Nov 30, 20225 min read

Weed Removal from a Lawn - Novel Approaches for Difficult Weed Infestations

Updated: Aug 13

I have been weeding since I was a small child, even if sometimes the plants I pulled out weren't weeds! This has been a bit of an obsession. Lawn is particularly difficult to do weeding.

Background and Common Methods

For a while I have been weeding lawn by hand, which was a lot of work, even with a weeding tool. Unfortunately for some of the species of weed this was unavoidable, like this dark green grass growing like matchsticks out of the lawn:

Green matchstick grass growing from couch grass — Matchsticks be gone

The above grass was time consuming to remove, however fortunately it took up a relatively small amount of lawn. In fact, it's almost wholly shown there and was completely removed by hand since. It involved loosening the area with a pitch fork then digging right into the lawn with my bare fingers and pulling out by the root. Some grew back from the neighbours and so will need to be done again.

A lot of bindii used to grow in this particular lawn - no more. It was pretty much eliminated using selective weed killers from Bunnings.

The wintergrass is possible to kill using wintergrass weed killer, but it has to be poisoned in about April, when the weeds are still tiny shoots. This needs to be done repetitively about every 2 weeks, according to the instructions on the bottle.

Wintergrass in couch grass — Ugly grass that only lasts six months of the year

Much harder to kill with poison are two species of weed - creeping oxalis and corn speedwell. For these weeds, our options are very limited. This is creeping oxalis.

Creeping oxalis in couch grass — Evil, horrible weed to find in your lawn

This is corn speedwell. When flowering, it has tiny blue flowers. So far, I had been controlling it by pulling out by hand.

Corn speedwell in couch grass with some creeping oxalis — Eek! Get it before it seeds, if you can

Experimenting with Roundup, Black Plastic and Boiling Water

I did experiments and tried covering the lawn with black plastic and pouring on boiling water from a steel watering can. First, I did a small test. Here is the test area on 6 August 2021:

Heavy infestation of creeping oxalis in couch grass — Hold the napalm but we'll try something nearly as brutal for this. No, not chemicals!

As you can see, it is heavily infested with creeping oxalis, with more creeping oxalis than couch grass! Not much lawn to lose here, so ideal for an experiment.

On the right I covered with black plastic, on the left closer to the fence I used boiling water and further from the fence I used Roundup. Taken 15 August 2021:

Boiling water and black plastic for killing weeds — Welcome to my lab

And just after removing the plastic six weeks later:

Grass and weeds treated with black plastic, boiling water and glyphosate — Looks desolate, but look closer, that grass ain't dead yet

The wintergrass and creeping oxalis kept re-appearing, and so was treated with repeated applications of boiling water. Nonetheless, covering with plastic seemed to be the most effective option - as can be seen, it nearly wiped out the creeping oxalis. Therefore, I proceeded to use plastic for the rest of the lawn.

Larger Trial of Black Plastic

I kept the time I covered the lawn to about 2 weeks, finding that most effective for the late spring and summer period when sunlight is the strongest. This photo is taken 27 November 2021.

Using black plastic to kill creeping oxalis in couch grass — Die, die, die

When I stuck to 2 weeks, the lawn generally grew back quite well. Generally, the creeping oxalis was hit much harder than the lawn. However, much to my dismay I found the lawn really struggled to regrow under the eaves of the house and near the large bushes. This photo was taken 5 April 2022, over four months later, with weeds starting to take advantage of where the lawn was weak. Elsewhere, the creeping oxalis was much less than before.

Lawn with a few dead patches — Mostly good

More unfortunate was near the carport. This was taken 12 April 2022, 11 weeks after the plastic was removed. Here, the lawn has fared worse off.

Long Term Results of Small Scale Test

However, there is still hope! Remember what the lawn looked like before in my test area? This is what it looks like as at 20 December 2022, with no boiling water treatment since 29 October 2021 (selective weed poison has been used for the past five years or so, this was continued):

Couch grass heavily infested with creeping oxalis

Couch grass with creeping oxalis mostly killed off

Some years later and with the continued use of selective herbicide, on 26 March 2023, the lawn was still relatively weed free. What can be seen of the creeping oxalis would have grown from seed.

Lawn with some creeping oxalis and a garden — Creeping oxalis mostly gone!

Larger Trial of Boiling Water

I applied boiling water on the lawn the other side of the rainwater tank on 7 October 2021.

Boiling water applied to creeping oxalis in a lawn — Poached Creeping Oxalis

The weeds did come back faster and a bit more than with the black plastic, but so did the lawn. In fact, the lawn fully recovered in less than 2 months.

Comparing boiling water and black plastic weed treatment in a lawn — Boiling water is less brutal than black plastic but somewhat less effective

Not Watering

In our backyard we had the situation where the lawn retic stopped working from about August 2023 to March 2024. A few months in we resorted to manual watering from a tap sprinkler but it wasn't nearly enough. The lawn suffered but this 'mechanical drought' has had the effect of nearly wiping out the creeping oxalis.

In October 2023 the creeping oxalis, a normally perennial weed, was already dying off.

Lawn and weeds suffering from lack of water — Creeping oxalis already dying

On 1 December 2023:

Lawn suffering from lack of water but with almost no weeds — Weeds dying faster than the couch grass

On 4 March 2024 it was almost completely gone. The retic was repaired mid February.

Lawn with no weeds but recovering from six months of little water — Success! Let's see how long it lasts...

Fortunately, even on 13 August 2024 and after just one application of selective weed killer over winter, it is still relatively free of creeping oxalis.

Lawn with some wintergrass and corn speedwell — Looking good, with just a few winter weeds

However, problem areas unfortunately needed more selective weed killer, due to the sprouting seeds of creeping oxalis.

Lawn with some creeping oxalis — Needed more Multiweed

We live in Mandurah, Western Australia and the capital Perth has a very similar climate.

Summary and Recommendations

Normally, selective weed killer like Multiweed is your best bet. For creeping oxalis, try black plastic, boiling water, starving of water or selective weed killer depending on your situation.

For corn speedwell, I'm still experimenting as at August 2024 but I would strongly consider boiling water for heavy infestations. I'm yet to try herbicides that claim to specifically target it.

If you want to try boiling water for your lawn, you can either do using a hot water urn and steel watering can (slow) or hire a contractor to shower boiling water on using a professional machine. Such machines do exist specifically for killing weeds, like from GreenSteam. When I asked one contractor they gave a disclaimer saying they do not know what the long term effects will be for the lawn. I can say with 90% confidence that the lawn does recover quite well, but expect some disappointment with killing weeds like creeping oxalis.

You can try black plastic but I don't recommend using it where the lawn is in shade or near trees or large bushes.

If you have a situation like mine and don't have much to lose, why not give boiling water a go. It is a very old and proven method for killing weeds. It will set you back a few hundred $ for a contractor but based on my testing so far it has very strong potential. I would have it applied in the middle of summer and apply fertiliser to the lawn afterwards so it grows back the fastest. When I applied the boiling water in late winter, the weeds grew back before the lawn so I had to keep re-applying.

Have fun getting rid of those weeds! 🔥🚿🌈