With a general move away from plastics in the garden it is often difficult to know the direction to proceed. A good first step to environmental sustainability is to look towards the options that are available to us, the gardeners, right now.
Very few products can claim to have zero environmental impact, or carbon footprint as it is sometimes termed. Even Coir Pots, which are made from compressed coconut fibres, must be sent by ship from places as far as Sri Lanka and the Caribbean. These product also come without the benefits we have become so used to from plastic, convenience, predictability - and primarily - reusability.
I have discovered that there are many benefits to polystyrene seed trays that people just are not aware about. It is easy to see their bulky appearance and assume they are just as damaging to the environment as black plastics, but I hope by the end of this page you will be surprised by just how suitable and sustainable they are for gardening.
Expanded Foam
Polystyrene is made by stringing together, or polymerizing, styrene, a building-block chemical used in the manufacture of many products. Styrene also occurs naturally in foods such as strawberries, cinnamon, coffee and beef.
When made in to a foam material, called expanded polystyrene, the product is made from more than 95% air. If we consider for a moment that there were two seed trays of the same size, side by side, the one made from plastic would contain 20 times more plastic than the same polystyrene tray next to it!
Recycling
One of the biggest benefits of polystyrene is the availability of recycling. Although they last for many decades, there may become a point when you wish to recycle your seed trays and the knowledge of being able to do so can help alleviate some of the concerns about environmental sustainability.
We have a great website in the UK that helps us find out if certain products are available for recycling and where to take them. It's called Recycle Now and I really recommend that you bookmark it. Simply find the material you wish to recycle and enter your postcode. It will provide a list of local places which accept your materials.
Food Safety
On my page about stopping black plastic use, I provided a link to the Independent newspaper report about micro plastics making their way back in to our food chain via the application of fertilisers. Plastic entering our food and our bodies is a primary concern.
For polystyrene, many studies have been conducted by the US FDA, the Food and Drug Administration, a Federal agency responsible for protecting human health. They have concluded that polystyrene products are extremely safe for food service and food packaging. You can find more information about it at Chemical Safety Facts.
Of course although we are not using polystyrene seed trays for direct food use, we do use them to begin growing our own food.
Durability
From my "About Us" page I told the story about renovations on our former house, and how the garden was overgrown. In an old shed we found hundreds of plastic pots crumbling after years of disuse, but found a stack of polystyrene trays still in good condition.
This is one of the main points that makes polystyrene seed trays so sustainable. Not only are they 95% air, but they are extremely durable. In one of his videos, Charles Dowding, a No-Dig Guru, makes a point about having used his polystyrene seed trays for many decades.
Rigidity and Strength
With most garden plastic products such as seed trays, their strength lies in being a monocoque structure - the rigidity and strength comes from the product as a whole. Once there is a break in part of the structure though, the integrity is lost and the product rendered useless. Even a small crack will propagate quickly, making plastic seed trays disposable and poor for sustainability.
Polystyrene seed trays however are not subject to the same problems. Expanded polystyrene maintains structural integrity even if part of the tray breaks off. If a few seed cells are broken off then there is no reason to throw away the tray. The remainder remain useable and strong. You can simply tidy the tray with a sharp knife or a saw.
Insulating Properties
Being made from 95% air, the insulation properties of polystyrene seed trays protect your seedlings at their most critical time of growth. They help to steady temperature changes and limit the effect of temperature extremes. This helps to promote good seed germination.
This applies not only to vegetable growing but also to seed growth generally. There are various articles written by Monty Don about the benefits of maintaining a constant temperature by use of a greenhouse or a porch, but remember that until the summer is here, it can get pretty cold at night even in a greenhouse. Limit this temperature extreme with a polystyrene tray to help prevent germination from stalling.
Drainage
Polystyrene Seed Trays sold here come with a drainage hole at the bottom. This helps prevent the plant from becoming waterlogged but also helps, with the aid of a pencil, to push the seedling out of the tray when it is ready for planting.
Plants growing in each cell need the nutrients to last as long as possible. This way you can transplant them outside without having to pot them on. For this I would recommend you apply some compression of the compost when you fill the tray. The strength of polystyrene seed trays makes this possible, and if you wanted to, you could even slightly over fill them so that the compost will settle over time.
Compressing the compost has the added bonus of removing air pockets, which may inhibit rooting, as well as aiding to hold the compost together when the plug is removed from the cell. Just before you remove the plug for planting elsewhere, make sure you check the moisture levels. A dry compost will fall apart as it is removed, so water as necessary first.
In conclusion
Totally natural products are not always as sustainable as one imagines, and often do not have the benefits we have become accustomed to. Sometimes the most suitable alternative product can be the one that seems the least likely. I hope that after reading this page that you have been presented with new information and had your own views challenged. The most important thing is that we are all taking steps to sustainability with a broad understanding of the options.
Happy gardening.