Alcalthaea: Planting, Pruning, Care

Description and botany

The (x)Alcalthaea suffrutescens is a semi-shrubby perennial from the Malvaceae family, like Lavatera, hollyhocks, Mallow or Malva, or hibiscus. This hybrid plant results from cross-breeding between hollyhock (Alcea), a perennial, and the tree mallow or Rose of Sharon (Althea), a bush, creating an inter-genus species named (x) Alcalthaea. First appearing in a Hungarian garden in the 1970s, this hybrid has only been available from French nurseries for about a decade. Today, only a few varieties are available for purchase, including ‘Freedom’ with pale pink flowers fading to light garnet, ‘Parkrondell’ with mauve-pink flowers with grey hues, and ‘Parkallee’, with semi-double flowers. They are distinguished primarily by their flowering colours.

Alcalthaea forms a beautiful bushy clump with an upright shrubby habit. It reaches 1.50 to 3 m in height, sometimes up to 5 m, with a spread of 80cm to 1m, and lives for 5 to 6 years. Large, downy stems rise from the stump, which has a taproot. They develop deciduous foliage reminiscent of Althea. The leaves, arranged alternately along unbranched stems, are lanceolate and more or less lobed with dentate and undulate edges. Slightly downy, they are soft green or ashen grey. The foliage is very healthy and proves resistant to rust, a common issue often associated with hollyhocks.

Alcaltheae Parkallee (© Gwenaëlle David)

Alcalthaea boasts the brilliance and abundance of hollyhock flowering. From June to September, and often until the first frosts, the flowers—more numerous than those of its relatives—adorn the villous stems. Large flower buds bloom gradually from the bottom upwards. The flowering takes the form of single or semi-double corollas, similar to hibiscus flowers. They open into wide, well-spread cups, measuring 4 to 6 cm across. Their petals are delicately ruffled.

They display very nuanced and delicate shades depending on the variety, even more vibrant than those of its two parents, ranging from pale pink tinged with light garnet to mauve-pink with grey hues in ‘Parkrondell’, and ivory white with chamois highlights tinted apricot and a raspberry-stamened, frizzy heart in Alcalthaea ‘Parkallee’. Like all Malvaceae, the flowers of Alcalthaea are edible and can be added to summer salads for a colourful touch.

Though unscented, these delicate corollas compensate with remarkable floribundity, blooming continuously throughout summer until autumn. Melliferous, they attract bees, butterflies, and other pollinating insects. Alcalthaea is a valuable ally for garden biodiversity.

The flowering stems hold up well in vases and make beautiful bouquets with fresh hues.

Main varieties

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Planting Althaea

Where to plant it?

Hardy down to -15°C (Zone 7b), Alcalthaea can be grown throughout France. Like all Malvaceae, it thrives in sunny spots with light soils, even poor, chalky or stony ones, but will flower more abundantly in rich soil. It requires well-draining soil where it will prove more long-lived, as it dislikes heavy clay soils. It will tolerate summer drought in regions north of the Loire. In southern regions, it will manage with light shade. While it copes with occasionally dry soils, it will be more floriferous in soil that retains moisture in summer.

It is also well-equipped to withstand wind. Choose an open position to encourage it to reach its full height.

With its fresh colours and simple yet elegant flowering, it is ideal for naturalistic or romantic gardens. It can be used for hedging, lining paths, enriching mixed borders, as a standalone specimen, at the back of perennial beds, or to adorn a wall. Its tall silhouette adds charm and vertical interest.

When to plant it?

Alcalthaea is best planted in spring, from March to May, after the last frosts.

How to plant it?

Allow 1 plant per m². For a beautifully floriferous shrub, plant in groups of five or seven. The soil should be light and well-drained. In heavy soils, mix in sand and gravel (1/3) and compost to improve drainage.

• Dig a hole 2 to 3 times the size of the pot
• Spread a 5-10 cm layer of gravel at the base for drainage
• Position the rootball level with the collar
• Plant using compost mixed with garden soil
• Firm gently with your foot
• Water thoroughly after planting and until established

Maintenance, Pruning and Care

Alcalthaea is easy to maintain once well established in well-drained soil. Water regularly but moderately, two or three times a week during the first summer after planting. Afterwards, it will tolerate short periods of drought. Water mainly during dry spells.

Remove faded flowers regularly to encourage flowering.

In autumn, in very cold regions, you can apply a mulch (straw or dead leaves) to protect the stump from severe frosts.

Being short-lived, plants disappear within 5 to 6 years. We recommend renewing one in three plants each year to maintain well-filled flower beds.

Unlike other Malvaceae, Alcalthaea is not prone to rust, the fungus that causes orange pustules on leaves.

When and how to prune Alcalthaea

Pruning is recommended as it stimulates growth by encouraging new shoots and therefore flowering.

1. In March, carry out hard pruning to promote more abundant flowering later. Using pruning shears, cut back old wood to 80 cm from the soil to maintain a compact habit and a height not exceeding 1.50 m. Remove any overcrowded or poorly placed stems.
2. In June, you can lightly prune again by cutting back shoots to encourage them to become ramified.
3. Every 2-3 years, pinch stems during May-June to maintain a compact shape.

Propagation

Successful Propagation by Cuttings of Alcathea

Alcathea propagates very easily by propagation by cuttings. Carry out between April and September on current year’s growth.

• Choose vigorous shoots
• Using disinfected pruning shears, take 10-15 cm stems
• Remove the base leaves and any flowers or buds
• Insert the cuttings into buckets filled with a slightly moist mix of sand and compost
• Place in partial shade
• Keep them under a cold frame throughout winter, maintaining moist but not waterlogged substrate
• The following spring, transplant them to their permanent location

Pair

Alcalthaea naturally finds its place in naturalistic gardens, country gardens and English cottage gardens. With its soft hues, it works well in romantic borders. It looks magnificent at the back of borders or to clothe the walls of an enclosed garden.

In a bed of summer-flowering bushes, it will complement the flowering of Hibiscus or Althaea, Abelia, Buddleia or simply old-fashioned roses.

In a bed of summer-flowering perennials, it pairs well with tall pink or blue asters, Gaura, hollyhocks and mallows, alongside Salvia nemorosa ‘Amethyst’. It benefits from being paired with more structural flowering plants like the spherical heads of Allium ‘Globemaster’ or the slender stems of Verbena bonariensis.

A few clumps of ornamental grasses, the delicate Melica altissima ‘Atropurpurea’ with its slightly wild appearance, Pennisetum alopecuroides, Miscanthus or Stipa tenuissima will add softness as a counterpoint to Alcalthaea’s upright habit.

Alcalthaea ‘Parkallee’, Verbena bonariensis, Pennisetum alopecuroides ‘Hameln’ and old-fashioned rose ‘Blush Rambler’

→ Discover more inspiration and ideas for pairing Alcalthaea in our advice sheet!