Knifeleaf Tree Care – Learn How To Grow Knifeleaf Acacia Trees
By: Bonnie L. Grant, Certified Urban Agriculturist
Acacias are one of the wonders of the savannah. In Australia, these magnificent plants are called “wattle” and Knifeleaf acacia trees are an outstanding example of native flora. It is such an attractive plant that many gardeners are growing Knifeleaf wattle as an ornamental. Some background on the tree will help you decide if the plant is right for your landscape.
What is a Knifeleaf Acacia?
Scented blooms, lovely blue-green leaves and statuesque appeal characterize Knifeleaf acacia (Acacia cultriformis). What is a Knifeleaf acacia? It is an arid, warm zone plant in the legume family that can fix nitrogen in soil. Additionally, the trees are easy to grow, have lasting appeal and Knifeleaf tree care is uncomplicated in appropriate sites.
Knifeleaf acacia is a small tree or bush that can achieve between 10 and 20 feet (3 to 6 m.) in height with a rounded to vase-like form. The name for the plant comes from the pointed leaves, which resemble the blade on a small dagger. Actually, the leaves are technically modified foliage called phyllodes.
It has numerous branches decorated with dark brown bark. The flowers are fragrant, bright yellow and look a bit like tiny pompoms. As a legume, acacia produces pods that are 1.5 inches (3.8 cm.) long and become dry and leathery over time.
How to Grow Knifeleaf Acacia
The plant is suitable for USDA zones 9 to 10. It needs full sun in clay, sand or loam and tolerates either slightly alkaline or acidic soils. The key element is that the soil drains very well, as the plants are not tolerant of soggy roots for long periods. In fact, this is a very drought tolerant plant once established.
Gardeners with deer problems might try growing Knifeleaf wattle, as it is not on those browser’s menu. Knifeleaf acacia trees grow slowly and can survive for up to 50 years. The fruit may become a nuisance, but they are very ornamental when attached to the tree.
Knifeleaf Tree Care
This is a very uncomplicated plant. Young trees will need supplemental water until they establish a good root zone. Thereafter, water plants during the hottest periods but allow the soil to dry out before irrigating anew.
They do not need much fertilizer, as they draw nitrogen from the air and store it in their roots. Knifeleaf acacia doesn’t need pruning either but is tolerant of some trimming to keep it in a tidy habit and out of the way of paths.
It makes and interesting screen or hedge and has several seasons of interest, making it well worth growing in arid, warm areas. As an added bonus, birds and pollinators are very attracted to the flowers and fruits.
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What Is A Knifeleaf Acacia Tree: Tips On Growing Knifeleaf Wattle - garden
Acacia is a very large genus of plants in the family Mimosaceae, a subfamily of the Fabaceae or Pea family. Acacia species are found throughout the world however most of the species that are cultivated in California come from Australia. There are many diverse forms within the genus. These range from large trees to prostrate shrubs. Many of the plants in cultivation are noted for their form, foliage color, flowering accent, adaptability and cultural tolerances. In many cases all of these attributes combine to make an exceptionally attractive and useful plant.
There are several cultural considerations when planting Acacia.
It is wise to select the appropriate plant for a given situation but in many cases the ultimate form of an Acacia is determined by pruning a large shrub species can be trained into a small tree or a tree species can become a shrub. To form shrubs out of trees, top out the main leader to create a tree from a shrub, prune out the side branches and stake. Shaded branches tend to dieback and should be pruned out. Pruning to open an acacia shrub or tree will minimize this as well as produce sturdier branches which will prevent wind damage - deadwood is easily felled by strong winds.
Acacias can withstand a fair amount of cold weather, but most cannot tolerate long periods below 20 ° F.
Irrigation and Fertilizing
Newly planted Acacias require an infrequent yet deep watering to produce a well anchored, deeply rooted plant. Once established usually within the first year in the ground, most Acacia can thrive with just rainfall and a few deep waterings. More frequent watering can promote root rot which can kill otherwise healthy plants. Acacia do not require heavy amounts of fertilizer and in some cases can be harmed by the addition of fertilizers high in phosphorus.
In general Acacias are relatively short lived, lasting anywhere from 20-40 years though older specimens of some species, such as Black Acacia (Acacia melanoxylon) are noted in the wild and in cultivation. This short life span is often compensated for by rapid growth and youthful vigor. Acacias also will often flower within their second year so by the time a tree or shrub is planted it is usually of flowering age.
The following Acacia are grown at San Marcos Growers:
Acacia adunca - Wallangara Wattle
Native to Queensland and New South Wales. Fast-growing, small (to 23 feet tall) tree with slightly weeping branches and long, dark dull green leaves and fragrant yellow flowers that form in terminal clusters winter through early spring. Requires full sun and good drainage for its best growth will grow in part shade. Frost and fairly drought tolerant once established. (We are currently not growing this plant)
Acacia baileyana - Fernleaf Acacia
Native to New South Wales. A fast-growing small (20-30 feet tall) tree with silvery gray/blue gray, feathery leaves, wide-spreading (20-40 feet) canopy, and weeping branches. Bright golden yellow small rounded flowers bloom late winter through early spring. Requires full sun to filtered shade, well-draining soil and regular watering for its best appearance. Not fussy about soil type. Once established it is frost and moderately drought tolerant.
Acacia baileyana 'Purpurea' - Fernleaf Acacia
Similar cultural and growth habits as A. baileyana other than the new growth has a purple tinge to it and its canopy is not as wide-spreading.
Acacia boormanii - Snowy River Wattle
Native to New South Wales and Victoria. A fast-growing multi-stemmed shrub to 16 feet tall. Composed of thin, graceful, silvery stems/branches and small, narrow gray-green leaves that are evenly spaced along the branches giving the plant a light feeling. Bright yellow rounded flowers form in clusters in early spring. Very tolerant of cold, it will withstand some drought, but looks better if placed on a regular watering schedule. Plant in full sun to filtered shade. Requires good drainage. A great specimen plant to bring in the vertical plane of your garden plan noticeable, but not overpowering, it can be used with many other plants to bring out their unique characteristics.
Acacia cognota - River Wattle
Native to Australia. A quick-growing, small (20-30 feet tall), graceful tree or shrub with narrow, drooping bright green leaves and weeping branches. Small rounded yellow flowers appear in pairs in spring. Requires full sun to light shade and very good draining soil. Protect from salt-laden air along the coast and hot sun inland.
Acacia cognota Cousin Itt ['Mini Cog'] - Little River Wattle
Native to Australia. A dwarf selection of a tree forming species that get 2-3 feet tall with narrow, drooping bright green leaves and weeping branches. Requires full sun to light shade and very good draining soil. Protect from salt-laden air along the coast and hot sun inland.
Acacia covenyi - Blue Bush
A fast growing shrub or small tree to 20 feet tall from Southern New South Wales, Australia with dark brown stems and silver-blue 1 to 2 inch long elliptic leaves (phyllodes). In spring appear the ½ wide globular bright yellow flower heads that are held in upright racemes about the length of the phyllodes. Plant in full sun in a well drained soil. Requires little to no irrigation. Tolerates hard frosts and temperatures below 15° F.
Acacia craspedocarpa - Leather-leaf Acacia
Dense, rounded shrub to 8 feet tall by 5 feet wide with leathery 1 inch long broadly elliptical gray leaves (phyllodes) finely netted in green. The stems and new growth have coppery tones and the bark is somewhat fissured. The flowers in short, golden spikes (rods) appear in spring and various times through the year and are followed by attractive flat, rounded bright green seed pods. It is very drought tolerant once established but also tolerates regular irrigation in well drained soils. Cold hardy to 15° F. Some sources in the desert southwest list it as a tree to 15-20 feet but it is so slow growing here along the coast that this is hard to imagine. It is an interesting small tree or shrub for a low screen. Plant not currently in production
Acacia cultriformis - Knifeleaf Acacia
Native to Queensland and New South Wales. A drought resistant bushy shrub growing to a little over 13 feet tall, and as wide. 1 inch long gray-green triangular leaves tightly hug the drooping gray branches. Fragrant bright yellow rounded flowers form in clusters in spring. Not fussy about soil type, but it requires good drainage. Plant in full sun to light shade. Frost resistant. Foliage can be used in flower arrangements. An unusual plant for its leaf-shape and color. Can be used as a specimen in a mixed drought garden as a hedge or screen.
Acacia glaucoptera - Clay Wattle
Native to Western Australia. A highly unusual acacia that can be used selectively as a ground cover in a small area. Growing to a little over 3 feet high, it has a sprawling habit twisting branches clothed with continuously overlapping, flat pointed, gray-green leaves giving the appearance of a zigzagging stem (think of what is commonly depicted as a dragon's back). The new growth of this acacia is red or bronze and in the cooler months the foliaged stems take on an almost purple tone. Large, rounded yellow flowers form along the stems in spring. Grows in full sun to light/filtered shade. Requires very good draining soil. A good plant for under open trees. Prune back if it becomes straggly looking. An oddity in the garden, it nevertheless, will always command a viewer's attention, wonderment and praise. Plant not currently in production
Acacia iteaphylla - Willow Acacia
A dense tall shrub to 10-13 feet tall with intricate angular branching structure and somewhat drooping tips and bearing many soft narrow blue-green leaves (phyllodes) Stems and leaves are have a distinct red tinge when young. Pale yellow fragrant flowers emerge in late winter to early spring. A very adaptable shrub that tolerates most soils. Noted to be drought and lime tolerant and tolerant of coastal planting outside of severe exposure to salt spray.
Acacia longifolia - Golden Wattle
Native to New South Wales, Victoria, Tasmania and Southern Australia. A very hardy, fast-growing bushy shrub/small tree to a little over 16 feet tall, and as wide. Long (3-6 inches), light green leaves and fragrant round, golden yellow flowers that form along the plant's branches in winter/early spring make up this plant's appearance. While very adaptable to soil types it does require good drainage. Plant in full sun to light shade. A good plant for seaside conditions. Frost and drought resistant. Use as a screen, windbreak or quick coverage of an unsightly object. Used a lot in the freeway plantings of southern California.
Acacia melanoxylon - Black Acacia
Native to Queensland, New South Wales, Victoria, Tasmania and South Australia. A very quick-growing, pyramidal tree growing to almost 40 feet tall and 20 feet wide, with a straight, vertical trunk and long (2-4 inches) dark gray-green leaves. Creamy flower clusters form in winter and spring. Plant in full sun to filtered shade. Should have a deep soil that drains well. A good tree for erosion control. Be cautious in using this tree as it has aggressive roots and can lift sidewalks or damage foundations if planted too closely. Its branches are considered brittle and the plant suckers easily.
Acacia merinthophora - Weeping Myall
An open shrub reaching 9 to 12 feet tall with a weeping habit from Western Australia. The phyllodes are long (up to 8 inches), curved , narrow and are gray-green in color. The stems of the branches change direction at the points where the phyllodes occur producing a zigzag shape. The branches are very attractive in dried arrangements. Short, rod-shaped flower clusters (about 1-2 inches long) are produced in the phyllode axils in early winter to early spring. They are bright yellow in color and are followed by slender, curved seed pods. Although native to Western Australia, Acacia merinthophora has been successfully cultivated in humid areas where many other western species fail. It is best grown in a well-drained, sunny position and, once established, will tolerate extended dry periods. Cold hardy to 25-30° F, possibly lower once established. The botanical name aacia is from Greek acis, a thorn. The specific epithet, merinthophora with long, thin phyllodes.
Acacia pendula - Weeping Myall
Native to Queensland, New South Wales and Victoria. A small, slow-growing tree to almost 40 feet tall in its native habitat, usually to around 25'x 15' in most gardens, with weeping branches clothed in narrow, 2-3 inch long, silvery blue-gray leaves. 1/4 inch wide pale lemon flowers appear irregularly. Drought resistant, but the soil should be slightly fertile and well-draining with some moisture. An excellent choice for a drought tolerant garden, mixed with grasses and phormiums, rock gardens and as an accent plant. Plant not currently in production
Acacia podalyriifolia - Pearl Acacia
Native to Queensland and New South Wales, Australia where it is commonly called Queensland Silver Wattle or Mount Morgan Wattle. A small tree or medium to large shrub to 15 feet tall by nearly as wide. The soft gray rounded phyllodes have a conspicuous central vein. Bears masses of bright yellow fragrant flowers in late winter. Needs good drainage to perform well. Responds well to heavy pruning after flowering. A very beautiful Acacia.
Acacia redolens - Prostrate Acacia
A low-growing acacia that is readily used as a groundcover, especially on slopes. Grows to 3-5 feet tall and 15 feet wide. Dense, the plant is heavily branched and covered with narrow, gray-green leathery leaves. Small yellow flowers appear in spring. Plant in full sun to light shade. Not fussy about soil type, but requires that it be good draining. Tolerant of drought and frost once established. Plant not currently in production and only growing the variety 'Low Boy'
Acacia redolens 'Low Boy' - Low Boy Prostrate Acacia
A selected very low-growing prostrate acacia that stays under 1 foot tall. This cutting grown plant is vastly superior to the seedling grown plants commonly found. A good groundcover, especially on slopes. As with the species it can grows 15 feet wide. Dense, the plant is heavily branched and covered with narrow, gray-green leathery leaves. Small yellow flowers appear in spring. Plant in full sun to light shade. Not fussy about soil type, but requires that it be good draining. Tolerant of drought and frost once established.
Acacia spectabilis - Mudgee Wattle
attractive small tree to 12 feet tall that has silvery-white branches that weep at their tips holding feathery bright green bipinnate leaves - uniquely, these fine bipinnate leaves stay with the plant for its entire lifespan and do not mature into the phyllodes common with most other Acacia. In late winter (March in our garden) appear the attractive small balls of 20 to 35 tiny lightly scented bright yellow flowers whose weight bows down branch tips. Grows well inland and along the coast where it can be plant in full sun to part shade in most any soil from well drained to clay that drains well and is not overly alkaline. Irrigate occasionally to very little - quite drought tolerant once established. It is hardy to around 18°F. Responds well to pruning to shape when young and a light shearing of spent flowers to encourage denser foliage, but avoid cutting into hard wood as this will not resprout. It is an attractive plant year round because of its delicate foliage and beautiful flowers.
Acacia stenophylla - Shoe-string Acacia
An open, quick-growing tree to 30 feet tall by 20 feet wide. Very long (to 16 inches), narrow and slightly twisted drooping leaves lightly clothe pendulous branches. New bark is maroon. Small, rounded creamy yellow flowers appear in late winter/spring followed by clusters of pendulous chocolate brown seed pods. A wonderful tree for light shade for a mixed succulent/drought tolerant garden for the unusual shadows cast by its structure. Drought and frost tolerant once it is established. Plant in full sun to light shade in deep, well-draining soil. Stake until it is deeply rooted.
Acacia vestita - Hairy Wattle
A dense growing tall shrub to 12-15 feet tall by equal spread. The long pendulous branches are covered with oval soft pubescent gray green leaves. The golden yellow flowers are held in clusters at the branch tips in the spring. A very adaptable shrub which tolerates both short periods of soggy soil and dry periods. Prune carefully so not to destroy the beauty of the pendulous branches.
Scottish botanist George Don described Acacia cultriformis in 1832,  calling it the cultriform-leaved acacia.  The species name relates to the knife-shaped phyllodes. 
Queensland botanist Les Pedley reclassified the species as Racosperma cultriforme in 2003, in his proposal to reclassify almost all Australian members of the genus into the new genus Racosperma  this name is treated as a synonym of its original name however. 
Common names include knife-leaf (or -leaved) wattle, golden glow wattle, half-moon wattle or dog-tooth wattle. 
Acacia cultriformis is a woody shrub with an upright or spreading habit that grows to 4 m (13 ft) in height. Branchlets may be bare and smooth or covered with a white bloom.  The mature trees do not have true leaves but have phyllodes that are crowded along the stems. The green to green-grey phyllodes are asymmetrical, with one leaf margin angled so the overall shape is triangular. They are 1–3 cm long by 0.6–1.5 cm wide.  There is a nectary at the widest part of the phyllode.  Flowering takes place from August to November,  and can be prolific on upper parts of the plant.  The rounded inflorescences are bright yellow and occur in groups of 2 to 25 in axillary 1–8 cm-long racemes. The later developing pods are straight or slightly curved, 3 to 10 cm long and 0.4 to 0.75 cm wide. They are flattish, with raised segments over the seeds.  The oblong seeds themselves are 3.5 to 4.5 mm long, black and shiny, with a clavate (club-shaped) aril. 
The related Acacia semilunata is similar in appearance but has hairy branchlets and narrower seed pods.  Acacia pravissima can look similar but has a secondary longitudinal vein in its phyllodes. 
Acacia cultriformis is found in central New South Wales and southern Queensland on the western slopes of the Great Dividing Range, from Wagga Wagga and Narrandera northwards, and west of Denman and Singleton,  to Stanthorpe and Inglewood.  It grows in clay-loam or sandstone soils in Eucalyptus woodland,  often on rocky ridges. 
Its bright flowers and attractive leaves make Acacia cultriformis one of the most popular wattles in cultivation. Adaptable to the garden, it is grown in a wide range of soils and can tolerate frosts. It grows in sun or part shade.  Drought tolerant, it can be used to combat soil erosion.  Acacia ‘Cascade’ (RN: ACC154) is a prostrate form that spreads to a diameter of 2 m (7 ft). It was registered on 18 November 1982, having been propagated by Bill Molyneux at his nursery in Montrose, Victoria from a selected seedling.  It makes an attractive cascading plant in a rockery. 
A. cultriformis is used as cut flowers.  The flowers are edible and they are an ingredient used in some fritters.   Yellow dye is extracted from the flowers and green dye is extracted from the seed pods. 
- Keep the soil evenly moist after planting established Acacia needs moderate to little water.
- Deep watering will help Acacia develop deep roots which will anchor plants on banks and slopes.
- Fertilize Acacia with an all-purpose fertilizer. Acacia grown indoors or in the greenhouse can be fertilized with a dilute solution of fish emulsion every two or so weeks.
- Protect plants in winter by adding mulch to the drip-line
- Acacia is easily shaped to a screen by removing the central leader or limbing up.
- In cold-winter regions, Acacia can be grown in the greenhouse if given ample light.
Map is from The Atlas of Living Australia web site, licensed under Creative Commons Attribution 3.0 Australia License
Dogtooth wattle, Half-moon wattle, Golden-glow wattle
The Knife-leaf Wattle is tree or shrub with upright or spreading habit growing to a height of about 4 m. The twigs are sometimes covered in a whitish bloom. The phyllodes green or grey-green and triangle-shaped, growing to about 3 cm long and 1.5 cm wide. The phyllodes are crowded along the stems. The plant flowers in late winter and spring from August to November. The bright yellow flowers are produced on flower spikes from 1 cm to 8 cm long. The seed pods are straight or slightly curved, about 3 cm to 10 cm long and about 0.5 cm wide. They are fairly flat, with bumps over the seeds. The seeds are black, shiny, about 4mm long and oblong shaped.
It grows in Eucalyptus woodland in clay-loam or sandstone soils.
Acacia cultriformis is found in central New South Wales and southern Queensland on the western slopes of the Great Dividing Range. It has also naturalised in New Zealand, Asia, Africa, North America and South America. The Knife-leaf Wattle is a popular garden plant.
Height (m): 2 - 4
Spread (m): 2 - 4
Soil Texture: loam, clay
Soil pH: acid soils, neutral soils, alkaliine soils
Frost Sensitivity: resistant
Minimum Rainfall (mm): 450
Flower Colour: yellow
Flower Season: winter, spring
(source: SA State Flora Catalog)
attracts seed-eating and insect-eating birds