Solarising Garden Soil to Reduce Pests and Disease

Solarising garden bed soil under clear plastic

Our local growing conditions are typically temperate and wet. It’s brilliant for growing, but also perfect for weeds, pests, disease, and fungus. Blah. Between my personal desire to minimise chemicals in the garden, safety for our pets, and trying to keep things bee-friendly, it is a constant struggle to maintain a healthy garden. Solarising garden soil uses heat as the treatment instead of chemicals. The objective is similar to fumigation or drenching, but using completely (plastic aside) natural methods. Here’s an introduction to solarisation and how to solarise garden soil at home.

Introduction to Soil Solarisation

Soil solarisation is a hydrothermal soil treatment. It uses solar heat to raise the temperature of garden soil above the viable temperature of soil based pests and problems. Thin clear plastic is used to amplify the natural heating of the sun through to the soil beneath and to improve heat retention. 

How Does Solarisation Help to “Clean” Soil?

In effective solarisation, the upper layer of soil will reach 50C and above. This is hot enough to bake many pests, pathogens, and plants. The hotter the better! The greater the sizzle, the shorter the length of time needed for good results. 

Even with effective soloarisation, the treatment works best in the upper 10-15cm. Soil temperatures will be the hottest at the upper surface of the soil, decreasing with depth. Unfortunately, problems that are active below the solarisation zone or that are readily mobile can evade your efforts, but the same is true for many other treatments.

How Well Does Soil Solarisation Actually Work?

If temperatures are hot enough, you can use solarsation to kill insects and nematodes, cook out weeds, and reduce or eliminate bacteria and fungal pathogens. The results will depend significantly on the weather during your solarisation period, the sun exposure, how effectively you can trap the available solar heat, and the depth and mobility of the problem being treated.

It can be difficult to know if solarisation is working. Signs of success include die-off of weeds under the cover during solarisation. Unlike black plastic “weed killing” cover, clear plastic transmits light. Unless things are sizzling, weeds may continue to germinate and grow. Hardcore weeds might make a decent effort even with heat, but if things are growing normally under your plastic, it’s a good indicator that temperatures aren’t hot enough for effective treatment. 

If you are getting hot sunny weather and good exposure, look for heat leaks. Check for holes (small holes can be patched with duct tape) and check your edge seals. If there are no quick-fixes, decide whether it’s worth persevering.

Patience is a Key Ingredient for Success

Soil solarisation is relatively simple and inexpensive to try, but does have two key pitfalls. The major downside is that you need to clear and cover the area for at least six weeks (minimum) in hot sunny weather. This means losing a big chunk of prime growing time. Time is a big sacrifice, especially for gardeners who have limited growing windows. 

If you’re keen to avoid the chemicals, it’s an easy method to try in your garden with little to be lost other than prep and growing time. It’s worth noting that, if the solarisation is effective, you will cook off the beneficial organisms and well as the baddies. Time will restore things and you can use inoculations to accelerate recolonisation. 

How to Solarise Garden Soil

Selecting a Suitable Plastic Covering

To solarise the soil, you’ll need a suitable clear plastic. Black plastic gets quite hot, but doesn’t transfer the solar heat into the soil like clear plastic. Select a clear outdoor plastic sheeting that is thick enough to avoid punctures from contact with the soil or breaking from UV embrittlement before your solarisation is complete. Ideally, you’ll be able to clean, store, and reuse it. It needs to be big enough to fully cover the area being treated plus extra for anchoring, sealing, or burying the edges. 

Preparation for Garden Soil Solarisation

  • Clear the soil of all visible sticks, rocks, weeds, and other unwanted material.
  • Add any (non-living) soil amendments (manure, compost, etc) you’d like to work into the soil concurrent with treatment. It’s a win-win for boosting the soil temperatures and accelerating nutrient break down for availability when you resume planting.
  • Cultivate the surface so that it relatively smooth. This will reduce the air gaps between your plastic and the soil surface. If rainwater pooling is a concern, you can raise the soil slightly in the middle to facilitate run-off from your plastic. If you’re working to eliminate a pest/pathogen, remember to wash your tools after use.
  • Wet the soil. Wet soil is more effective at conducting heat. and steam is necessary for some treatments.  Keep things moist, but not soggy or muddy. Since moist soil works best, depending on what you’re treating, you may need to access and re-wet the soil during this process. Only do this is you really need to and use hot water if feasible. Cool water will cool the soil to the detriment of your solarisation efforts. 

Solarising the Soil

  • Lay clear plastic sheeting on top of your soil. Get it as tight to the surface as possible. You may need to use weights (bricks, scrap timber, etc) to help press the plastic close to the soil if treating large areas.
  • Cover the edges to secure and anchor the plastic. Ideally, bury the edges or backfill with soil all along the edges to create a stable anchor and semi-seal the plastic onto the soil. Heat leakage will reduce the effectiveness of your solarisation efforts.
  • Maintain the cover for 4-6 weeks of solarisation (minimum, but the longer the better). 
  • Remove the cover and inoculate the soil to restore beneficial fungi, bacteria, and other organisms before planting. Time will help to restore the balance.

Our Experiences (And a Few Complications)

In applying solarisation to my small garden beds, I ran into complications. Removing the soil and bagging it to bake (a good option for containers and small beds) was volume prohibitive, but the confined area of my raised beds meant that it wasn’t feasible to backfill the edges with soil. 

Getting a Suitable Seal

The beds were built with raised edges above soil level for looks, convenience, and containment (we get wicked wind and rainstorms). They also have high corner posts which are great (so great!) for attaching string, mesh, or supports for taller plants. Not so great for covering the beds to solarise. 

The insides are sealed with black plastic and the sides get quite warm in the sun, so I treated them like a giant container that I couldn’t wrap and focused on the top. As a compromise, the edges of my plastic were tucked as best we could into the soil and long pieces of scrap timber were used to reinforce the seal and provide anchoring weight along the edges as close as possible to the walls of the bed. It was definitely hot and steamy under there!  At least at the beginning…

Keeping the Soil Most

The beds are fully exposed to baking sun, cross flow, and lots of wind. The covered soil started to dry out at several points during solarisation, evident by the lack of condensation on the inside of the plastic and clear view of dry soil below. Fortunately, I was only solarising small raised garden beds, so using solar heated water to re-moisten was possible.

The Final Verdict?

Was it a soil solarising success? To be honest, it’s really hard to know for sure. The weather was cooperative, weeds and plants thwarted (at least for a while…), but I can’t say with absolute certainty whether solarising fully cleaned the soil. Since we weren’t able to achieve a full seal for the bed, I know we had some heat and moisture leakage. It was definitely hot under there, but temperatures and depth may not have been optimal. That said, the beds have since been inoculated, planted, and things are currently growing happily. No issues have been sighted so far, so it’s a win in my books!

Solarising Garden Soil to Reduce Pests and Disease

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