Knife leaf acacia is an attractive evergreen shrub with symmetric rows of triangular leaves that aren’t as dangerous as the name implies. Here’s everything you need to know about growing this resilient Australian native in this complete care guide.
Knife leaf acacia (Acacia cultriformis) is a one of a kind desert shrub. Long graceful branches are lined with rows of flat, leathery, gray-green leaves. In the spring, it explodes with a profusion of brilliant yellow puff-ball flowers.
Once you see this captivating shrub, you’ll never forget it. I saw my first one 15 years ago — and I’ve been pining for one ever since!
In spite of the word “knife” in the name, this shrub is unarmed and has no thorns or spikes to stab you. The leaves look more like rows of teeth than knives and the overall effect of this shrub is whimsical and airy.
It’s foliage and flowers together make such a striking combination that they’re sometimes used in floral arrangements!
Why I Like This Plant
- Evergreen gray-green foliage
- Bright yellow puffball flowers in spring
- Low maintenance
- No thorns, low litter, non-toxic
- Attracts bees, butterflies
What to Watch Out For
With all of these attributes, you would think this would be a popular plant, but you don’t see them very often. Mainly because very few nurseries stock them. So if you want one, and happen to see one, grab it while you can. You may luck upon one at Home Depot which carries the Australian Outback Plants brand.
Optimal Growing Conditions
If you’re thinking of adding a knife leaf acacia to your garden, you need to find a suitable place that will keep your shrub healthy and looking good… while minimizing maintenance for you.
Here are the key factors to keep in mind.
Temperature
Knife leaf acacia should be grown in USDA Hardiness Zones 8 – 11. It thrives in extreme heat and reliably survives temperatures down to 20℉, but reportedly can survive cold snaps down to the mid-teens.
Sun Exposure
Knife leaf acacia is a sun-loving plant that thrives in full sun but will tolerate some light shade.
Size and Growth Rate
Knife leaf acacia is a fast growing shrub that reaches a mature size of 6 to 12 feet high and wide. This plant’s growth rate and ultimate size depends somewhat on how much water it receives and how it’s pruned.
Unlike some acacias, knife leaf acacia is relatively long-lived, lasting up to 50 years.
Soil
Knife leaf acacia is not fussy about soil. It grows well in any desert soil — rocky, sandy, or clay — provided it’s well-draining.
Knife Leaf Acacia:
The Essentials
| Common Name | Knife leaf acacia |
| Scientific Name | Acacia cultriformis |
| Origin | Eastern Australia |
| Plant Type | Evergreen shrub/tree |
| USDA Zones | Zones 8 – 11 |
| Cold Hardiness | To 20℉ |
| Flower Color | Yellow |
| Flower Season | Spring |
| Mature Size | 6-12’ high x 6-12’ wide |
| Growth Rate | Fast |
| Sun Tolerance | Full, part sun |
| Water Needs | Low |
| Garden Friendly | No thorns, non-toxic |
| Wildlife | Attracts butterflies, bees Deer resistant |
How to Plant
The rule of thumb when planting any shrub in the desert is to dig a hole three times as wide as the root ball but no deeper. As tempting as it is to add amended soil, this is not recommended. Counterintuitively, backfilling with only native soil helps your plant develop a better root system.
When to Plant
The best time to plant knife leaf acacia is in the fall. This gives your plant three seasons to grow roots and get established before the following summer.
The second best time is in spring, the earlier the better. This still gives your plant time to get established before the intense heat of June arrives.
How to Care for Knife Leaf Acacia
Whether you’ve recently planted a new knife leaf acacia or have an existing one in your yard, here’s how to take care of it to keep it healthy and looking its best.
How to Water
Knife leaf acacia’s water needs are low, but plants still need watering, especially while getting established.
There are two factors to consider when watering any desert shrub.
- First is how often to water. This will vary with the seasons.
- Second is how much water to give your plant with each watering. This depends on its current size.
To determine the ideal watering schedule for any desert shrub, you’ll find everything you need to know, including easy-to-use charts, at How to Water Desert Shrubs: How Often? How Much?
Should You Fertilize?
There is no need to fertilize knife leaf acacia which evolved to grow in poor soil. And since it is a member of the legume family (Fabaceae), it fixes nitrogen, which naturally improves the surrounding soil.
How to Prune
Knife leaf acacia has a naturally attractive form and rarely needs pruning unless you want to grow it as a multi-trunk tree.
If you chose to shape it as a tree, I urge you to hire a certified arborist (NOT a landscaper) to prune it. Correctly shaping a new tree is a special skill — it’s easy to go wrong and ruin the shape forever.
Plant Lover Facts
Knife leaf acacia is native to the eastern Australian states of Queensland and New South Wales. It has naturalized in India, parts of Africa and South America, and in California in the US.
Knife leaf acacia also goes by the names knife acacia or knife leaf wattle. Acacias are commonly called wattles in Australia. The word wattle comes from a quick house construction method used by the early English settlers that involved using acacia wood.
Knife leaf acacias technically don’t have leaves — they have phyllodes which are flattened leaf stalks that look and function like leaves. Phyllodes are common in acacias, especially in the Australian species.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for over 15 years.
Photo Credits
Stickpen, Public Domain, Wikimedia Commons
Consultaplantas, CC BY-SA .0, Wikimedia Commons