Prickles in Your Hamilton Lawn? Spray Onehunga Weed Now, Not in December

The prickles that stop you walking barefoot across a Hamilton lawn in January come from Onehunga weed, and the only time you can do anything about them is right now. Spray it with a selective lawn weedkiller between September and early October, while it is still small, soft and growing. Once the seed heads have hardened, spraying kills the plant and leaves every prickle exactly where it was.

This is the one lawn job in the Waikato that has a real deadline. Most things you can put off a month and pay for it with a bit more work. This one you put off a month and you have missed it for the whole summer.

We have been mowing Hamilton lawns since 2007, and every December the same thing happens. Someone steps off the deck, gets a foot full of prickles, and rings around asking what to spray. The honest answer in December is nothing. Here is what to do instead, and when.

What Is Actually Stabbing Your Feet

Onehunga weed. Everyone in New Zealand just calls it prickle weed, and every garden centre sells something with that name on the bottle.

Right now it does not look like a problem at all. It is a small, flat, ferny rosette sitting low in the grass, a bit like a tiny piece of parsley or carrot top. It is soft. You can walk on it. Most people never notice it, because in September it is hiding under the first flush of spring growth.

Then it flowers, sets seed, and each seed grows a hard spine. By early summer the plant itself dies off and leaves the spiky seed heads behind, sitting right at ground level where your feet land. That is the prickle. It is not the weed hurting you. It is what the weed left behind.

Why December Is Too Late

Weedkiller works on a living, growing plant. The plant takes the spray in through its leaves and dies over the next couple of weeks.

By December, Onehunga weed has already done its job. The seeds are set, the spines have hardened, and the plant is on its way out anyway. Spraying it at that point does nothing to the prickles that are already in the lawn. They stay there until they get raked, mown, or trodden into the soil to grow next year’s crop.

So the whole game is timing. Kill it before it flowers, and there are no prickles. Kill it after, and you have a dead weed and the same sore feet.

The Hamilton Window: September To Early October

In the Waikato, the spray window is short and it is now.

The weed germinates over autumn and winter, sits there growing slowly through the cold months, and gets ready to flower in October. September is the sweet spot: the plants are big enough to hit with a spray, still soft enough to die easily, and have not started flowering.

The other reason not to wait is that Hamilton grass goes vertical from about the middle of September. Once the lawn takes off, that flat little rosette disappears under the grass and you cannot see what you are spraying. The Waikato lawn care calendar puts broadleaf spraying in October, and for daisies and dandelions that is right. Prickle weed is the exception. It needs to be done a few weeks ahead of the rest.

How To Find It Before It Finds You

Get down on your hands and knees. You will not spot it standing up.

Look in the sunny, dry, thin parts of the lawn first. Onehunga weed likes the opposite conditions to moss. Moss wants shade and wet. Prickle weed wants open ground, hard compacted soil and full sun. Check:

• The berm. Parked on, walked on, never watered, never fed. Prickle weed loves a berm, and once it is on the berm it walks up the driveway on your shoes.
• Along the driveway and path edges. Hot, compacted, and the grass is always thin there.
• Where the dog runs. The track along the fence, the spot by the back door.
• Any bare or patchy area the grass never quite filled in over winter.
• Wherever you got prickles last summer. The seed drops where the plant was. Same spot, every year, until you break the cycle.

What To Spray, And How

Buy a selective lawn weedkiller labelled for Onehunga or prickle weed. Any garden centre in Hamilton has one on the shelf, usually two or three. Selective is the word that matters. It kills the broadleaf weed and leaves the grass standing.

Do not use Roundup or any other glyphosate spray on the lawn. Glyphosate kills everything it touches, grass included. You will trade a prickle problem for a bare patch, and bare patches are exactly where next year’s prickle weed germinates.

Pick a still day with no rain forecast for 24 hours. Wind puts the spray on your roses. Rain washes it off before the weed takes it in.

Leave the mower in the shed for a few days either side. The weed needs its leaves to take the spray in. Mow it off the day before and there is nothing to spray. Mow it off the day after and you have cut off the leaf before the chemical has moved down into the plant.

Keep the kids and the dog off until it is dry. Read the label for how long. It is usually a few hours.

Give it two to three weeks. The rosettes yellow, then brown, then disappear. If a patch is still green and growing after three weeks, hit it again. A second application in early October is common on a heavy infestation.

Small patch? Pull it. Right now, in September, Onehunga weed comes out of damp ground with a hand fork in seconds, root and all. If you only have a dozen plants along the driveway, that is faster than mixing a sprayer and there is nothing to keep the dog off.

Why It Is In Your Lawn In The First Place

Onehunga weed is an annual. It does not have a permanent root system waiting under the lawn. Every single plant grew from a seed that landed on open ground last autumn.

That tells you the real problem. A thick, healthy lawn does not have open ground. The seed lands on a mat of grass, never touches soil, and nothing happens. A thin lawn with bare patches, compacted ground and grass that has been scalped every fortnight is a seedbed.

The seeds travel on feet, on the dog, on the mower, and on the neighbour’s mower if they mow the berm for you. Which is why once a street has it, most of the street has it, and why the berm is usually where it starts.

Stopping It Next Year

Spraying fixes this summer. A thicker lawn fixes every summer after it. Four things do most of the work:

• Feed it in spring. A lawn that has not been fed in years is thin, and thin is the whole problem. A spring feed is the cheapest thing on this list.
• Stop scalping it. Grass cut too short lets sun straight onto the soil, which is what a prickle weed seed is waiting for. Lift the mowing height a notch over summer so the ground stays shaded. There is more on that in how often to mow in spring.
• Sow seed into the bare patches in autumn. Every bare patch is next year’s prickle patch. Fill it with grass before the weed seed gets there, and if the ground is hard and squelchy, aerate it first.
• Keep the catcher on when seed heads are about. If you missed the window this year and the prickles have formed, mowing with the catcher on collects a lot of the seed instead of spreading it across the whole lawn for next year.

And do not stop mowing. A lawn that gets cut properly every fortnight, at a sensible height, all year round, stays thick. A lawn that gets hacked down to dirt once a month in summer and abandoned in winter comes into autumn wide open, and that is when the seed lands.

Where We See It Most Around Hamilton

It is not the same across the city.

The newer subdivisions in Rototuna, Flagstaff and Huntington are built on cut and fill sections with thin topsoil over compacted clay. The lawns went down fast, the berms went down faster, and a berm that was rolled flat by a digger and never watered is prickle weed heaven. On those streets it tends to be a berm and front lawn problem.

The older streets around Nawton, Dinsdale and Hamilton East have deeper soil and thicker lawns, so it is usually a patch here and there along a driveway or where the grass has worn thin, rather than the whole lawn. A lot of those older lawns are heavy in kikuyu, which is dense enough over summer that prickle weed struggles to get a foothold at all.

Frequently Asked Questions

What causes prickles in a lawn in New Zealand?

Almost always Onehunga weed. It is a small, flat, ferny-looking weed that grows through winter and spring, then sets hard, spiky seed heads in early summer. The plant dies off, the prickles stay. It is the reason you cannot walk across the lawn barefoot in January.

When should I spray for prickles in Hamilton?

September to early October. The weed has to be sprayed while it is small, soft and growing, and before it flowers. In the Waikato that window is short because spring growth comes on fast from mid September and hides the rosettes.

Is it too late to spray prickle weed in December?

For this summer, yes. Once the seed heads have formed and hardened, killing the plant does not remove the prickles that are already there. You have missed it until next winter, when the new crop germinates.

What do I spray on Onehunga weed?

A selective lawn weedkiller labelled for Onehunga or prickle weed. Every Hamilton garden centre stocks one. Selective means it kills the broadleaf weed and leaves the grass. Do not use a glyphosate product like Roundup on the lawn, because that kills everything, grass included.

How long does prickle weed take to die after spraying?

Two to three weeks. Do not mow for a few days either side of spraying, pick a still day with no rain forecast for 24 hours, and keep kids and dogs off until it has dried. If the patch is still green after three weeks, do a second application.

How do I stop prickles coming back next year?

Grow a thicker lawn. Onehunga weed is an annual that needs bare or thin ground to germinate in each autumn. Feed the lawn in spring, do not scalp it, mow a little higher over summer so the soil stays shaded, and sow seed into bare patches in autumn. A dense lawn leaves it nowhere to start.

Happy mowing!