Tree mallow (Malva arborea) prefers to grow on exposed coastal areas, as it has a high salinity tolerance and can excrete salt through its leaves. It produces vibrant purple flowers and is native to the British Isles, the western coasts of Europe, the Mediterranean, and parts of North Africa.
I. Appearance and Characteristics
Malva arborea (previously known as Lavatera arborea, or, more recently as Malva eriocalyx), the tree mallow, is a species of mallow native to the coasts of western Europe and the Mediterranean region, from Ireland and Britain south to Algeria and Libya, and east to Greece.
It was first described in 1753 by Carl Linnaeus as Lavatera arborea, but was transferred to the genus, Malva, in 1836 by Philip Barker-Webb and Sabin Berthelot.
Although long considered a species of Lavatera, genetic and morphological analysis by Martin Forbes Ray, reported in 1998, suggested it was better placed in the genus Malva, in which it was named Malva dendromorpha M.F.Ray. However the earlier name Malva arborea (L.) Webb & Berthel. was validly published and has priority over Malva dendromorpha.
It is a shrubby annual, biennial or perennial plant growing to 0.5–2 m (rarely 3 m) tall. The leaves are orbicular, 8–18 cm diameter, palmately lobed with five to nine lobes, and a coarsely serrated margin. The flowers are 3–4 cm diameter, dark pink to purple and grow in fasciculate axillary clusters of two to seven.
Malva arborea tolerates sea water to varying degrees, at up to 100% sea water in its natural habitat, excreting salt through glands on its leaves. This salt tolerance can be a competitive advantage over inland plant species in coastal areas. Its level of salinity tolerance is thought to be improved by soil with higher phosphate content, making guano enrichment particularly beneficial. It grows mainly on exposed coastal locations, often on small islands, only rarely any distance inland.
Invasive spread
Tree mallow seeds may be transported between separated coastal areas by the floating fruit, and seabirds are considered a likely means of spread. The seeds are encased in an impermeable outer case, and can remain viable for years, even after extended immersion in saltwater.
The tree mallow’s recent increased range among Scottish islands has raised concerns that it is displacing native vegetation, and is reducing Atlantic puffin (Fratercula arctica) populations in affected areas. The plant forms dense stands along the island coasts that inhibit breeding, and causes the puffins to abandon their burrows.
Its spread among Australian islands in recent decades is thought to be reducing biodiversity, soil retention, and seabird habitat. According to one study, “Crested and Caspian Terns that nest in the open are closed out by the canopy, and Little Penguins (Eudyptula minor) and Bridled Terns (Sterna anaethetus) are excluded by the lack of undergrowth”.
In New Zealand it has established on several seabird-dominated islands where it is considered to be a serious ecological weed and for which there are active eradication programmes, such as on North Brother Island in Cook Strait.
II. How to Grow and Care
Tree mallow is known for being a hardy coastal plant with a tolerance for salt-laden winds, making it relatively easy to manage. Key care needs include ensuring well-draining soil and full sun exposure to promote healthy growth. A special care point for tree mallow is its requirement for occasional pruning to maintain a desired shape and remove dead stems, which can help prevent rust disease. Regular watering during extended dry periods will also support its vigorous growth.
Sunlight
Tree mallow thrives best in full sun conditions, where it can receive unfiltered sunlight for the majority of the day. This intense light exposure is crucial for the plant’s optimal growth and flowering. While tree mallow can tolerate some variations in light, deviations from full sun may lead to suboptimal growth and reduced bloom production. In outdoor environments, tree mallow should be positioned in open spaces away from larger plants and structures that could create significant shade. Insufficient light may result in leggy stems or pale foliage as the plant strives for more light.
Temperature
The tree mallow can grow under a broad range of temperature conditions, with bushier varieties usually able to handle warmer temperatures. This is a hardy plant, able to withstand a winter that drops down to -40 ℃. Some varieties, like the Tree tree mallow, can handle drought well once established, and will burst back into life when the rains return.
Watering
Originating from the Mediterranean region, tree mallow is accustomed to a climate with periodic dry spells. This species exhibits moderate drought tolerance and prefers well-drained soil, reflecting its coastal origins where water availability can be inconsistent. Watering every week will maintain its health without over-saturating the roots. As an outdoor plant often found in coastal landscapes, tree mallow thrives when its watering schedule mimics the natural rainfall patterns of its native habitat, ensuring it receives adequate moisture during its active growing season.
Soil
The tree mallow is not picky about its growing substrate. It can handle both clay and sandy soils and will adapt to most gardens. It doesn’t necessarily require rich organic soils, and fertilizers are not a must. It will grow in neutral soil, but most will thrive if the soil pH is around 7.0.
Fertilizing
For tree mallow, apply a balanced nutrition fertilizer to support robust growth and vibrant blooms. Fertilize bi-monthly during the growing season, tapering off in late summer to prepare for dormancy. Use recommended label quantities, typically a quarter to half strength, to prevent nutrient overload. Adapting the regimen to tree mallow’s lifecycle stages ensures balanced vegetative and reproductive development. Always water tree mallow thoroughly after fertilizing to distribute nutrients and avoid root burn. A proper fertilization strategy enhances tree mallow’s health, resilience, and ornamental value.
Pruning
The tree mallow, especially the bushier varieties, benefits from occasional pruning. Trimming should be done in late winter or early spring when the frosts have passed. If freshly pruned plant tissue is exposed to freezing temperatures, it can be damaged by frostbite. Trim the shoots down to the first set of leaves, which will stimulate new and healthy dense growth. Use clean, sharp shears and always disinfect your tools in between the cuts to reduce the chances of possible cross-contamination.
Propagation
Tree mallow, known for its vibrant blooms and robust growth, propagates effectively through cuttings. Gardeners should select healthy, non-flowering stems for the best chance of success. Using a sharp, sterilized tool, make a clean cut below the leaf node and strip the lower leaves. Dip the cutting end into rooting hormone and place it in a well-draining soil mix. Consistent moisture and warmth aid root development without introducing rot, ensuring a healthy foundational start for tree mallow.
Transplanting
For the best establishment of tree mallow, transitioning them from late winter to the cusp of summer grants them the perfect growing momentum. Choose sun-kissed, well-drained locations, and remember, gentle handling of roots underscores a successful move.
III. Uses and Benefits
- Ornamental uses
Although tree mallow is a biennial plant, in gardens, it is commonly grown as an annual because it flowers only during its first year. That is why it is used in containers and as an annual bedding plant, rather than within any lasting garden arrangements. The flowers that are its primary value are hibiscus-like, trumpet-shaped, purple to pink in color. Additionally, tree mallow makes a fine cut flower.
- Other uses
The leaves of the species are used in herbal medicine to treat sprains, by steeping them in hot water and applying the poultice to the affected area. It is theorised that lighthouse keepers may have spread the plant to some British islands for use as a poultice and to treat burns, an occupational hazard. Thought to have been used as an alternative to toilet paper. The seeds are edible and are known in French as “petit pains”, or “little breads”.
Tree mallow was considered a nutritive animal food in Britain in the 19th century, and is still sometimes used as animal fodder in Europe. For human consumption, some sources describe the leaves of tree mallow as edible, although not as palatable as common mallow, unless cut very thinly, because of the very velours-like hairy mouth-feel.
Malva arborea has long been cultivated in British gardens, as described in the 1835 self-published book British Phaenogamous Botany, which used the then-common name Sea Tree-mallow: “This species is frequently met with in gardens, where, if it is allowed to scatter its seeds, it will spring up for many successive years, and often attain a large size. The young plants will, as Sir J. E. Smith observes, now and then survive one or more mild Winters; but having once blossomed it perishes”.
While sometimes detrimental to seabird habitat, management of tree mallow (both planting and thinning) has been successfully employed to shelter nesting sites of the threatened roseate tern, which requires more coverage than common terns to impede predation.
