You misunderstand me @pierre_de_gaye ... we all make mistakes ... I’ve made many ... what annoys me is when plant catalogues etc give misleading information (which seems to have happened in this case) resulting in people spending their hard earned money on plants that aren’t going to meet their needs.
No reason why you should admit you got it wrong ... you were given poor or inadequate information. As I said, you could grow it in a large container but it would still need substantial support 6’ - 8’ wide.
But it’s pretty hard when I and others are trying our best to help and we get told we’re wrong and called confrontational when we’re trying to help by giving you the correct information ... if the plant you have is going to need sturdy support and a lot of space there’s nothing we can do to change that. 😢 We have the choice of ignoring your question or trying to give you the right information
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Ive not grown Loch Ness so have no experience to pass on. However the information here looks hopeful https://www.ashridgetrees.co.uk/loch-ness-blackberry-plants?utm_source=bing&utm_medium=cpc&utm_campaign=Soft%20Fruit_products&utm_term=%2BLoch%20%2BNess%20%2BBlackberry&utm_content=Loch%20Ness%20Blackberry%20Plants
However I would add that the top growth will benefit from as much space as you can give it.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.