Monday, January 29, 2018

Solarising Garden Soil to Reduce Weeds, Pests, Pathogens, and Disease

Our local growing conditions are typically temperate and wet - brilliant for growing, but also perfect for 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. Solarisation uses heat instead of chemicals to treat soil, similar to fumigation or drenching, but using completely (plastic covering not withstanding...) natural methods.

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

It's 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, meaning you loose a big chunk of your prime growing time.  This is a big sacrifice, especially for gardeners who have limited growing windows.  If the solarisation is effective, you will cook off the beneficial organisms and well as the baddies, but time will restore things and you can use inoculations to accelerate recolonisation.

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.  To solarise an area of your garden:

  • Source suitable clear plastic. Black plastic gets quite hot, but doesn't transfer the solar heat into the soil like clear plastic. Select a suitable 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. Compromise on thickness to reduce the tearing risk but still maintain good solar transmission. The piece(s) used will need to be big enough to cover the area being solarised plus extra for anchoring/sealing/burying the edges. 
  • 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 better 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 for the surface, 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.
  • Wet the soil. Wet soil is more effective at conducting heat. Keep things moist but not soggy or muddy. Since moist soil works best, depending on what you're treating, you may want to access a few points and re-wet the soil during this process. Only do this is you really need to, as it will cool the soil to the detriment of your solarisation efforts. 
  • 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, as 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 bacteria and fungi before planting. 

In effective solarisation, the upper layer of soil will reach 50C and above, which becomes hot enough to "bake" many pests, pathogens, and plants.  The hotter the better, and 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 of the soil as 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 easily evade your efforts, but the same can be said for many other types of treatment as well. 

It can be difficult to know with solarisation is things are going as hoped. Signs of success include die-off of weeds under the cover during solarisation. Unlike black plastic "weed killing" cover, clear plastic transmits light so, unless things are sizzling, weeds will continue to germinate and grow. Hardcore weeds might make a decent effort despite the heat, but if things are growing normally under your plastic, it's a good indicator that your temperatures aren't hot enough for effective soil pathogen and pest treatment.  If you are getting hot sunny weather and good exposure, look for heat leak holes (small holes can be patched with duct tape), check your edge seals, and if there are no quick-fixes then decide whether it's worth persevering or trying again when conditions are more favourable.

In applying solarisation to my smaller garden beds for treatment, I ran into a few complications. Removing the soil and bagging it to bake (a good option for containers and small beds) was volume prohibitive, but the relatively confined area at the surface of my raised beds meant that it wasn't feasible to backfill the edges with soil. Hubby built the beds with raised edges above soil level for looks, convenience, and containment (we get wicked wind and rainstorms) as well as using 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! My raised beds are fully exposed to baking sun, cross flow, and lots of wind. The covered soil dried 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 areas so cheating things by using solar heated water to re-moisten was possible.  Was it a success? It's hard to know. The weather was cooperative, weeds and plants thwarted (at least for a while...), but I can't say whether it helped my pest and fungus control efforts. That said, they have since been inoculated, planted, and things are currently growing happily so it's a win in my books!

Have you tried solarisation in your garden? Have any helpful tips? Success stories? Tales of woe?  Leave us a comment. :) We'd love to hear your stories and ideas. If you have your own blog/website or a favourite read you'd like to share you can leave a live link in the comments using the following format as a guide: <a href="">text you want shown for your link</a>

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