January 21st Comparing Of The Spuds

Because we only  have a handkerchief sized section, our garden is minimal as well. Therefore if we want to grow Potatoes, our only option is to grow them in a bag. We have two of these bags. The first bag we harvested for Xmas Day, and for yesterday's dinner, the 2nd bag was harvested.

I remembered that I had a photo of Harvest number one, so I decided to compare the two harvests and to see how the same crop of potatoes compared with the passing of 4 weeks?

On the left is 25th December and on the right is 21st January. Obviously a larger size which is to be expected, but a different colour? Are there any scientific boffins out there who can explain why the colour change (the potatoes decided to match my hair?) and even more so, why when we boil these, the colour totally disappears?