Plot #59 Trials and Observations

Growbag Trial Update – The Harvest Results Are In

This year I’ve been trialling the new SylvaGrow Peat-Free Planter vs a bog-standard peat-based growbag, and now I’m pleased to report the results of the trial.

This year I’ve been trialling the new SylvaGrow Peat-Free Planter from Melcourt, one of which I was very kindly sent for the purposes of this trial, alongside a bog-standard 50% peat, 50% “forest gold” growbag (which happens to be from Bulrush), which I bought at a garden centre.

So far I’ve made a note of the trial setup, a few early growth observations, the onset of fruit-set and ripening and now, I’m pleased to report the results of the trial.


This was not a scientific trial, by any means. I haven’t precisely controlled any of the inputs, or taken measurements at exact intervals, or anything as detailed as that. It’s just a small-scale growing trial, in allotment greenhouse conditions, carried out by a busy bloke who, for instance, hasn’t always remembered to water everything exactly when he thought he ought to. In other words: probably typical home or allotment growing conditions.

Nevertheless, I have done my utmost to give all six tomato ‘Maghrebi’ plants in the trial – two grown in peat-free, two grown in standard 50% peat mix and two ‘control’ plants grown in soil-plus-manure – as equal a treatment regime as possible. So when one was watered, they were all watered and by roughly the same amount. When one was fed – which I only did twice, during fruit development – they were all fed with roughly the same concentration of organic liquid tomato feed. When one was pruned to remove some of the lower leaves, they all were and by the same number of stems. And so on.

Whilst this means the results and observations can’t be called conclusive or definitive in any way – a much larger trial would have to be carried out in much better controlled circumstances for that to be the case – I think they’re still valid for the scale and method of the trial and they’re still interesting in their own right.

Here we go…

Weight of Harvest

This has become the main measure of comparative success for this trial. I was hoping to assess the merits of the three methods in terms of flavour, but to be perfectly honest I couldn’t detect much difference between the fruits obtained from the three growing methods.

Weight-wise, here are the totals harvested, picked over the course of a couple of months as and when they were either ripe enough or conditions dictated an early picking:

Growing Medium Total Harvest Spoiled Harvest Edible Harvest
Bulrush 50% Peat 3,266g 713g 2,553g
Slyvagrow Peat-Free 4,098g 0g 4,098g
Control Soil 4,143g 550g 3,593g

Fruit Quality

These photos are representative samples of the fruits from the peat-based Blurush (left) and peat-free Sylvagrow (right) growbags. I’ve tried to be as un-biased as possible, taking a broad selection of fruits from good to not-so-good at peak harvest for both growbags.

Conclusions, Notes and Observations

  • The peat-free growbag definitely offered better water-retention than the peat-based bag:
    • During the plants’ full-growth phase I noticed that the plants in the peat bags were prone to wilting whilst the peat-free plants stayed fully turgid.
    • In addition, the spoiled fruit on the peat-based plants were all damaged by blossom end rot, a physiological disorder caused by insufficient water supply during the development of the fruit.
    • This was despite the shape, size and orientation of the growbags meaning that the peat-free bags actually had a larger exposed surface area open to potential evaporation, as well as a larger volume of growing medium for the water to soak into in the first place.
  • The peat-based growbags were the earliest to provide a decent batch of ripe fruits, by around five days.
  • The plants in the control soil ripened significantly later – around three weeks later – than the plants in either of the growbags.
  • The spoiled fruit on the control soil plants were all damaged by a late attack of tomato blight. The greenhouse stayed fortunately blight-free all summer and it wasn’t until the last couple of weeks of the trial, when the wet October weather set in, that blight reared its ugly head. It ended up affecting around half a kilo of the fruit, and would have taken more if I hadn’t spotted it and called an halt to the trial.
  • As a result of the early halt, at least half the total harvest from the control soil was still green (chutney-making time…)
  • As you can see from the pics above, whilst the peat-free tomatoes were by no means blemish-free, the peat-based fruits showed much more variability in terms of shape, size, discolouration and other blemishes, including the aforementioned blossom end rot and a few instances of skin-splitting, which again tends to be caused by uneven irrigation. So in terms of general fruit quality, the peat-free fruits were more consistent.
  • In terms of overall harvest weight, the winners were the soil control plants, followed by the peat-free plants, with the peat-based plants noticeably lagging. However, the blight-damage to the fruit of the soil control plants, and the subsequent need to harvest early, meant they fell back to second place in terms of the edible quantity of fruit harvested and to third place, behind the peat-based plants, in terms of quality.
  • The best performers in this trial were therefore the peat-free plants, followed by the peat-based, then the soil control plants. In another growing year, with different weather conditions, the control plants may have flourished and put on a late burst of ripening, resulting in them taking the laurels. But based on the issues with water retention demonstrated during the growing season, I suspect the peat-based plants would still have come in well behind the peat-free plants. Although of course, other peat-based growbags are available, with varying mixes of peat and other materials. So perhaps a 100% peat growbag would have performed better. Or worse. Again, a larger-scale trial would be needed to draw any firm conclusions.

In summary then: next time someone tells you “you can’t grow anything in peat-free”, please do point them in the direction of this here blog post. Or, even better, suggest they ruin their own trial, because there’s nothing like seeing the results for yourself to convince you that peat-free is a viable option.

I for one am satisfied that you can indeed grow very good crops – certainly of tomatoes – in a peat-free growing medium and, given the very real environmental concerns surrounding peat extraction, I won’t be buying peat-based composts for my own use again in future. (Yes, I know what the knee-jerk response to that will be – “but peat-free compost is so expensive!” – and I have another blog post lined up to explain why that’s not actually the case at all.) If you’re interested in looking into the peat-free options, then this article from should help you broaden your knowledge on the subject.

In the meantime, I hope you’ve found this series of posts interesting, and if you have any questions about the trial, or observations of your own to make (feel free to criticise my method and suggest improvements for the benefit of anyone else who might be reading, bearing in mind that I’ve already acknowledged that my methods are far from scientific) – then please do so, via the comments.

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