My parsnips grew large but were very dumpy and twisted, just like octopus tentacles entwined together. So I planted the pregrown plants in a potato barrell, to ensure no stones, and I mixed top soil with garden centre compost. I fed with tomato food and watched the lush greenery grow. However, on harvesting I was astonished to see no roots longer than 2" and no thicker than a pencil. What can I do to grow my favourite vegetable, and what is going wrong?