Bathtub solar ovens, by Ian Dillon

Experiment in progress. Please join in!

  • Turn problem weeds into a resource!
  • Reduce methane emissions
  • Dry out and kill tough, invasive weeds during warm weather (October to April)
  • Speed up composting the rest of the year

Kill things dead

There are plenty of stories that show just how resilient weeds, especially their seeds, can be. Everything has its limits though! Testing is ongoing but it seems weeds cooked in these ovens end up properly dead. They’re not just pretending and lying dormant. In warm weather (27oC or higher), without too much cloud cover, the top 30 – 40 cm of air beneath the Perspex lids of the tubs reliably heats up to 60oC. That’s the recommended temperature for killing weeds in compost heaps over a few days. On a sparsely cloudy, 30oC day the air inside reaches over 80oC! That should be hot enough to kill most weed seeds in only a few hours. On cooler days below 27oC the inside air hasn’t dipped below 40oC. If you don’t have a thermometer for the outside temperature you can use the Bureau of Meteorology app or website. The temperature given for Carindale is usually a degree cooler than the location of the tubs in the carpark.


I hope you will use these ovens and experiment with different weeds, temperatures and the length of time left in the tub. An initial and ongoing germination test with Cyperus rotundus (nutgrass) nuts and Neonotonia wrightii (glycine) seeds is being done. Nuts and seeds left in a solar tub for several days (with the air inside reaching over 60C for several hours during those days) has produced zero sprouts 3 weeks after sowing (as of 17/11/23). More tests are being done including just two hours in a tub on a 30oC day. After two hours the glass jar with the seeds in it was too hot to hold in my bare hands.

Why is this being done?

1. Methane. 2. Pointless transport emissions. 3. Mulch and compost is always needed.

If you put tough to kill weeds and weed seeds in the red bin, they go to landfill and make methane during anaerobic decomposition. There’s a ridiculous amount of variation in statements about how much extra heat methane traps in the atmosphere compared to CO2, even on websites that should be reliable. It ranges from 28 – 100 times as much. Let’s just say methane is a lot worse than CO2. Methane only sticks around in the atmosphere for 12 years on average but given the huge inertia in switching away from fossil fuels, keeping methane out of the atmosphere is a priority. The truck transporting your red bin contents also burns fossil fuel. On top of that you don’t get to use the weeds you pulled out for any useful purpose!

Why join in at B4C?

It’s a good idea!!!! I’m also moving a few 100 km to look after family. I’ll probably set up something similar where I move but please don’t let my efforts here go to waste!!!. Ask Leigh for my contact details if you’re interested in any further developments I haven’t already passed on to B4C.

How to make it work

It’s not as tricky as parenting compost bacteria but you need to be methodical. 

Trap the heat. Make sure hot air isn’t escaping much from beneath the Perspex. The cotton sheets are for insulation, filling air gaps and reducing air movement. The cast iron tubs don’t hold on to heat very well so the sheets hanging around the outside reduce air movement. The fibre-glass tub holds on to the heat best. There is a whole in the Perspex lid on the fibreglass tub that you need to keep outside the hose ring seal.

Minimise soil. A coating of dirt around a nutgrass nut or a plant’s roots will insulate it from the heat.

Use cooler days for drying out weeds. Not every day is hot and sunny. The water in your weeds takes a lot more energy to heat up than air. On a very hot day it doesn’t matter much if your weeds are dry or wet (you can help verify this) but you can’t predict when those days will come along.

There’s a limited amount of space. Will it resprout? In the warmer months, only put in plants that are likely to survive and resprout after cool composting.

Be selective. Only put in the part of the plant that might resprout. For example, just put in the fruit of tomatoes, not the whole plant. Put the whole plant in if it’s an invasive vine such as  Passiflora suberosa (Corky passion vine), Ipomoea cairica (Mile-a-minute) or Neonotonia wrightii (Glycine). Dyschoriste depressa that has seeds the whole length of its stems should all go in too. I haven’t tried Anredera cordifolia (Madeira vine) but that would definitely be the whole plant. I don’t know if the nuts and seeds of nutgrass are the only parts that that will resprout / germinate so I’m putting the whole plant in at the moment.

Environmental mindset. I can’t count the number of times I’ve seen a whole pot plant (including the pot) thrown in a red bin. That has included house plants that aren’t even environmental weeds. If you see that happen where you live and can’t separate the pot / plant / soil at home, perhaps you could bring it to B4C and put it in the compost / solar tubs / pot cleaning pile as appropriate?

Why not just compost tough weeds?

Compost heaps often don’t reach the recommended 55oC needed to kill seeds and pathogens. Getting the right ratio of water, carbon (browns) and nitrogen (greens) to feed the thermophilic bacteria that assist decomposition is tricky. You always seem to have too much or too little greens. Even a happy, steaming pile is only hot enough in the middle and requires turning over every 3 days [1]. Compost piles also produce some methane. The less well aerated they are the more methane they produce. I’m not at all against composting. It’s just not appropriate in some circumstances. It can be the next step after killing the weeds in the bathtubs.

Are the ovens environmentally friendly?

If they are used, yes. The Perspex is unwanted left-over sheets from the fern house. The bathtubs and wooden planks are having a second life. The length of garden hose was too short for practical use. The sheets are leftovers from enviro-bag donations that couldn’t be used. The contents are NOT being transported in a fossil fuel powered truck to go into landfill to break down anaerobically into methane.

How did I measure temperature?

Ideally you would stick your thermometer inside the tub and read the temperature by looking through the Perspex. You can’t always do that due to condensation on the underside of the Perspex. I got around the problem by using a non-contact temperature gun and lifting the Perspex to get a quick reading before the heat escaped. I’ve been using an Ozito IFT-100 IR thermometer I bought for another purpose years ago. It seems to be accurate to within 1 – 2oC. The current model at Bunnings is the IFT-520 for $35. They have other brands.

Household thermometers only go up to 50oC. I don’t own an oven or food testing (probe) thermometer. You aren’t supposed to get oven thermometers wet and the humidity can be very high inside the tubs, even when there is no condensation on the underside of the Perspex.

Thank you

Thank you to Leigh Weakley and the many volunteers who shared ideas and gave assistance.

Thank you to Leigh Weakley and the many volunteers who shared ideas and gave assistance.


Let’s wipe out weeds in Brisbane BCC weed identification website

[1] “To kill weed seeds, the internal temperature of your compost or mulch must reach 55oC for 3 consecutive days before each turning of the heap.” Queensland Government Environment, Land and Water: Preventing weed spread: advice for gardeners and outdoor enthusiasts

As is often the case with detailed knowledge that can be valuable to the general public the Australian Standard AS 4454-2012 Composts, soil conditioners and mulches is behind a paywall at Sai Global and costs $219.67.

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