If you’ve got a large patch of weeds in a flat area, you’re in luck because there’s a chemical-free solution that actually works. The method is called solarisation and involves heating the soil while depriving weeds of water and sunlight, effectively killing them and other soil diseases.
It’s a great solution if you have difficult weeds, like nut grass and onion grass, taking over your garden. Here’s what you need to know.
What is solarisation?
Solarisation in gardening is a process where you use the heat of the sun to kill weeds or other ground cover by laying down sheets of plastic to trap heat in the soil. The main equipment you need are black or clear plastic and something solid to fix it in place, such as rocks, soil, bricks or metal. Over time, heat from the sun will build up under the plastic, effectively treating the area without the need for pesticides or excavation.
You can also use solarisation to improve your overall soil quality, as heating the topsoil can also remove pests and pathogens.
How to solarise weeds in 6 steps
First, keep in mind that solarisation is most effective in areas that are exposed to the sun for all or most of the day. You could try solariation in a partly-sunny area, but would need to allow more time for the process to work.
Once you’ve selected your patch, follow these step-by-step instructions on solarising weeds.
1. Clear the area
Remove as many existing weeds, rocks and other matter as possible from your chosen location. The solarisation process works best on cleared soil but if you have left weeds there, factor that into the time you leave the plastic in place.
2. Water the soil
Thoroughly water the soil to ensure it’s moist. This helps conduct heat better and enhances the effectiveness of solarisation.
3. Cover the soil
Cover the moist soil with thick, black or clear plastic sheeting. Use a heavy-duty, UV-resistant plastic to maximise the heat absorption. Ensure the plastic completely covers the soil without any gaps.
4. Secure the edges
Bury the edges of the plastic sheeting with soil or use rocks to secure it in place. This prevents heat from escaping and creates a greenhouse effect under the plastic.
5. Leave to cook
Allow the plastic to remain in place for an extended period, ideally 4 to 6 weeks during the hottest part of summer. This helps to raise the soil temperature sufficiently to kill weeds, seeds, and pathogens.
Note: You can use solarisation in cooler months, providing the area gets enough sun. In fact, research published by the CSIRO (pdf) in 2024 found that sunlight had more of an impact on soil temperatures than the ambient temperature, stating “soil solarisation can be planned around sunlight projections (e.g. UV index) rather than temperature projections.”
Remove the plastic
After the solarisation period, carefully remove the plastic. Be cautious of any trapped heat or steam. Remove any dead weeds or other matter and allow the soil to cool and dry before planting.
How to prevent weeds in your garden
When weeds take over your garden, prevention is better than a cure. Once you’ve planted into the soil, you can do a couple of things to prevent the growth of pesky weeds.
Add a weed barrier
A weed barrier can help reduce weed growth. You can buy ready-made weed barriers or make one out of cardboard if that’s a suitable material for your area (check for pests that it may attract, such as termites, first).
Add a layer of mulch
Mulch is a fantastic way to stop weeds from germinating. Apply a layer (about 5-10cm), and let it work its magic, preventing weeds in your garden.
