Alcalthaea : planting, pruning, maintenance

Description and Botany

The (x)Alcalthaea suffrutescens is a semi-shrubby perennial from the Malvaceae family, like the Lavatera, the althaeas, the Mallow or Malva, or the hibiscus. It is a hybrid plant resulting from the cross-breeding between the hollyhock (Alcea), a perennial, and the tree mallow or Tree Mallow (Althea), a bush, giving rise to an inter-genus species named (x) Alcalthaea. Appearing in a Hungarian garden in the 1970s, this hybridisation has only been offered by French nurseries for about a decade. Today, only a few varieties are available for sale, ‘Freedom’ with pale pink flowers fading to light garnet, ‘Parkrondell’ with mauve-pink flowers with grey highlights, and ‘Parkallee’, with semi-double flowers. They are distinguished mainly by the colour of their flowering.

The 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 80 cm to 1 m, and lives for 5 to 6 years. Large, downy stems rise from the stump, which consists of a taproot. They develop deciduous foliage reminiscent of the Althea. The leaves, arranged alternately along unbranched stems, are lanceolate and more or less lobed with dentate and undulate edges. Slightly downy, they are tender green or ashen grey. The foliage is very healthy and proves resistant to rust, a sensitivity often attributed to hollyhocks.

Alcaltheae Parkallee (© Gwenaëlle David)

The Alcalthaea boasts the brilliance and abundance of hollyhock flowering. From June to September, but often until the first frosts, the flowers, more numerous than those of its cousins, are scattered along 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 cups, 4 to 6 cm across, with delicately ruffled petals.

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 highlights in ‘Parkrondell’, and ivory white with chamois highlights tinged with apricot and a raspberry-stamen heart in Alcalthaea ‘Parkallee’. Like all Malvaceae, the flowers of the alcathea are edible and can be added to summer salads for a colourful touch.

Unscented, these delicate corollas compensate for this lack with their remarkable floribundity, renewing continuously throughout the summer until autumn. Melliferous, they attract bees, butterflies, and other pollinating insects. The alcathea is a precious ally for the biodiversity of our gardens.

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

Main varieties

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Planting the hollyhock

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 and 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 can tolerate some summer drought in regions north of the Loire. In southern regions, it will manage with light shade. While it can handle occasionally dry soils, it will be more floriferous in soil that retains moisture during summer.

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

With its always fresh colours and simple yet elegant flowering, it is ideal for country and natural gardens or romantic settings. It can be used for hedging, lining a path, filling out a mixed border, as a standalone feature, at the back of a perennial bed, or to adorn a wall. Its tall silhouette adds charm and verticality.

When to Plant It?

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

How to Plant It?

Allow for 1 plant per square metre. For a beautifully flowering shrub, plant in groups of five or seven. The soil should be light and well-draining. In heavy soils, be sure to add 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 to 10 cm layer of gravel at the bottom of the hole for good drainage
• Position the root ball level with the collar
• Plant, mixing compost with garden soil
• Firm gently with your foot
• Water thoroughly at 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. After that, it will tolerate short periods of drought. Water mainly during dry spells.

Remove faded flowers regularly to stimulate flowering.

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

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

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

When and How to Prune Alcalthaea

Pruning is recommended, as it stimulates growth by encouraging the development of new shoots and thus flowering.

1. In March, perform a hard prune to encourage more abundant future flowering. Using pruning shears, cut back the old wood to 80 cm from the soil to maintain a compact habit and a height not exceeding 1.50 m. Remove any excess or poorly placed stems.
2. In June, you can lightly prune again by cutting back the branches to encourage them to ramify.
3. Every 2-3 years, pinch the stems around May-June to maintain a compact shape.

Multiplication

Successful Propagation of Alcathea by Cuttings

Alcathea propagates very easily by cuttings. Carry out the process from April to September on shoots from the current year.

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

Associate

Hollyhock naturally finds its place in naturalistic, country, and English cottage gardens. With its soft hues, it suits romantic borders perfectly. It looks stunning at the back of borders or to adorn the walls of an enclosed garden.

In a bed of summer-flowering bushes, it will complement the blooms 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, Gauras, hollyhocks, and mallows, alongside a Salvia nemorosa ‘Amethyst’. It will shine when paired with more graphic blooms 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 the upright habit of the hollyhock.

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

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