[AlaninCarlisle]

I've been growing them for many years but they are never as long and free from soil as those in the supermarket which are about 18" long and totally clean compared to my squat 10" mucky ones. How do I make them compete with ASDA's?

[Snow]

Try a long variety  . Bulgarian giant made really long pale green ones for me. They weren't very hardy though so got rather damaged by the deep frosts we had, the hardy winter/spring ones are all quite short varieties

[mumofstig]

You could use some lengths of drain pipe pushed into the soil around them, to give you a longer blanched stem, but I found it attracted slugs inside, so I had to use pellets regularly along the rows.