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 you 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 branched : less than a 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 nearly 10 m tall in their natural habitat, such as Prostanthera lasianthos. In short, the genus Prostanthera is vast and diverse.

The foliage is evergreen and releases a strong camphor or mint scent when crushed. This fragrance has earned it the nicknames “Bush mint” or “Australian mint.” The leaves are single and opposite and grow abundantly along the stems. Note that the branches and stems often have a square cross-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 typical lips of the Lamiaceae family are clearly distinguishable. The upper lip forms a helmet with two fused lobes, while the lower lip, marked at its base by a yellow spot, has three well-defined lobes. Depending on the species, the 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

The Prostanthera cuneata is an atypical 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

The Prostanthera ovalifolia 'Variegata' forms a rounded but light mass, woven with small grey-green marginate leaves edged with creamy-white, and becomes spectacular in spring, disappearing under a multitude of small tubular flowers, gathered in blue-purple clusters.

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

See the product sheet

Prostanthera rotundifolia - Mint Bush

The Prostanthera rotundifolia forms a light, rounded but airy mass, woven with small leaves that become greyer as the soil dries, and is adorned in spring with a multitude of small tubular flowers, gathered in purple clusters.

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

See the product sheet

Prostanthera rotundifolia Variegata

The Prostanthera rotundifolia 'Variegata' forms a light, rounded but airy mass, woven with small olive-green or slightly grey variegated leaves edged with cream. It becomes covered with a multitude of small tubular flowers, gathered in purple 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 acidic, ideally around pH 6. Australian mints dislike calcareous soils.

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

Prostanthera are relatively frost-sensitive, hardy only to -6°C for most varieties. They should therefore be planted in the ground preferably in the southern and south-western parts of France or grown in pots to be sheltered from frost during winter. However, Prostanthera cuneata can withstand snow and temperatures as low as -12°C. This species can be planted in the ground even in northern regions, provided the soil is well-drained.

Note that Prostanthera tolerates sea spray relatively well but dislikes cold winds. As for 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), avoiding frost periods. If winters are cold (frosts 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. Feel free to mix coarse sand or gravel into the soil to improve drainage. You can also mix some 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 surface;
4. Fill the hole with the excavated soil and lightly firm it down with your fingers;
5. Water thoroughly to moisten the soil and eliminate any potential air pockets between the soil and the roots.

In a Pot

1. Choose a pot with a minimum volume of 6 to 10 litres and equipped with drainage holes at the bottom;
2. For the substrate, a mix of 1/3 heather soil and 2/3 good-quality universal 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. 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 it down with your fingers;
5. Water thoroughly.

Maintenance and pruning

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

In regions with harsh winters, bring the bush indoors if it’s 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 plants 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 known as “whiteflies”) and red spider mites (which are actually mites) may infest the plant. Daily misting will help prevent this.

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

Multiplication

By sowing

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

By cutting

Cuttings should be taken in July-August. Take semi-hardened terminal shoots about 15 cm long. Plant them in pots in a light substrate, such as a special sowing and cutting compost. Keep the compost moist but not waterlogged. Place your cuttings in partial shade. You can plant them in the ground 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 the coastal heathlands of Australia: the Melaleuca squamea, a small evergreen bush with flowering clusters resembling tiny brushes. 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 throughout winter into spring in mild climates, with clusters of bright red flowers.

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

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