What Do I Sow My Seeds In? Pots or Coir Pellets?

Back in April we did a film about some of the different ways we can all sow our seed. A month later and we want to talk about germination and general health of plants. We chose three different ways to sow cosmos and two different ways to sow nasturtiums. Cosmos Xanthos was sowed in trays, plastic modules and coir pellets. Nasturtium Alaska was sowed in pots and coir pellets. Remember that this is not a definitive trial, we just want to see whether coir pellets are the way forward for us to slowly phase out plastic in favour of something that is more sustainable. First, let's talk about germination, all seeds were sowed on the same day under the same conditions, a blend of coir and organic multipurpose compost was used in 50:50 ratio in the pots, trays and modules. During cool days and nights, propagator lids and fleece were added to all sowed seeds. Germination for Cosmos Xanthos in the tray was 0% and in the plastic modules only thirteen out of thirty five seeds germinated, that's 37% germination. Now, gardeners with limited space should be encouraged to grow their own but with such low rates of germination in traditional forms of growing you can see why some gardeners buy plants in. We all buy plants but with Brexit looming and worries about climate change we have to question where our plants are grown, and how far they have travelled to get to us. That aside, I would have been really upset at such low germination rates for what I know is a variety that germinates well. Sadly, the Nasturtium Alaska also had low rates of germination in pots, with only 1 out of 5 pots germinating. Sadly plant health was no better in the plastic modules, with seedlings failing to achieve the vigour found in the coir pellets. Now, we know there are variables here, there could have been additional nutrients in the coir pellets that were not found in the organic compost but we think there is more at play here, and it comes down to the actual containers.

coir vs compost 

Cosmos Xanthos in the coir pellets had a germination rate of 86% with only four coir pellets not germinating. The surprise was Nasturtium Alaska which had a germination rate of 100%. In terms of health, the seedlings quickly became plug plants and left our greenhouse a whopping two weeks before the plastic module plants (in fact the plastic module plants never left the greenhouse as they succumbed to the heat during the recent heatwave). This again makes us question the importance for gardeners with limited space. Two weeks is a long time and means that you could have sowed another crop in the space freed up. Let's talk about containers, the things we sowed our seed in and why we think plastic may be the problem here.


Now, you can see that I place the coir pellets into a seed tray. However, even though the pellets are packed together there is still air space between them, allowing for the movement of oxygen - a vital thing for plants and plant roots. This movement of air can do two things, trap in warmth as the foliage grows or allow slow cooling when needed e.g. it responds to the environment faster, you open the door of your greenhouse the plants cool down quicker and grow harder. When you plant into black plastic you have to consider the heat conduction of the colour black, fast to heat up, slow to cool down. This means that in a warm greenhouse soil within plastic modules heats up fast but fails to cool down at the same rate it heated up, this could lead to a baking effect or even dampening off as water fails to drain through baked soil/compost (it changes the composition of it, meaning friable compost becomes like a pan). There is no movement of air around the root structure and the drainage is reliant on one hole at the bottom of the module unlike the coir pellets that can drain all round (this mimics the plant in the ground). This inability of black plastic to drain excess moisture, to cool down and receive oxygen to the roots means plants will be grown softer, more sappy and inevitably more likely to succumb to heat changes and pests.

life on pig row, nasturtiums

This is not a definitive trial but does reveal to us that we will be going to coir pellets next year and ditching many of our pots as they fail (we are not advocating throwing away plastic, we will choose to phase them out as they break). For me as a disabled gardener they mean a decrease in using shop bought compost that come in bags I cannot carry, the coir pellets come dry and are easy, and light, to carry, they are soaked in water and achieve their full size quickly (you can see how that happens in the film here). The germination rates, plant health and plant growth speak for themselves. The coir pellet grown Cosmos Xanthos and Nasturtium Alaska grew faster and where hardened off quicker than the plastic modules. For me, it's a simple case of my own personal health and ease of movement. The coir pellets were easier to handle and involved no need to mix my own compost. Now, let's talk economics, the cost of compost vs coir pellets. We're going to start with the trays, it's been a few years since I bought any plastic but I know a rough figure for both, the green plastic trays holding the coir compost cost roughly 60p each compared to the module tray which only cost 40p. However, the green plastic trays have lasted me eight years and are still going strong, the module tray has lasted three years and broke only this week. This means the green plastic is a better investment, easier to store and clean. Let's do seed, we'll do one pack, Cosmos Xanthos which cost £2.09 a packet, we used two packets for this trial at £4.18. There are 35 planting stations in the plastic modules (we'll take this down to 24 in the final sums so we can compare like for like) and 24 in the coir pellet trays, then we had one large plastic seed tray (again costing roughly 80p). Each packet contains 30 seeds. Meaning we used 60 seeds. That's 6p a seed. Now, let's look at growing mediums, the coir pellets were £7.47 for 100. That's 7p a coir pellet. The organic compost were 3 bags for 10.00. That's £3.33 a bag, we used a quarter of a bag on 24 modules mixed with coir, giving us a rough costs of 83p. This was the same for the tray. Let's add it up:

Coir pellets
Cosmos Xanthos at 6p x 24 = £1.44
Pellets used at 7p x 24 £1.68
Plastic used, one tray 60p

Cost of tray £3.72 divided by 20 germinated plants = 18p a plant.

Plastic modules (at 24 stations so we can compare like for like)
Cosmos Xanthos at 6p x 24 = £1.44
Modules filled, 24 = 83p
Plastic used, one module tray 40p

Cost of tray £2.67 divided by 9 germinated plants = 29p a plant*

Seed tray
Tray = 80p
Compost = 83p
Cosmos Xantho seeds x 30 sowed = £2.09 (full packet) 

Cost of tray £3.72 divided by 30 = 12p a plant (no germination). Loss of £3.72.

* technically as we have lowered the ratio to included like for like, 37% germination of 24 modules is 8.8, we have rounded this up.

The figures speak for themselves. Yes, we do have to factor in that we will be buying in coir pellets each year but this is cancelled out in the saving of not buying in compost and with that alone I am in pocket to the tune of just over £2.50. Next I'm going to see how well they cope with something like salads, that'll be the make or break for how we grow in the future but I think we will be saying goodbye to plastic modules..

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