While I have been writing The Peat-Free Diet I have also been conducting some small experiments on the side. One of those was on seed germination and seedling development in different growing mediums. It was time to sow some more seeds for plants later in the year, so I sowed two identical batches of seeds – one in 100% coir compost with no additives and one in a 50/50 mix (by volume) of coir compost and vermiculite.

I hadn’t used coir compost for seed sowing before, and it’s pretty nice stuff. I put a large block in a bucket of water and left it to soak up the water and soften. They say you should leave it for half an hour or so… I left mine for several days as other things got in the way. In the end I had a bucket of nice compost, fine textured for the most part (with the odd lump, mostly easy to break up) and clean to handle. The dark colour makes sowing most seeds easy, too.

For the second tray of seeds I mixed up a batch of 50% coir, 50% vermiculite. It’s not easy to sow seeds into – the mixture of colours means both dark- and light-coloured seeds disappear, and the chunks of vermiculite make the texture significantly more lumpy.

I sowed the seeds on 6th July, and made note of the first germinations on 9th July – ‘Petit Posies’ brassicas and leaf beet. I forgot to make any further notes of dates, but today at least some of each type of seed have germinated. I used a mixture of new seeds and old, packets and home-saved.

Pure coir
Seedlings grown in pure coir compost. The empty row on the right hand side is from home-saved (and pretty ancient) dwarf marigold seeds, so the problem is almost certainly a lack of seed viability.

Coir and vermiculite
As you can seed, only one of the marigold seeds germinated in this mixed growing medium.

For completeness, these are the germination results as of today:

The number out of 7 is the number of cells in each row in which seeds have germinated – in some cases there was more than one seed in each cell, which means I don’t know how many seeds in total there were (it was tricky to get one in each cell!).

When you look at the results in the table it appears that germination may be better in the 50/50 mix, although it’s hard to state that definitively with such a small experiment.

Seedling development
It’s not easy to see from the top-down photos, but there is a noticeable difference in seedling development:


That’s the 50/50 mixture on the left and the pure coir on the right. As you can see, the nasturtiums are about twice as tall in the pure coir. The honesty seedlings are also larger. It’s not as noticeable with the smaller seedlings, but I think they are more vigorous in the pure coir as well. There are no obvious signs of nutrient deficiencies, so the stronger growth could just be down to the difference in water availability (both trays received the same amount of water, into the reservoir at the base).

So… from this (albeit limited) experiment it seems that there are pros and cons to both mixes. If you want the highest possible seed germination rate then it seems that adding vermiculite to the mix does help – and if you’re pricking out once the seedlings are large enough that may be the option to go for. If you’re aiming for vigorous seedling development in modules then perhaps pure coir is the better option.

Another mix (perhaps with sand instead of vermiculite) would be slightly different, so it is worth doing side-by-side comparisons if you’re mixing your own growing mediums, to get the results you’re looking for.

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