Gardening using plastic buckets or other plastic gardening infrastructure has gained in popularity over recent years. However, one aspect that should be assessed when using plastic is whether it is safe to use when growing vegetables and other food crops. This is because plastic is often made with additives, such as phthalates and bisphenol A (BPA), that can leach from the plastic into the soil or plant media.
These plastic-leaching chemicals may enter food crops growing in the plant media, translocate to the edible parts of the plants and end up on our dinner plates. Since some of these compounds have been linked to adverse health effects in humans, it is important for us to consider what this means for human health.
In this post, we characterise the different types of commonly-available plastic that might be used in the garden. We focus mainly on what chemicals might be leaching from them, especially when exposed to the heat and ultraviolet (UV) light of the sun. By doing so, we can usually avoid harmful plastics and use alternative safer ones or even switch to other materials entirely.
When it comes to plastics, the vast majority we encounter are commodity plastics (the ones we use everyday as opposed to engineering or structural plastics). These fall under seven different polymer categories identified by their resin identification code (RIC). The categories of the RIC consist of the following:
1 - PET (PETE)
Polyethylene terephthalate (PET), commonly used for plastic single-use drinking bottles and food packaging, has gained some popularity in gardening in recent years due to its availability. Unfortunately, PET plastic can leach chemicals, in particular, antimony and BPA (bisphenol A). In addition, PET exposure to heat or UV radiation can enhance the release of these substances. Antimony is thought to negatively affect health in a number of ways while BPA is known for its potential endocrine-disrupting properties. As a result, PET containers should not be used for food crop growing in soil and especially not for hydroponic systems.
2 - HDPE (PE-HD)
High-density polyethylene or HDPE plastic is also commonly found in the everyday consumer goods we buy, such as milk jugs and detergent bottles. This plastic is not known to release chemicals into the foods or liquids it is in contact with. In addition, HDPE is resistant to heat. However, although coloured HDPE is resistant to UV, natural HDPE, which is the white and translucent material often used for milk containers, is not and will fall apart after a few months of sunlight. As a result, only coloured HDPE plastic is safe for gardening, both using soil and hydroponically.
3 - PVC (V)
Polyvinyl chloride (PVC) plastic is known to release an additive that it is often made with. Phthalate plasticisers are sometimes added to PVC to improve its flexibility. These can be released by the plastic into the environment. Studies have shown that phthalates are taken up by plants. However, although their translocation to the plant leaves is quite limited, this is probably due to their rapid metabolism into other derivatives. Some of these derivatives do translocate to the leaves and their potential harm to human health remains unclear. As phthalates themselves have been linked to adverse health effects in humans, its metabolites may be problematic as well. Therefore, as a precaution, PVC should be considered unsafe for vegetable growing and other food crop gardening.
One point to note, however, is that not all PVC contains phthalates. uPVC (PVC-U; unplasticized PVC) does not contain phthalates. Therefore, although the use of PVC in the food garden is generally frowned up, if PVC has to be used, it should be of the uPVC variety.
4 - LDPE (PE-LD)
Like HDPE plastic, natural low-density polyethylene (LDPE) has low UV resistance but is usually UV-stabilised with additives during manufacture. As for heat-resistance, LDPE has a lower melting point than HDPE but it is still quite heat-resistant with a melting point of around 110ºC. LDPE is considered a very safe plastic as it is not known to leach any chemicals into foods or liquids. This characteristic ensures that the plants grown in LDPE containers do not absorb any unwanted substances. Consequently, LDPE plastic is considered safe for food crop gardening. Because LDPE plastic is naturally flexible, it tends to be used in garden applications that require bending or shaping, such as drip lines and extra flexible plastic pots.
5 - PP
Polypropylene (PP) plastic is probably the most commonly-used plastic in gardening, particularly for outdoor pots and hydroponic systems. PP is fairly heat-tolerant and UV-resistant, making it a good plastic to use for outdoor applications.
Although some studies have suggested that PP containers can leach quaternary ammonium biocides and oleamide, the amounts released are so small as to be negligible. In fact, quaternary ammonium biocides are the same stuff you find in antibacterial surface cleaners or wipes, and oleamide is a compound that naturally occurs in the human body. A tiny release of these compounds from PP plastic will be inconsequential compared to the 'background' levels we are normally exposed to. Consequently, PP plastic is considered a very safe plastic for food crop gardening.
6 - PS
Another commonly available plastic is Polystyrene or PS. PS is most familiar to us as take-out food containers and parcel packaging. Polystyrene is sometimes used for outdoor gardening and some components in hydroponics. However, PS is thought to release potentially harmful chemicals (suspected carcinogens and neurotoxins) especially when exposed to heat. Since direct exposure to sunlight can increase its temperature, this can be problematic in a gardening or hydroponics context. In addition, PS has poor resistance to UV and breaks down when exposed to sunlight. As a result, it is not recommended to use PS plastic for growing food crops.
7 - Other
The resin identification code (RIC) number 7 (Other) represents all other commodity plastics that do not fit into one of the first six categories discussed above. Consequently, the number 7 code comprises of a number of different plastics, often making it difficult to know exactly which type of plastic you are dealing with. The most common members of this category that you will likely encounter include Polycarbonate (PC), Polylactic Acid (PLA), Acrylonitrile Styrene / Styrene Acrylonitrile (AS / SAN), Tritan, and Acrylonitrile Butadiene Styrene (ABS).
Key features as they relate to gardening
Leaches BPA - do not use
Safe bio-plastic but has poor UV resistance
AS / SAN
Safe to use but has poor UV resistance
Safe to use but has poor UV resistance
Safe (if BPA-free certified) but has poor UV resistance
Each plastic in this category has different properties, but most, if not all, no. 7 plastics are inappropriate for gardening in some way or another. PC has been associated with high levels of BPA (bisphenol A) leaching. Consequently, PC should not be used for food crop gardening at all. The other common number 7 plastics, PLA, AS / SAN, Tritan, and ABS, on the other hand, appear to be safe to use in a gardening context but most do not do well with prolonged exposure to UV. Therefore, it is generally accepted that most no. 7 plastics are not suitable for use in the garden.