Not a straight answere, Cmw. Being a salad potato Belle de Fontenay is really a maincrop but it can be harvested early! In the summer use your fingers to gently expose some tubers and, when they are large enough, you can start harvesting. They are supposed to be a very tasty, waxy potato.

Belle de Fontenay is an old French salad variety from around 1885. It is a maincrop producing long, yellow-skinned tubers with light yellow flesh. Tubers are waxy with a fine taste.

hope that helps!

I grew these last year,and they were lovely.Good flavour,and no damage on them at all.Like Bulbaholic says,they can be harvested in summer,I started lifting mine from August onwards.

Thanks everyone!