garden marshmallow, is a funnel-shaped upright shrub that grows up to three metres high and 1.5 to 2 metres wide. It grows slowly and does not sprout until late spring. Its shiny, medium-green leaves are pointed, egg-shaped and trilobate. From August onwards, the striking, widely open bell-shaped flowers open at the branches. They are in the species simple and purple. In addition, there are numerous varieties with white, pink and red or violet to blue coloured single or partially double flowers. Often the flowers have a conspicuous dark red spot on the inside, which ends radiating towards the edge. Also conspicuous are the stamens, which, as in many mallow plants, have grown together tube-like, from which the three-branched stylus of the flowers protrudes. The pollinated flowers develop into small fruit capsules, which often remain on the plants throughout the winter. The rose marshmallow has striking, brightly coloured flowers ranging in colour from white, yellow and orange to red and pink. The pretty houseplants and potted plants are available with filled and unfilled flowers as well as leaves of different shapes. The cultivated forms of the perennial hibiscus are not yet widespread here. They grow to a height of between 120 and 200 centimetres and show striking flowers from July to October, which can reach a diameter of up to 30 centimetres. The colour spectrum ranges from white, pink and red to multicoloured flowers.
Location and soil
The shrub marshmallow wants a fully sunny, protected place, for example near terraces or in inner courtyards. Hibiscus needs well-permeable, fresh to moderately dry soil with a high nutrient content, whether in the garden, in the tub on the terrace or in pure indoor culture. If the soil contains too few nutrients, flower formation suffers. It thrives best in sandy-loamy substrate, which is slightly acidic to alkaline. If the drought persists, the bud may drop. However, the garden hibiscus is quite afraid of water. Although it loves sufficiently damp soils in summer, it does not tolerate waterlogging.
Also the flower splendour suffers with continuous rain, above all with filled varieties. The marshmallow, on the other hand, needs watering almost daily, especially during its flowering period. If one holds his hibiscus in the room and in the pot, then he needs a very bright place here also all year round. A sunny windowsill is well suited, only in midsummer you should make sure that the plant is not too exposed to the full midday sun. Normal room temperature is perfect, in winter it should be a few degrees less.
If possible, hibiscus should only be planted in the garden in spring so that it is well rooted by the first winter. The planting hole may be more than twice the size of the root ball. Before refilling, mix the excavated soil with some nourishing compost. Be careful when you step on the ground so that no roots are damaged. A mulch layer in the root area is advisable. Water the hibiscus well and also make sure that the young plants are watered in good time during prolonged dry periods, otherwise buds that have already set can fall off.
During its growth phase, the marshmallow needs some liquid fertilizer once a week. You can use conventional flower fertilizers for this. In winter, one fertiliser application every two to three weeks is sufficient. Plant your hibiscus – if its pot has become too small – best in spring or summer. For older, larger specimens, it is often sufficient to change only the substrate. In the case of the garden hibiscus, fertilizing is completely stopped in winter.
With the garden hibiscus, you can shorten all last year’s shoots to about five leaf knots in spring to bring the plant into shape. Dead shoots frozen after budding are thinned out. If necessary, the shrub also tolerates a radical rejuvenation cut into the old wood in late spring. However, it takes a while for the slow-growing shrub to grow back into a stately shrub. At the same time, the rose marshmallow is thinned out and the shoots slightly shortened.
Hibiscus can also be cultivated as a tall stem. However, it takes several years until this growth habit is fully developed. All lateral branches are removed every spring and only the strongest main shoot is left standing. Once it has reached the correct height, the tip is cut off to advance the bud drive. The topmost of the new page branches is now drawn as the trunk extension. Tie him to a staff to guide him straight up. The remaining three to four branches gradually form the crown. Shorten them regularly by about half so that they branch out nicely and densely.
Wintering or winter protection
Most hibiscus species are not hardy and have to move to their winter habitat very early in autumn for safe hibernation. This also includes the rose marshmallow. If he stands outside with his pot, temperatures of twelve degrees Celsius and above become too cold for him. Check your plant for pests and remove any dead or wilted parts of the plant before you put it away. A bright location is also indispensable in winter, otherwise the hibiscus loses its leaves. But a small leaf fall is normal. Moderately heated rooms (16 to 18 degrees Celsius) or a place in the cool winter garden are now ideal – this also applies to plants that are kept all year round in room culture. In an environment that is too dry or too warm, the risk of spider mite infestation increases. Watering is only moderate, it is sufficient if the root ball does not completely dry out. Fertilisation is completely omitted during the hibernation period. Starting in spring, you can slowly increase the amount of water you are given and add liquid fertiliser every two weeks. From May, the hibiscus can be outdoors again.
Many types of garden marshmallow, on the other hand, are hardy and can be planted in the garden. Especially young hibiscus plants should be protected with a winter protection like a thick mulch layer of leaves and brushwood in the root area. Alternatively, well-grown groundcovers also keep the heat in the ground. If one of these hardy specimens is kept in a pot, bring the plant container close to a protected house wall in autumn and place it on an insulating wooden or polystyrene base.
Hibiscus spreads tropical flair and creates a holiday atmosphere – inside and outside. A particularly beautiful sight results from the planting together of different hibiscus varieties. Suitable companions in the garden are bed roses, lavender (Lavandula), hollyhock and bushmalve (Lavatera). Besides, hibiscus is also a bee pasture and attracts numerous bumble bees and bees as well as useful insects. Underplanting the garden marshmallow with evergreen ground coverers protects the soil from drying out in summer and from frost in winter.
Among the different hibiscus varieties, those with single flowers have proved to be more flowering and more resistant than those with semi-double or double flowers. The garden marshmallow Hibiscus syriacus ‘Helena’, for example, has particularly beautiful simple white flowers with a red dot in the centre which ends in a star shape. Depending on how much space you have available, we recommend vigorous garden marshmallow varieties such as ‘Lady Stanley’ or low-growing varieties such as ‘Red Heart’. The blue flowering garden marshmallow variety ‘Blue Bird’ (Hibiscus syriacus ‘Blue Bird’) has proven to be very hardy and therefore also suitable for colder regions. A rarity among the rose marshmallows is the variety Hibiscus rosa-sinensis ‘Cooperi’. Although its red flowers are relatively small, its idiosyncratic white-green leaves make the hibiscus a very special eye-catcher on the windowsill. But beware: ‘Cooperi’ needs a relatively high humidity.
The varieties of the garden marshmallow are usually propagated by grafting. Sometimes, however, hibiscus also sows itself in the garden, whereby the flower colour and shape of the seedlings can later deviate from the mother plant. It is also possible to propagate by using wood from one-year-old woody shoots in autumn, but the failure rates are high: of ten, only one usually grows. Important is a shady, evenly moist propagation bed with very humus-rich, slightly loamy soil. It should be covered with fleece until the onset of winter.
The rose marshmallow can be propagated with non-woody head cuttings. These roots grow at a soil temperature of at least 22 degrees Celsius, better more, in standard commercial breeding soil.
Diseases and pests
Unfortunately, all hibiscus species and varieties are often attacked by aphids, which suck on the young shoots and flower buds. Especially in room culture, but also in winter quarters, spider mites often become a plague. Take care of cool temperatures and air that is not too dry.