Prostanthera - Australian Mint Bush: Planting and Growing

Botany and description

The genus Prostanthera comprises 90 species, all native to eastern or southeastern Australia (particularly New South Wales). Australian mints or bush mints belong to the Lamiaceae family, well-known to gardeners, especially for the most cultivated aromatic plants: mint, sage, thyme, rosemary… In Australia, Prostanthera are commonly called “Mint bush”. The genus name Prostanthera comes from the Greek “Prosteke” meaning “appendix” and refers to the protuberances on the anthers where pollen forms.

Prostanthera rotundifolia (botanical plate circa 1840)

The most common species are Prostanthera cuneata, Prostanthera ovalifolia, Prostanthera rotundifolia, but one can also find Prostanthera caerulea, Prostanthera lanceolata, Prostanthera nivea var. induta, Prostanthera lasianthos, Prostanthera melissifolia, Prostanthera staurophylla (Tenterfield Mint Bush) and Prostanthera sieberi.

Prostanthera rotundifolia, Prostanthera cuneata (© color line) and Prostanthera lasianthos (© Leonora Enking)

Typically, Prostanthera are small bushes, usually very ramified : less than one metre high for Prostanthera cuneata. But some form beautiful shrubs up to 2 m tall like Prostanthera ovalifolia or Prostanthera caerulea. Others still form small trees up to 10 m tall in their natural habitat, such as Prostanthera lasianthos. In short, the Prostanthera genus is vast and diverse.

The foliage is evergreen and releases a strong camphor or mint scent when crushed. This fragrance earned it the nicknames “bush mint” or “Australian mint”. The leaves are single and opposite and develop in numbers along the stems. Note that branches and stems often have a square section, a characteristic of the Lamiaceae family.

Flowering occurs in spring or summer depending on the species and is usually arranged in clusters. Each flower is more or less bell-shaped, but the two lips typical of the Lamiaceae family are clearly distinguishable. The upper lip forms a galea with two fused lobes, while the lower lip, marked at its base by a yellow spot, has 3 well-defined lobes. Depending on the species, flowers can be white, yellow, green, blue, purple, mauve or even red.

Prostanthera incisa (© HQ), Prostanthera junonis and Prostanthera rose

The flowers are very nectariferous and attract a multitude of pollinating insects (bees, bumblebees, butterflies…), even in our latitudes.

The fruiting that follows flowering produces very small nutlets: a nutlet is a fruit with a very hard wall, like a hazelnut for example.

Our most beautiful varieties

Prostanthera cuneata - Mint Bush

Prostanthera cuneata is an unusual evergreen shrub, forming tapetums covered with tubular flowers, white washed with mauve and speckled with purple inside.

• Flowering time June to September
• Height at maturity 60 cm

See the product sheet

Prostanthera ovalifolia Variegata - Mint Bush

Prostanthera ovalifolia 'Variegata' forms a rounded yet airy mass, woven with small grey-green leaves marginate with cream-white, becoming spectacular in spring when covered with a multitude of small tubular flowers, gathered in violet-blue clusters.

• Flowering time May to July
• Height at maturity 2,10 m

See the product sheet

Prostanthera rotundifolia - Mint Bush

Prostanthera rotundifolia forms a light, rounded yet airy mass, woven with small leaves that appear greyer in drier soil, and adorns itself in spring with a multitude of small tubular flowers, gathered in violet clusters.

• Flowering time May, June
• Height at maturity 2 m

See the product sheet

Prostanthera rotundifolia Variegata

Prostanthera rotundifolia 'Variegata' forms a light, rounded yet airy mass, woven with small olive-green or slightly greyish variegated leaves with cream. It becomes covered with a multitude of small tubular flowers, gathered in violet clusters.

• Flowering time May, June
• Height at maturity 1,50 m

See the product sheet

Planting Prostanthera

Where to Plant?

Prostanthera adapts to a wide range of soils: clay soils, sandy soils, stony soils, even granitic soils. The soil pH should be neutral to acidophilous, with an optimal pH around 6. Australian mints do not like chalky soils.

The soil should remain moist but well-drained. A heavy, wet soil in winter will significantly reduce the bush’s hardiness. However, Prostanthera rotundifolia tolerates drought well.

Prostanthera are relatively tender, hardy down to only -6°C for most varieties. They should therefore preferably be planted in the ground in the south and south-west of France or grown in pots to be moved to a frost-free location during winter. However, Prostanthera cuneata withstands snow and temperatures down to -12°C. This species can be planted in the ground even in the north, provided the soil is well-drained.

Note that Prostanthera tolerates sea spray relatively well but dislikes cold winds. Regarding exposure, Prostanthera enjoys full sun or partial shade in the South, but Prostanthera cuneata tolerates shade well.

Prostanthera cineolifera in its natural habitat (© Ruth P)

When to Plant?

Planting is done in autumn (September-October) or spring (March to May), outside frost periods. If winters are cold (frost down to -9°C) and wet in your area, it is better to plant in spring, between April and May.

How to Plant?

In the Ground

1. Once you have chosen the location, dig a planting hole, two to three times the size of the root ball;
2. Do not hesitate to mix coarse sand or gravel into the soil to improve drainage. You can also mix one part heather soil with the excavated soil to help lighten and acidify the soil;
3. Remove the bush from its pot and place it in the centre of the planting hole, ensuring the collar is level with the soil;
4. Fill the hole with the excavated soil and lightly firm it with your fingers;
5. Water thoroughly to moisten the soil well and to minimise potential “air pockets” that may form between the soil and the roots.

In Pots

1. Choose a pot with a minimum volume of 6 to 10 litres and with drainage holes at the bottom;
2. For the substrate, a mix of 1/3 heather soil and 2/3 good-quality multi-purpose compost will be perfect;
3. Place a layer of gravel or clay pebbles at the bottom of the pot to fill without blocking the drainage holes and prevent the substrate from escaping through the holes. Add a first layer of compost at the bottom of the hole;
4. Remove the plant from its pot and place it in the centre of the pot. Fill the remaining space with the compost and heather soil mix. Lightly firm with your fingers;
5. Water thoroughly.

Maintenance and pruning

Maintenance will involve watering the young plant regularly at its base during dry periods. Avoid wetting the foliage exposed to sunlight.

In regions with harsh winters, bring the bush indoors if potted or, if planted in the ground, apply a good mulch at its base (dead leaves, herbs…) to protect the stump, or even use winter fleece. This mulch will also help keep the soil slightly moist in summer.

Remove faded flowers daily!

Pruning should be done in March or just after flowering. Pruning is not really necessary, but you can trim the branches slightly to maintain a compact shape, especially in pots. You can also regularly remove diseased or dead wood.

Prostanthera linearis

Diseases and pests

Prostanthera rarely suffer from diseases. However, in damp and poorly drained soil, the roots may rot due to an attack by a fungus of the genus Phytophtora. New selections of Prostanthera cuneata are more resistant to this disease.

If you grow your Australian mint in a greenhouse or conservatory, whiteflies (commonly called “whiteflies”) and red spider mites (which are actually arachnids) may infest the young plant. Daily misting will prevent this.

→ The advice sheet Whiteflies: identification and treatment and the advice sheet Red spider mite: identification and treatment may be useful in case of infestation.

Propagation

By sowing

Sow the seeds in April, in pots or trays, using a special sowing compost. Keep the compost moist but not waterlogged during germination and the seedling’s growth. Transplant the seedlings into individual pots in autumn and plant them out the following spring.

By cutting

Take cuttings in July-August. Select semi-ripe (semi-lignified) terminal shoots about 15 cm long. Plant them in pots with a light compost, such as a special sowing and cutting compost. Keep the compost moist but not waterlogged. Place your cuttings in partial shade. They can be planted out the following spring.

How to pair Australian mint?

Prostanthera should be paired with plants that require the same growing conditions: warmth, well-drained soil that remains moist.

A Prostanthera rotundifolia ‘Variegata’ will thrive alongside a bush from Australia’s coastal heathlands: the Melaleuca squamea, a small evergreen bush with bottlebrush-like flowering. Also native to southeastern Australia (and New Zealand), Leptospermum are stunning small bushes with aromatic evergreen foliage: Leptospermum scoparium ‘Martini’ blooms with single flowers, transitioning from pale pink to deep pink between April and June. The Grevillea juniperina, also known as the juniper-leaf grevillea, is a beautiful evergreen bush of Australian origin, capable of flowering all winter through to spring in mild climates, with clusters of bright red flowers.

A New Zealand-inspired pairing featuring Prostanthera rotundifolia ‘Variegata’, Leptospermum scoparium ‘Martini’, Melaleuca squamea (© JJ Harrison), and Grevillea juniperina

Of course, no one is obliged to pair a Prostanthera solely with plants of Australian origin. Mediterranean bushes (oleanders), plants from the desert regions of America (Leucophyllum frutescens), or even some African species (Gomphostigma) can also serve as excellent companions in the garden.