Mimosa: planting, pruning and care

Description and botany

Genus Acacia encompasses species present on the African and American continents but above all in Australia and on some Pacific islands.

Australian mimosas have the unusual habit of flowering out of season. Brought by the English to the Riviera around 1850, mimosa has, so to speak, naturalised around the Mediterranean basin and also along the Atlantic coast as far as southern England!

Genus Acacia belongs to family Fabaceae or legumes and to subfamily Mimosaceae. It includes more than 1,200 species, most of which do not withstand our temperate climates. Species naturalised in France originate from Australian mountains and are confined to the Mediterranean region, the Atlantic littoral and a few sheltered spots as far as the Paris region. Some species such as Acacia baileyana, dealbata and decurrens are even considered invasive there because of a strong tendency to form suckers and to displace native plants. As with all legumes, roots fix nitrogen from the air and they grow very well in poor soil.

Acacia dealbata – botanical illustration

Here, height of erect trees ranges between 3 and 8 m (up to 30 m in their native range). Some have a sprawling habit ideal for covering a bank (Acacia baileyana Prostrate, declinata or dealbata Pendula…). For container cultivation there are forms with a compact habit such as A. retinodes Lisette, A. covenyi… A. dealbata Pendula grafted on a stem has a weeping habit of about 2.50 m in all directions and suits small gardens well. Mimosas often grow very rapidly. Acacia dealbata, the commonest, for example can grow by 1 m per year during its first five years and flowers after two years. In five years a plant can reach 6 m in height with a crown of similar breadth. However, its lifespan rarely exceeds 10 to 15 years. Other species can reach a century, such as Acacia melanoxylon, a mimosa with creamy flowering.

Their running root system helps retain soil on banks but can have a strong tendency to produce suckers. That is why these species are grafted onto the ‘four seasons’ mimosa rootstock, which is also tolerant of clay and calcareous soils. Grafting also encourages earlier flowering.

Leaves can be bipinnate with the look of fine ferns or replaced by phyllodia arising from a flattened, elongated or triangular petiole as in Acacia pravissima, which has a very particular imbricate silvery foliage. The foliage, generally evergreen, takes various hues — dark green, grey-green, bluish to purple as in Acacia baileyana Purpurea — interesting to consider since flowering generally lasts only between 3 and 6 weeks per year.

Flowering extends from December to March depending on species but also on mildness of the climate and exposure. Small round flowers are glomerules of bright yellow stamens gathered in axillary clusters. Their scent is unmistakeable: a fresh, herbaceous opening gives way to a divine honeyed aroma, a powdery fragrance.

Fruits are small brown pods that split down the middle and release round, flattened, rather tough seeds.

Various mimosa foliage: Acacia dealbata, Acacia retinodes (photo Forest and Kim Starr-Flickr), Acacia pravissima (photo John Tann)

Be careful: the term “Acacia” commonly designates a perfectly hardy North American tree, the black locust (robinia faux-acacia — Robinia pseudo-acacia), which is in the same family Fabaceae but has fragrant white pea-flowers in clusters. Conversely, the name Mimosa in Latin designates the sensitive plant (sensitive — Mimosa pudica), this suckering undershrub whose compound leaves close as soon as they are touched. It also forms small glomerules of stamens but pink and can be grown as an annual.

Main varieties of Mimosa

Mimosa varieties with fern-like leaves

Mimosa varieties with entire leaves

Seeds of sensitive mimosa for sowing

Acacia dealbata Gaulois Astier

Acacia dealbata is prized for its generous clusters of highly fragrant flowers and good hardiness. This cultivar has very finely divided foliage of a deep, glossy dark green, brighter than the species. Grafted, hardy to -6°C (-10°C on own root), tolerates calcareous soil and does not sucker. Superb specimen for a large pot that can be wintered away in cold areas.

• Flowering time February to April
• Height at maturity 5 m

See the product sheet

Acacia baileyana Purpurea

Young foliage with a very attractive feathery appearance in a fantastic grey-blue-silver colour, tinged with purple and lilac from March to November before turning green, adding depth to the garden and enhancing other foliage colours. Flowering in golden-yellow pompoms, sparsely in January–February but forming a lovely contrast with the silvery-green foliage. This large bush with a trailing habit copes well with calcareous soils, drought and frosts down to -8/-10°C, and does not sucker.

• Flowering time February to April
• Height at maturity 4 m

See the product sheet

Acacia retinodes Lisette

This evergreen mimosa with very elongated oval leaves is a more floriferous variety, offering clusters of yellow, fragrant flowers more generous than the type, flowering several times a year. Large, very supple bush and not very hardy (-6°C), tolerates calcareous soils and drought, and does not sucker. Fine specimen for planting in open ground or for a large pot, to be wintered away in cold regions.

• Flowering time June to December
• Height at maturity 4 m

See the product sheet

Acacia pravissima

Among the most cold-resistant (-10°C) and indifferent to soil pH, this small evergreen tree offers a graceful, slightly trailing habit with original foliage composed of small triangular phyllodia of grey-green colour. Generous flowering, buds tinged red before opening into pleasantly scented lemon-yellow stamen pompoms.

• Flowering time April, May
• Height at maturity 5 m

See the product sheet

Mimosa nuttallii Pink Sparkles

Undershrub grown in pots in our climates for its globular, downy pale-pink flowering and stature reaching 60 cm. It has retained its ability to react to the slightest stimulus by rapidly folding its leaflets. It can now more easily be grown in open ground.

• Flowering time July to October
• Height at maturity 60 cm

See the product sheet

Planting mimosa

Where to plant mimosas?

Sun-loving plant par excellence, choose a spot receiving at least 3 hours’ sun per day, ideally close to the house to enjoy the fragrance and to shelter it from prevailing winds.

Acacias offer very high drought resistance, particularly the group of mimosas with phyllodia such as the four-seasons Acacia. Mimosas prefer sandy or stony, poor soil, tending to be acidic or neutral. However, young plants grafted onto Acacia retinodes help them better tolerate lime, which causes foliage yellowing. Grafting also limits suckering.

Acacia dealbata, considered one of the hardiest (-10 to -12°C), readily resprouts from the stump and produces suckers even if its aerial parts have completely frozen. However, do not hesitate to replant the suckers in a more sheltered spot because young plants are even more sensitive to frost.

When to plant?

If you buy a mimosa in flower, it is preferable to wait until spring to put it in the ground. Keep the young plant in its pot, making sure it is watered, in the coolest part of the house.

How to plant?

This plant is easy to grow provided you prepare a well-drained planting hole to increase your mimosa’s tolerance to cold.

• Plunge the bucket into a tub of water to thoroughly moisten it.
• Dig a hole three times as wide as the root ball.
• Place a draining layer of gravel at the bottom if your soil is heavy, then mix in horticultural compost (no fertiliser) or even heather soil if your soil is calcareous.
• Insert a stake then position the tree without burying the graft union.
• Backfill with soil and firm gently.
• Water.

For planting in a pot:

A 40 cm-sided container is suitable for a mature tree. Use heather soil with an acidic pH according to the requirement of the variety, or ordinary soil or compost that is not too rich.

Care

Water regularly during first 2 years. Keep soil moist, especially during flowering. Apply a flowering plant fertiliser from March to September to potted plants.

Mimosas can be pruned after flowering to maintain a denser, rounded habit. This also reduces wind exposure and increases floriferous shoots. Don’t hesitate to gather stems for bouquets when young plant has enough vigour!

For potted plants, overwinter your mimosa in a bright, cool, frost-free room or protect the branches with a double layer of horticultural fleece if location is sheltered, and mulch the stump up to the graft.

Repot every 2 years into a container slightly larger but proportional to tree size.

Mimosas are not very prone to diseases and pests. Chlorosis of foliage is caused by excess lime in soil.

Find out more in our care sheet : Diseases and pests of mimosa.

Also read our advice : How to protect a mimosa in winter?

Propagation

Simplest propagation is to separate suckers on suckering species, but sowing in spring is also possible. Grafting requires some technical skill; it is carried out on non-suckering species, such as mimosa of the four seasons, which tolerates calcareous soil but confers low hardiness.

Sow mimosa

• Soak seeds in hot water or scarify the epidermis with sandpaper.
• Sow them in sowing compost and keep the sowing warm.

Uses and companion plants for mimosa in the garden

In mild climate, medium-sized mimosas such as Acacia dealbata make fine open-ground specimens to train on a trunk. Light foliage does not cast excessive shade in winter and it is advisable to plant it sheltered by a wall or an evergreen hedge to protect buds from frost. However take care with non-grafted species that tend to sucker, as is the case with florist’s mimosa. Its running roots easily spread to other parts of garden, especially after tree has been cut back by frost.

Regularly pruned mimosas during or after flowering retain a low rounded habit as seen with producers of cut flowers and can be used to stabilise soil on a bank.

An example of a planting combination: Leptospermum scoparium ‘Martini’, Acacia retinodes ‘Lisette’, Escallonia ‘Apple Blossom’ (©Hortival Diffusion), Callistemon

Mimosa easily makes a screening and windbreak hedge when planted with Retama, Buddleia or Olearia for example, but control of suckers is required with suckering species. Small grafted cultivars are preferable to create beds of bushes with exotic appearance alongside Abutilon Suntense, Caesalpinia gilliesii, Jacaranda mimosifolia (blue jacaranda), Sesbania punicea, Callistemon, Leptospermum, Escallonia or Coleonema album.

To accompany early spring flowering of mimosas, plant alongside Prunus triloba, Japanese quince or Teucrium fruticans ‘Azureum’.

Did you know?

For enthusiasts, note that the national mimosa collection is located in Bormes‑les‑Mimosas at Gérard Cavatore Nurseries.

Mimosa was introduced to the French Riviera for scent and for bouquets around 1850 by the English. Plant has naturalized there to such an extent that some species have become invasive, such as Acacia baileyana, A. dealbata and A. decurrens. Navigator James Cook brought back the first seeds of Acacia verticillata from Australia.