There’s a huge range of plastics in common use in the garden. Here’s the breakdown:
PET1: Polyethylene Terephthalate: marked with a 1 on the triangle. Very commonly recycled, can break down when exposed for long periods of time to light or heat.
Used in: Practically nothing in the garden. It’s the stuff drinks bottles are made of and too unstable for most uses as it breaks down to easily to withstand sunshine and weather.
HDPE: High Density Polyethylene (Polythene): marked with a 2 on the triangle: doesn’t break down easily, resists UV rays and very heat tolerant, coping with -100 to 80C – one of the most common plastics used in the garden because of these properties.
Used in: most black, brown and green rigid plant pots; seed trays; the containers you buy pesticides, herbicides and fertiliser in; insect-proof mesh
How to avoid: Use biodegradable pots; use wooden seed trays; make your own pesticides, herbicides and fertiliser from the plants you grow; and I am still experimenting with alternatives to insect-proof mesh. Calico is looking expensive, but promising.
PVC: Polyvinyl Chloride: marked with a 3 on the triangle. Used in plastic pipes and irrigation. Most contain chemicals known as phthalates, helping the PVC to be more durable, flexible etc. But phthalates are very harmful to humans.)
Use in gardens: hosepipes
How to avoid: rubber hosepipe might seem a good alternative, but many are synthetic rubber which is basically plastic. Better just not to use a hose: use a metal watering can instead.
LDPE: Low Density Polyethylene (Polythene): marked with a 4 on the triangle. Single use plastics used in things like plastic produce bags and bin liners. It came along after HDPE and shares many of the same characteristics – so very safe in a wide range of températures and doesn’t Leach into the soil.
Use in gardens: Single-use packaging around mail-order items
How to avoid: Buy from nurseries and garden centres or find a supplier who doesn’t pack in plastic (easier said than done…)
PP: Polypropylene: marked with a 5 on the triangle. Used in products that require injection moulding like straws, bottle caps, food containers. Not as tolerant to heat as HDPE or LDPE, generally safe for use with food and the garden.
Use in gardens: Some more flimsy plant pots (eg seedling modules); Horticultural fleece is made from woven polypropylene.
How to avoid: Use newspaper pots instead of modules; wrap plants in hessian and straw instead; or just avoid growing particularly tender plants.
PS: Polystyrene: marked with a 6 on the triangle. One of the most widely used types of plastic, but one of the most harmful to the environment as it breaks down very quickly into very small particles, and tends not to last very long.
Use in gardens: Bedding plant trays and packaging for mail order plants
How to avoid: Raise your own bedding plants, or buy from a supplier who doesn’t use polystyrene trays. Buy from nurseries and garden centres or find a supplier who uses biodegradable cornstarch packing instead.
Polycarbonate: marked with a 7 on the triangle (this actually means any plastic other than those listed above – but usually polycarbonate or polylactide). Polycarbonate is the most harmful plastic we have ever created: proven repeatedly to leach BPA (Bisphenol A) – shown to cause reproductive problems in animals and linked to cardiovascular disease and diabetes in humans.
Use in gardens: Plastic non-breakable greenhouses and cold frames.
How to avoid: Use glass instead.