Growbag Trial: the Tomatoes Are In

This year I’m trialling the new SylvaGrow Peat-Free Planter from Melcourt, one of which I was very kindly sent for the purpose.

Last weekend I set the planter up in the allotment greenhouse, alongside a control / comparison in the form of a bog-standard growbag.

I use the term ‘bog-standard’ deliberately because the Bulrush growbag (I didn’t pick this brand for any specific reason, it’s just what they had in stock at the last garden centre I visited) contains 50% peat. Not something I’d be happy about in normal circumstances, as I’m trying my hardest to go peat free from now on, but for the purpose of the trial it’s a necessary inclusion. The other 50% of the Bulrush compost is something they call ‘Forest Gold’ which, from what I can gather via the label is basically wood-chip (and/or leaf-mould? hard to tell…)

Here’s a handful of the Bulrush compost:

The Sylvagrow bag contains “a unique blend of fine bark (a by-product of sustainably managed British forests), coir (from a single, known source), green compost (a carefully-sourced, certified ingredient) and balanced organic nutrients sufficient for the first 3 – 4 weeks of growth.” Which is about the length of time you’d expect a grow-bag to last before you had to start feeding hungry plants like tomatoes. I have a bottle of organic tomato feed and concentrated comfrey tea standing by for next month onwards.

Here’s a handful of the SylvaGrow:

First impressions: the SlyvaGrow is denser, darker (suggesting there’s more humus in the mix) than the positively fluffy-feeling Bulrush. Whilst both should provide a nice, loose medium (the Bulrush was more compacted after storage and transport, so took more breaking up) for the plants to get their roots into, I suspect that the Sylvamix will retain water better and not dry out as quickly, but we’ll see.

For the trial participants I selected four ‘Maghrebi’ tomato plants. These have been grown this year from seed supplied last year by Garden Organic‘s Heritage Seed Library for seed guarding. We grew a few plants at work but unfortunately didn’t have chance to isolate them and so we couldn’t really send back any seed. I’m hoping to make amends this year by saving as much seed as I can from the trial and sending a package back to the HSL at the end. By way of a bonus, last year’s Maghrebi tomatoes were heavy croppers (and made a delicious passata!) so I’m hoping for good result this year as well.

I’ve used the end-on method of planting: fold each growbag in half and cut it across the middle to make a deeper ‘pot’ than by laying it flat. This gives you the chance to plant tomatoes deeper – leading to the formation of more adventitious roots from the main stem – and allows the roots to reach deeper as they develop, which adds stability.

One thing to note at this stage is the relative sizes of the two growbags: whilst the Bulrush is the standard 33 litres the Sylvagrow is around a third larger, at 45 litres. So you get more growing medium for your money, and because of the difference in shape, a roomier ‘pot’ for your plants. It’s also a bit easier to dig out a deep planting hole for your toms, without having to empty half the bag then back-fill it afterwards.

The four plants I picked were of roughly the same size. Two were a bit squatter and a deeper green colour than the others, so I put one of each (as it were) in each mix of compost – so there’s a (dark-sylvamix and light-sylvamix vs dark-bulrush and light-bulrush). They’re supported by a cane and one of those up-and-over, metal cane holders.

I watered them in with the same amount of rainwater in each planter and will continue to water them at a similar rate, unless it really does become obvious that one type of growbag is retaining significantly more moisture than the other, in which case I’ll chalk that up as a point of difference and adjust accordingly.

Here they all are with their companion Tagetes (marigold) flowers, which will hopefully help to deter aphids and attract pollinators. (Unless that’s Calendula and I picked the wrong species up at the garden centre… entirely possible).

I’ve already spotted the first truss of tomato flowers forming on all four plants, just a week after planting out. I’ll be keeping an eye out for the flowers opening, how well they seem to set, and anything else I can think of. Longer term I’ll be recording the number, size and mass of the tomato fruits, all being well.

How about you? What’s your preferred tomato growing medium and/or method? Do let me know, via the comments!

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