Growing food in plastic containers is on the rise, but is this a safe practice? What about the chemicals that leach out of plastic – are they absorbed by the soil or the food? Do they cause a health risk?
There is a great movement towards organic gardening to grow healthy food locally, and for smaller back yards and balconies it’s attractive to grow food in small containers. I’ve even seen pictures of people growth vegetables right in the bag that contained the soil they bought. This is all so simple but is it a healthy way to produce food? Is it still organic if you grow in plastic?
Understanding Plastic Containers
Before we can have an intelligent dialog it is important to understand that there are many different kinds of plastic and even within a class of plastic there are variations.
To help standardize this process, many types of plastic are now given a recycle code along with an abbreviation for the plastic type. For example #1 is PET or PETE (Polyethylene Terephthalate).
Each type of plastic is made from different chemicals, has different properties, leaches different chemicals and breaks down differently. Any general discussion about plastic and safety is of limited value if you don’t identify the actual plastic being discussed.
Does Plastic Leach Chemicals?
The answer to that is a clear yes, for almost all consumer plastics. We buy food in plastic, we store it in plastic and yet it all leaches chemicals.
I remember seeing some lab results 35 years ago, on the wall of the Ministry of Environment in Toronto, comparing chemicals in city drinking water to those in plastic bottles. The bottled water had more chemicals both in terms of type and quantity even though many people thought the city water was not as good. If this information had been made more public we might have stopped the habit of purchase bottled water.
Can Plastic be Food Safe?
If all plastic leaches chemicals, why is this not a big health risk?
To understand the answer to this question and the issue of growing food in plastic you have to understand two important concepts; dose and chemical processes.
I’ve talked about dose before. Any chemical is safe provided that we are not exposed to high levels of it, and the level is different for every chemical. It is important to understand which chemicals are leaching and the safe dose for each one.
The second part is to understand the processes that happen as chemicals move from plastic containers to our bodies.
The plastic container is filled with soil and plants are added. It is then watered, fertilized and exposed to sun. Chemicals leach from the plastic into the water and the soil, are absorbed by roots, then translocated into stems and leaves, and finally we eat them. But how efficient are these processes?
Plastic Chemicals in Soil
The chemicals in plastic are all organic, as defined by chemists, and the microbe community is able to decompose them. How quickly this happens depends on the chemical, the type of soil, temperature, and the microbial community.
A chemicals ‘half-life’ in soil is a measure of how quickly it is removed. A half-life of 10 days means that after 10 days half of it is gone.
Watering also has a big effect on how much of the chemical stays in soil. Containers are watered frequently and it is common to water until it runs out the bottom. This helps wash away any chemicals, including fertilizer.
Another process that takes place in soil is the absorption of chemicals by organic matter. A higher organic level results in less of the chemical being available to roots.
What all this means is that much of the chemicals leaching from plastic never make it to the roots.
Do Plants Absorb Plastic Chemicals?
If a chemical is present in soil, but it is not absorbed by the roots, it won’t get into the food we eat and therefore it poses no health risk when you eat the food.
In some cases the chemical is absorbed by the roots, but little if any is translocated to the above-ground parts of the plant. Eating roots may provide a higher dose than eating leaves.
Phthalates, often called plasticizers, are found in many soft plastic consumer products including plastic bags and garden hoses. Phthalates and its metabolites have been shown to be absorbed by lettuce, strawberry, and carrots. They were found in all parts of the plant, but leaves have a much lower level.
Phthalates are of a particular health concern. Most of our exposure to them is from eating food but we also inhale them and are exposed through our skin. The tolerable daily intake (TDI) has been set at 50 μg/kg body weight.
Phthalates have been identified in most food groups, including fruits and vegetables. A Canadian study found phthalates in all of 252 cosmetic products tested (fragrances, hair sprays, deodorants, nail polishes, body lotions and creams, skin cleansers, and baby products).
Clearly we want to limit our exposure, but would a bit more leaching from a PVC plastic garden hose or plastic container really make much of a difference considering all of our other exposures? It might be safer to skip the deodorant and eat the lettuce?
It’s important to understand dose. Just because scientists are able to find a chemical, does not mean they found a significant amount. Our testing instruments are now so sensitive we can find a chemical almost anywhere.
Bisphenol A (BPA)
BPA is found in polycarbonate plastic which is used for containers to store food and beverages, such as water bottles.
BPA is not normally a problem in food grade plastics but it, as well as lead, has been found in the new fabric pots.
Although BPA is not found in food grade plastic, it has been highly studied and it provides insight into how other plastic chemicals might behave. Due to health concerns, BPA is sometimes replaced with two similar chemicals BPS and BPAF. Testing of all three of these shows that BPA and BPS have a half-life of less than a day. They don’t last long in soil. BPAF has a half-life of about 30 days, which is still short compared to many other chemicals, but not nearly as good as for BPA.
This same study compared two soil types and found that the half-life in soil with a higher organic matter level was 60% higher, showing that it absorbed onto the organic matter which protected it from degradation. Presumable the organic matter also keeps BPA away from roots.
Biodegradable plastic is good for the environment because it breaks down, and over time disappears. But you have to remember that when anything breaks down it is converted into other chemicals and it is always important to know which chemicals are being created. They might be more toxic than the original plastic.
Plastic That is Safe For Food
Some types of plastic have been identified as being safe for food. This includes #1, #2, #4 and #5.
These plastics have been judged to be safe because the chemicals that leach from them have either low toxicity levels or the amount leached is very small. If they are safe for storing food, they should also be safe for growing it. If anything, the soil and water reduces exposure.
Recycled Plastic Containers
I went around the house looking at some plastic containers that could be reused to grow plants and these are the recycle codes.
- Peanut butter, ketchup, juice jars – all #1
- Tupperware – #5
- Large Rubbermaid – #4
- 4″ plant pot – #6
- Home Depot 5 gal pail – #2
All the items except the flower pot was made from food safe plastic. Five gallon pails, marked as food safe are available from home hardware stores. In my experience the one above from Home Depot does not last very long when exposed to sun.
Our city sells rain barrels which are HDPE (#2).
PVC is used a lot for water lines and some people even grow food right in larger diameter pipes. It is especially popular for hydroponic growing.
PVC is a hard plastic and phthalates are added to make it softer and more flexible. It also contains some BPA. A form of PVC called uPVC or rigid PVC contains no phthalates and is considered food safe.
It is best for the environment if you don’t use regular PVC in the garden.
Heat and Light
Much of the published information about plastic deals with it at normal room temperature and light. In the garden, they are exposed to more heat and UV light from the sun. Both of these speed up chemical seepage and decomposition.
Plastic can release BPA 50 times faster when in contact with boiling water than water at room temperature. PETE (#1) plastic will release antimony in the presence of heat. Polypropylene (#5) has a high heat tolerance and tends to leach less than other plastics.
How much of an effect does heat and light have on plastics in the garden? That’s not clear.
Is Growing in Plastic Containers Organic?
You can grow in plastic and still be certified organic. That seems like a contradiction in terms for me. It is alright to grow in soil that contains chemicals from plastic, but it’s not OK to use synthetic fertilizer which is identical to the nutrients from organic sources?
Even plastic mulch can be used in organic farming.
Are Plastic Containers Safe?
Plastic does release chemicals into the soil and some are absorbed by plants. Most of these chemicals are at very low levels and considered perfectly safe.
Phthalates are considered a potential health issue because they are everywhere and you will be eating them in your food, even if you don’t grow in plastic containers. Reducing the use of cosmetic products will probably reduce your intake more than anything you do in the garden.
A very extensive study on BPA found, “global estimated BPA daily intakes were generally below the temporary tolerable daily intake (tTDI) recommended by the European Food Safety Authority (EFSA).”
Other chemicals leached from plastic are even less of a health concern.
Growing food in plastic seems to be quite safe provided you use plastic that is stamped as being food safe.