People often ask what lining materials are safe for wicking beds, especially when plastic films are involved. In this newsletter Colin Austin explains a practical, engineering-based answer: black polythene is generally fine, clear polythene can work if protected from sunlight, and flexible PVC films are best avoided unless you are confident they meet strict standards. The key risk is not “plastic in soil” in general, but how materials degrade and what additives may be present.
Newsletter Context
I try base my newsletters on questions people ask and also their comments, so keep them rolling in. For the time being I have given up on running a forum — maybe people are shy about putting their names on the web. I had requests to comment on the Gardening Australia self-watering bed, so that should have been the topic of this newsletter (next time), but I have to admit I have got myself into a bit of a problem.
I like to answer all questions directly and have had several people ask about the toxic quality of liners. I did give a reply to the first question but now cannot find the emails of the other enquirers, so apologies to these people — and I am making liners and toxicity the subject of this newsletter.
Short Answer
Well the short answer is: black polythene is fine — but for those wanting a more detailed answer, here we go.
Plastics — The Hope and the Reality
Before the Second World War the only plastics were thermosetting plastics like phenolics, better known as Bakelite. During the war a number of thermoplastics were developed — mainly polyethylene, nylon and PTFE (Teflon). Polythene, with its dielectric properties, played an important part in the development of radar, which was critical in the war.
After the war these materials were promoted as the wonder material of the age — they would replace sliced bread and aeroplane wings. In that era I was young and gullible (as opposed to my current state of old and gullible) and after University joined the plastics industry as an engineer to study their mechanical properties. I am not a polymer chemist, but I have sat at enough lunch table discussions to make some comments.
Like most new innovations which are going to change the world (including the iPad), the disadvantages only become apparent after they have cashed the cheque.
Plastics were promoted as chemically inert — not attacked by chemicals, safe and non-toxic. That is in fact true in many cases. In particular, polythene is a long-chain molecule of carbon and hydrogen and meets these claims. When polythene decomposes it gives off water and carbon dioxide.
The Side Attack: UV Degradation
But you are never caught out by the frontal attack — it’s what sneaks in from the side that gets you. In this case, that side attack was UV degradation. Anyone who has left a plastic milk bottle in the sun knows how it goes hard and brittle.
This matters because many “is it toxic?” worries really start as “is it breaking down?” worries. If a liner stays stable, stays underground, and stays out of sunlight, the practical risk profile changes compared to a film that is being cooked by UV day after day.
Polythene Liners: Clear, Green-Tinged, and Black
Polythene can be stabilised by chemicals or by carbon black. If the polythene is clear (with a whitish tinge) then this probably means it is unstabilised, so it will degrade rapidly in sunlight. I have used this as a liner where any waste plastic may not be disposed of properly. It does not degenerate under the ground protected from UV.
The chemically stabilised version has a slight green tinge as seen in tree guards — not a big problem, but it is still there. Carbon black stabilised polythene is very safe.
In plain terms: if you want a liner that stays stable and behaves predictably, black polythene is a sensible default. If you choose clear polythene, treat it as a material that must be kept away from sunlight and protected by soil cover. The stabilised “green-tinged” films sit somewhere in between: serviceable, but not the simplest choice if you want to avoid ambiguity.
PVC Film: Rigid Pipes vs Flexible Liners
The other common film material is PVC. PVC (as in pipes) is rigid: carbon is the backbone, but the side elements are chlorine, which is toxic when burned.
PVC can be made flexible by chemical additives (for example, swimming pool liners). The early additives were simply horrible. Lead and arsenic make excellent flexibilisers — which most reputable companies stayed away from — but some used antimony as a “safe option”.
These have now been banned for many years in Australia, but much of our plastics industry has shut down so most materials are now imported from Asia. Safety rules in Asia are not as tight and are regarded by some manufacturers as “optional”, particularly if they cost more money. So unless you are sure that any PVC film abides by Australian rules, then best to avoid it.
That is the key practical point: rigid PVC in regulated supply chains is one thing; flexible PVC films of unknown origin are another. When people ask “is the liner toxic?”, what they often really need to ask is “what additives are in this flexible film, and do I trust its provenance?”
Rubber Liners: A More Stable Option
Some pool liners are made from a rubber (such as EPDM). Rubbers are fundamentally different to plastics as they are cross linked, which makes them much more stable.
If you are determined to avoid the uncertainty of flexible PVC films, a cross-linked rubber of the correct grade is a reasonable direction to look. The phrase “correct grade” matters: materials are made for purposes, and you want a grade intended for water contact and outdoor durability rather than “whatever was cheapest”.
How Real Is the Danger?
In my mind, the dangers from toxic chemicals leaching out are minimal. The fact that drinking water is typically delivered through either polythene or PVC pipe would be a far bigger relative potential danger — but even here the risk is minimal.
In other words, it is easy to become alarmed by the idea of “plastic in a garden bed”, but the real-world exposure pathways matter. Stable plastics designed for water systems are already used widely. The bigger practical risks usually come from the wrong material in the wrong context — especially films that break down under UV, or flexible materials with unknown additives.
Conclusion: Practical Liner Guidance
So to conclude: use black polythene film if you want a stabilised version, or clear (white, not green-tinged) if you want it biodegradable in sunlight. Avoid flexible PVC (as is commonly used in cheap swimming pools), but cross linked rubber of the correct grade should be safe.
Hope that helps.
Colin
