For a few weeks each autumn the Alstonville tibouchina turns solid purple. Not scattered flowers - the whole canopy, so densely covered that the foliage disappears. It arrives in March and April when most gardens have gone quiet, which is exactly why it has become a fixture in Brisbane, northern New South Wales and Sydney gardens. The plant is Brazilian by origin but this particular cultivar is Australian, selected at Alstonville in the Northern Rivers.
Quick facts
| Detail | What to expect in Australia |
|---|---|
| Botanical name | Tibouchina lepidota 'Alstonville' |
| Mature size | 4-5 m tall, 3-4 m wide |
| Position | Full sun to light shade, sheltered from wind |
| Soil | Acidic, free draining, enriched. Dislikes lime |
| Flowering | Deep purple, autumn |
| Frost | Tender. Damaged below about 2°C |
| Growth rate | Fast - up to 1 m a year when young |
| Best climates | Brisbane, northern NSW, Sydney, coastal Perth |
Choosing a variety
Alstonville is the best known but there are a dozen tibouchinas available in Australia, ranging from four-metre trees to compact shrubs for pots. Size is the deciding factor for most gardens.
| Variety | Size | Flower | Best for |
|---|---|---|---|
| Alstonville | 4-5 m | Deep purple | Feature tree, the classic autumn display |
| Jazzie | 2-3 m | Purple | Smaller gardens, informal screening |
| Kathleen | 2-3 m | Pink | Pink alternative to the purple forms |
| Noelene | 2-3 m | White ageing to pink | Two-tone effect on one plant |
| Peace Baby | 1-1.5 m | White | Courtyards and pots |
| Little Beauty | 1 m | Purple | Pots, low borders |
| Purple Star | 1.5-2 m | Deep purple | Compact purple in a small space |
| Jules | 1 m | Purple | Dwarf shrub, containers |
Alstonville remains the choice where there is room for a four to five metre feature tree. For a suburban backyard, Jazzie gives the same effect at half the size. For pots and courtyards, Jules and Little Beauty stay around a metre.
Where it grows in Australia
- Northern NSW and southeast Queensland - its adopted home, and where it performs best.
- Brisbane, Gold and Sunshine Coasts - excellent, flowering heavily each autumn.
- Sydney and coastal NSW - very good in a warm sheltered position.
- Coastal Perth - workable with acidic soil management and summer water.
- Melbourne and Adelaide - marginal. Frost damages it, so grow the dwarf forms in pots that can shelter.
- Canberra and cold inland - too cold for reliable growth.
The two things that decide success
Soil acidity
Tibouchinas want acidic soil, ideally pH 5.5 to 6.5, and they show it immediately when they do not get it. Yellow leaves with green veins on a tibouchina almost always means alkaline soil locking up iron, not a shortage of fertiliser. Keep them away from concrete foundations and paths, which leach lime, and never apply garden lime or dolomite anywhere near them.
Shelter from wind
The wood is brittle and the flowers are short-lived once battered. A single windy week during flowering can strip a display that took a year to build. Plant where a fence, building or other planting breaks the prevailing wind.
How to plant
- Plant in spring once soil has warmed.
- Full sun for the heaviest flowering, though light afternoon shade is fine in hot inland areas.
- Enrich with compost and an acidic organic matter such as composted pine bark.
- Ensure drainage. Tibouchinas rot in wet soil, so plant raised on clay.
- Allow three to four metres of width for Alstonville.
- Water regularly through the first two summers.
Pruning: the key to next year's display
Prune straight after flowering finishes, cutting back by about a third. Tibouchina flowers on new growth, so pruning immediately after the display gives the plant a full season to build the wood that carries next autumn's flowers.
This also keeps the plant dense. Unpruned tibouchinas become leggy and open, with flowers only at the tips. They tolerate quite hard pruning and reshoot readily, so a neglected plant can usually be brought back.
Do not prune in late summer or autumn - you will be cutting off the buds.
Ongoing care
Watering
Consistent moisture through spring and summer while the plant is building growth, easing off in winter. Drought stress during summer reduces the autumn display noticeably.
Feeding
Use a fertiliser for acid-loving plants - the same products sold for camellias, azaleas and gardenias. Feed in early spring and again in midsummer. Stop feeding by late summer so growth hardens before flowering and before cold weather.
Common problems and fixes
- Yellow leaves with green veins is iron lock-out from alkaline soil. Apply chelated iron, then lower the pH with sulphur and acidic mulch.
- Few flowers usually means pruning at the wrong time, too much shade, or too much nitrogen.
- Leggy, open growth means it has not been pruned after flowering. Cut back by a third each year.
- Blackened growth after cold is frost damage. Prune to live wood in spring.
- Flowers dropping quickly is wind or heat. Shelter matters more than anything you can spray.
- Wilting with wet soil is root rot. Improve drainage.
Using it in the garden
Tibouchina works best where its autumn moment can be seen - near an entry, framing a view from a window, or against a pale wall that sets off the purple. Because it is fairly plain outside flowering, it pairs well with plants carrying year-round interest such as lilly pilly, cordyline or lomandra. The dwarf forms suit large pots, where controlling the potting mix also solves the soil acidity problem entirely.
Frequently asked questions
When does Alstonville tibouchina flower?
Autumn, mainly March and April in southeast Queensland and northern New South Wales. The display lasts several weeks and covers the entire canopy when the plant is well grown and sheltered.
Why are my tibouchina leaves turning yellow?
Almost always iron lock-out caused by alkaline soil rather than a lack of fertiliser. Tibouchinas need acidic soil around pH 5.5 to 6.5. Apply chelated iron for a quick fix and lower the pH over time with sulphur and acidic mulch.
When should I prune tibouchina?
Immediately after flowering finishes, cutting back by about a third. It flowers on new growth, so pruning then gives a full season to build next year's display. Never prune in late summer or autumn.
How big does Alstonville get?
Four to five metres tall and three to four metres wide. For smaller gardens, Jazzie reaches two to three metres and dwarf forms such as Jules and Little Beauty stay around a metre.
Will tibouchina grow in Melbourne?
Marginally. It is damaged below about 2°C, so in Melbourne, Adelaide or Canberra grow a dwarf variety in a pot that can be sheltered over winter rather than planting in an open garden.
Is tibouchina Australian?
The genus is Brazilian, but the Alstonville cultivar was selected in Australia at Alstonville in the Northern Rivers of New South Wales, which is how it got its name.
Ready to plant?
We grow the full tibouchina range, from Alstonville feature trees to dwarf forms for pots. Browse our top flowering trees, or plants for pots.
Related reading: gardenia care and soil types explained.
