I would use a loam based John Innes no 3 type compost mixed with about 1/3rd multi purpose compost to help with water retention.
The nutrients in teh compost will only last 80 to 90 days so you are going to have to give it a good top dressing of slow release fertiliser for clematis, roses or tomatoes every spring and then regular watering as it will be entirley dependent on yu for food and water. Adding some seaweed or tomato fertiliser to the water once a week when flower buds start to show will also help as they are gross feeders and do not lik eto be thirsty. Don't want to be drowned either
Vendée - 20kms from Atlantic coast.
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The nutrients in teh compost will only last 80 to 90 days so you are going to have to give it a good top dressing of slow release fertiliser for clematis, roses or tomatoes every spring and then regular watering as it will be entirley dependent on yu for food and water. Adding some seaweed or tomato fertiliser to the water once a week when flower buds start to show will also help as they are gross feeders and do not lik eto be thirsty. Don't want to be drowned either