Onion weed: how to kill this stubborn lawn invader for good

Onion weed is the lawn weed everyone underestimates. It looks soft and grassy. You pull a clump, see the small white bulbs come out, and think you have won. Three weeks later, more shoots are up. The truth: onion weed is one of the hardest lawn weeds to kill in Australia. No single spray or pull will do it. You need a multi-year, multi-attack plan — and even then, expect to chase stragglers for a few seasons. Here is the strategy that actually works.

What is onion weed?

The most common "onion weed" in Australian lawns is three-cornered garlic (Allium triquetrum) — a perennial bulb from the Mediterranean. Also called three-cornered leek or angled onion.

How to identify it:

• Smell — crush a leaf. Strong onion or garlic smell. This is the giveaway.
• Stems — distinct triangular shape, hollow inside. Three edges.
• Leaves — long, grass-like, slightly fleshy and folded
• Flowers — small white star-shaped with a green stripe. Spring (September–November).
• Bulbs — small white bulbs underground, each producing dozens of "bulblets" around it

The other plant called "onion weed" is nothoscordum — looks similar but with no smell. Same strategy works for both.

The big idea: onion weed survives because of the bulbs. Anything that does not kill the bulb just delays the problem. The whole plan is built around hitting the bulb.

Why is onion weed so hard to kill?

Three reasons:

1. Bulbs sit deep — often 10–15cm down. Most lawn herbicides do not travel that far.
2. It makes tiny bulblets. Each main bulb produces dozens of little ones. Dig one out and you usually leave bulblets behind. Each becomes a new plant next year.
3. The leaves are waxy. Herbicide sprays bead up and run off.

Plus it spreads by seed in summer. Every plant you miss drops dozens of seeds for next year.

Why don't Buffalo-safe weed killers work?

For Buffalo lawns (Sir Walter, Sapphire, Palmetto), Amgrow Bin-Die does almost nothing on onion weed. Buffalo-safe products target broadleaf weeds — bindii, clover, dandelion. Onion weed is more like a grass than a broadleaf, so these products miss it entirely.

The only herbicide that consistently works is glyphosate (Yates Zero, Roundup, or generic at Bunnings). Glyphosate kills nearly everything green — including your lawn where you spray. So the trade-off is real: kill the onion weed and accept small dead patches for a few months, or live with the onion weed.

The multi-attack strategy that works

The honest answer: no single method kills onion weed. You need to hit it from multiple angles, over multiple years. Here is the plan.

Attack 1 — Hand-dig in winter (the main attack)

The most effective single action. Why winter? The soil is wet and soft, so bulbs lift out cleanly. The plants are easy to spot. And most bulbs are still in first-year size, before making too many bulblets.

How to do it:

1. Wait for a day after good rain — soil should be soft.
2. Use a narrow garden trowel or weed digger.
3. Push it in about 15cm deep beside the plant.
4. Lever the whole clump out — leaves, stem, and bulb together.
5. Pull out any loose bulblets you can see in the hole.
6. Bag the lot in a sealed plastic bag and bin it. Do not compost or put in green waste — bulblets survive composting.
7. Fill the hole with soil and reseed if needed.

Slow work — a patch of 20 plants takes about an hour. But each plant removed with its bulb is gone for good.

Attack 2 — Spot-spray glyphosate in autumn

For larger patches, you need chemicals. Autumn (March–May) is the best time to spray — onion weed is pushing nutrients down to the bulbs, taking the glyphosate with it.

1. Buy a glyphosate product — Yates Zero, Roundup Bioactive, or any Bunnings store brand.
2. Mix per the label. Add a few drops of dishwashing liquid (helps it stick to waxy leaves).
3. Use a small spray bottle and aim directly at the onion weed leaves. Do not blanket spray.
4. Pick a still day, no rain forecast for 6 hours.
5. The surrounding lawn will yellow and die in a small circle within 7–10 days.

Repeat in 3–4 weeks on new shoots. Reseed dead spots in spring.

Honest note: you will get small dead patches. That is the trade-off. Fix them in spring and you are ahead of where you were with onion weed still there.

Attack 3 — Stop seed-set every spring

Even after you have hit the bulbs, surviving plants will try to flower in spring. Every flower head you let mature drops dozens of seeds. Those seeds stay viable for 4–6 years in the soil.

Walk the lawn weekly in spring (September–November). Snip off any flower heads you see and bag them. This single 10-minute job each weekend stops your future onion weed problem from getting worse.

What about boiling water or vinegar?

Both work on small plants but do not reach the bulb. You will scorch the leaves and feel like you have done something — then the plant regrows in 2 weeks. Skip these and stick with dig or spray.

Year-by-year onion weed plan (AU)

Realistic expectations: 2–3 years of effort to fully clear a moderate infestation.

Onion weed treatment by lawn type

What if it's in my garden beds, not the lawn?

In garden beds, you have more options. Glyphosate is still the most effective. Brush it directly onto the leaves with a small paintbrush to avoid hitting nearby plants. Or hand-dig — much easier in soft garden bed soil than in compacted lawn soil.

Whatever you do, do not let it flower. Garden bed onion weed is the source of most lawn reinfestations.

Frequently asked questions

Can I just mow it short and starve it out?

No. Onion weed has so much energy stored in the bulbs that it can keep sending up leaves for years. Mowing also chops the leaves before herbicide can work. If you plan to spray, do not mow for 2 weeks beforehand.

Will it come back even if I got all the bulbs?

Probably. Bulblets are tiny — sometimes 2–3mm wide — and easy to miss. Seeds in the soil also sprout for years. Plan for 2–3 years of follow-up.

Is onion weed dangerous to pets?

It contains the same compounds as cooking onions, which are toxic to dogs and cats in large amounts. A few nibbled leaves is usually fine but persistent eating can cause issues.

What about pre-emergent herbicides?

Pre-emergent products like Oxafert (a weed preventer that stops weed seeds sprouting) work on seedlings — useful for stopping new onion weed seeds from germinating. They do not touch existing bulbs. Use as a long-term layer once the main population is under control.

Let Lawnova help with the timing

Onion weed control is all about timing — hand-digging in winter, spraying in autumn, snipping flowers in spring. Lawnova reminds you when to do each step based on your region. No more missed windows.

Sign up here and get onion weed out of your lawn — for good.