A better test of readiness is to simply take away some soil to see what size the tubers are. If they look big enough, lift a plant and cook the potatoes. If they are too immature, they tend to be very soapy and are best left for a week or two to develop more dry matter.
Gerry, I really miss you off the radio, but it’s great to be able to ask you questions directly. I have so many…..I planted potatoes on 9 Mar. The British Queens which are described as 2nd Early are flowering, ahead of the Duke of York, which are supposed to be early. Is this alright? And when should they be ready for harvesting? (my wife is dying to get her teeth into some New Potatoes).
Flowering can be taken as a good sign of readiness for early potatoes, and they continue to mature after flowering. And some varieties do not flower well or at the same stage.