Knife Leaf Wattle

Knife Leaf Wattle (Acacia cultriformis) earns its place for the foliage long before it flowers. Stiff silver blue triangular phyllodes are arranged in overlapping ranks along arching stems, and in late winter and spring the whole shrub is hung with golden ball flowers. It grows fast to about 3m tall and 3m wide, and it copes with drought, frost, wind and thin soil without complaint.

Why you'll love it

• Silver blue foliage year round. The knife shaped phyllodes hold their colour through summer, so the shrub still works when nothing is in bloom.
• Gold flowers in late winter. Fragrant yellow balls cover the stems from late winter into spring, when the garden most needs colour.
• Fast cover on hard sites. It reaches 3m quickly and shrugs off drought, frost, wind and poor soil, which makes it a genuine problem solver.
• Feeds birds and bees, holds a slope. Small birds shelter in the dense stems, and the wide surface roots bind soil above retaining walls and on eroding banks.

Where it works best

Informal screening and windbreaks. Plant 2m apart for a screen that closes within two to three seasons. On exposed rural blocks a double staggered row at 2m filters wind rather than blocking it.

Banks and retaining walls. The arching habit spills over a wall face and the roots bind loose soil, so it suits batters and cuttings that are hard to plant.

Feature and mass planting. A single plant against a dark fence shows the silver foliage off well, or mass it at 2m spacing through a native garden.

Growing Knife Leaf Wattle in Australia

Position and climate. Full sun, always. It suits warm temperate, cool temperate, Mediterranean and arid or semi arid gardens, and being frost hardy it handles cold inland and tableland winters. It is not a plant for the wet tropics, where constant humidity shortens its life.

Soil. Drainage is the only real requirement. Poor, sandy and gravelly soils suit it; on heavy clay, plant on a raised mound. Skip the compost and skip phosphorus, which wattles resent.

Water and feed. Water weekly through the first summer, then only in extended dry spells. Feeding is rarely needed, and a light low phosphorus native fertiliser in spring is the most it wants.

Pruning. Trim lightly straight after flowering to keep it dense and stop it opening out at the base. Do not cut back into bare old wood, because wattles reshoot poorly from it.

What to expect when it arrives

Knife Leaf Wattle is grown as tube stock. Tubes are the right choice for wattles, because the root system never gets a chance to circle and young plants outgrow larger potted stock within a couple of years.

Expect little visible growth for the first six weeks, then a strong run once the roots are away, and first flowering in the second or third winter. Be aware that wattles are not long lived shrubs, often ten to twenty years, so plan for eventual replacement. Plants are delivered Australia wide, with delivery from $29.95 and a minimum order of $300 before delivery, and every plant carries our 30 day healthy plant guarantee.

Plant care guides and related reading

• The complete acacia and wattle care guide
• Browse Australian native shrubs
• Compare plants for windbreaks
• Planting above retaining walls and slopes

Common questions

How far apart should I plant it for a screen?

Two metres apart in a single row. Any tighter and the lower stems shade each other out, which leaves gaps at the base exactly where you want cover.

How fast does it grow?

Quickly. In a sunny, well drained position it usually reaches most of its 3m height within three to four years, faster in warm temperate gardens than in cold ones.

Can I keep it smaller than 3m?

Yes, with a firm trim after flowering each year it holds around 2m. Prune into green growth only, since bare wood rarely reshoots.