Swamp Mahogany

Swamp Mahogany (Eucalyptus robusta) is the eucalypt to plant where the ground stays wet. It grows quickly to about 15m tall and 8m wide in a garden, with a broad shady crown, thick corky red brown bark, and heavy cream flowers through the cooler months that draw lorikeets and honeyeaters. Very few large trees tolerate waterlogged soil, salt wind and humidity as comfortably.

Why you'll love it

• It grows in wet ground. Boggy corners, creek flats and clay that stays saturated after rain all suit it, in conditions where most trees drown.
• Fast shade. A broad crown develops quickly, so a bare paddock gets useful shade within about five to seven years.
• Serious wildlife value. The winter nectar flow is heavy and reliable, making it an important feed tree for nectar feeding birds on the east coast.
• Coastal and wind tolerant. Salt laden wind and exposed frontage sit within its range, which is why it holds the front line on coastal properties.

Where it works best

Shade trees on larger blocks. Allow at least 8m of clear space each side. This is a tree for acreage, a large lawn or a paddock, not a suburban courtyard.

Windbreaks. Given a mature width of 8m, set trees 6m to 8m apart in a row for a windbreak that filters rather than blocks, well back from buildings and services.

Wet sites and coastal gardens. Dam edges, drainage lines and low lying ground are its natural home, and the salt tolerance suits exposed coastal properties.

Growing swamp mahogany in Australia

Position and climate. Full sun. It belongs in tropical, subtropical and warm temperate gardens and takes light frost only, so cold inland and cool southern districts sit outside its range. Humidity, coastal exposure and wind cause it no trouble.

Soil. Anything from coastal sand to heavy clay, and it is one of the few large trees that positively tolerates wet feet. It also grows well in ordinary garden soil that drains freely, though it puts on height fastest with steady moisture.

Water and feed. Water deeply once or twice a week through the first two summers while the root system establishes, after which it looks after itself. A low phosphorus native fertiliser in early spring for the first few years is enough.

Pruning. Formative pruning only, in the first few years, to establish a single strong leader and lift the lower branches for clearance. Once established, large eucalypts should be pruned by a qualified arborist.

What to expect when it arrives

Swamp mahogany is sold here in one size, a 45L pot: an established young tree with a formed trunk and useful height on day one. Tease the outer roots out at planting so none are left circling, and stake only if the site is exposed.

A 45L tree is heavy. Advanced and large stock is freighted at cost. Plants are delivered Australia wide, with delivery from $29.95. Expect a quiet first season while the roots take hold, then rapid height from the second year. Every plant is covered by our 30 day healthy plant guarantee.

Plant care guides and related reading

• Browse Australian native trees
• Native plants that bring in birds
• Compare fast growing shade trees
• Other trees for a windbreak row

Common questions

How close to the house can I plant it?

Not close. The roots are vigorous and the tree reaches about 15m tall and 8m wide, so keep it at least 15m from buildings, paving, pipes and septic systems.

Will it really grow in waterlogged soil?

Yes. It grows naturally in coastal swamp forest and tolerates periodic flooding and permanently damp ground, which is unusual in a tree this size. Ordinary soil that never floods suits it just as well.

How fast does it grow, and does it drop branches?

Fast, given warmth and moisture: often 1m to 2m of height a year early on, slowing as it approaches around 15m. Like most large eucalypts it sheds bark and the occasional limb, so have it inspected by an arborist every few years once it passes about 8m.