|Well grown examples (if I do say so myself) of the blue-black fleshed potatoes Congo(left) and Négresse.|
—Négresse has a squarer shape than Congo (most obvious on fully mature tubers).
—Négresse tuberises slightly earlier (or at least gives more crop by early July when mine were killed by blight last year).
—Their foliage is noticeably different.
—Négresse has a shorter dormancy period.
I have said previously that I suspected Negresse was truly day-length sensitive (like oca); the plants will continue to grow without naturally dying back, until the first frost. Actually I now realise that very small tubers are formed by mid summer, so the plant is not day length sensitive. But they do need a very long growing season to reach the size of those in the top photo.
Here are my lazy beds in June last year, complete with authentic blight blackened foliage...
I was expecting to find no tubers at this time of year, but I was pleasantly surprised to find small tubers had already formed.
And then there is Vitelotte. Wikipedia says this is synonymous with Négresse. So do seed merchants Thompson and Morgan who are selling them as micropropagated mini tubers this year. Others say they are clearly different, so who knows. I certainly get larger tubers than those shown on the T & M site.
Changing the subject, an annoying problem with all these black potatoes is finding the well camouflaged tubers in the soil at harvest time. Missed tubers result in persistent volunteers which can be real pests amongst a following crop. No such problem with this new (as yet unnamed) variety...