Don't forget yer independant nurseries, I went to one for my seed tats and they were just £1.10 a kilo. then there are Wyevales, Haskins garden centres probably a bit dearer than the independants where you can buy just one tat if you want BUT no postage to pay. Don't know how near you are to a nursery. The tats in our B&Q looked a bit tatty to be honest.
I only ever growin great big pots (approx 24" wide) or garden tidy bags the pop up ones or year before last in old compost bags. All seemm fine to me, didn't use the compost bags last year cos they looked a bit untidy thats all. If you use bags or the pop ups I mentioned you can roll down bag and roll up as you go so they get some light, or the pop ups I tie down to make smaller via tying the handle to little tag on the bottom either side. Dry out quicker than in the ground so make sure remember to water. Found Charlottes gave me a great yield last year and Lady christl not so good a yield but hey different years could be different story. I also used Chicken poo as fertilizer as one of my independants sell big pots buy one get one free and each pot is 50% extra as well, they are outside at mo' so not sure of size (approx bucket size). I suppose you know about covering up the shoots with compost as they grow till you get to top of bag (accepted) or wherever you want to stop (run out of money for compost)
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30 reusable bags must have cost a lorra money if you mean the ones you put your shopping in?
Oh yeah a lot of peeps turn compost bags inside out so black outside absorbs the heat.