I do it too (well I use some bags from Wilko, but they work the same) - I do it for the taste and the sheer pleasure of it. But I always say they're the most expensive spuds I've ever eaten, by far! I've got four bags on the go at the moment, and each one will use about a sack.of compost.
'If you have a garden and a library, you have everything you need.'
I'd used the compost elsewhere so using the bag, soil from elsewhere in the garden and a couple of spuds that had been in the cupboard too long, didn't feel that expensive
I've grown potatoes in this way before and used a couple of old car tyres to put the bag inside to keep it upright, preferably using a large bag of compost (40 or 50 litres) and plant your seed potatoes in a couple of layers. It worked quite well.
I bought one bag designed to grow potatoes and used a couple of old compost sacks, initially rolled down to half their height. All took a lot of compost (used to mulch shrubs afterwards so not wasted) and a LOT of water. The compost bags produced a heavier crop than the purpose built and expensive bag, so I hope that you have tried the recycling method. Don't forget to add holes for drainage.
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