JackyB, the rhubarb is forced once it starts to poke through the earth by creating a microclimate that is warm and moist. The resulting stalks are thinner and weaker with a more delicate taste. They are less acidic and tart than the main crop and, because they have been hurried up, are earlier too. Yum yum.
With potatoes we chit them in a warmish, light dry place in order to give them a headstart when they are planted out. We do this in the light as we want fattish green with pink shoots that are strong, not the wiggly thin white things you get when you leave your potatoes in the dark.
This means the "early" potatoes have the opportunity to grow bigger quicker. For the "late" potatoes it means we can harvest them before the worst of the potato blight hits.
If you have "volunteer" potatoes appear in your garden you will get tubers from them but it will be a smaller crop. I think they tend to be more unusual shapes too. This might be because they can grow from very tiny tubers or even peelings so haven't been nurtured very well when they started off.
If you grew potatoes the previous year and had any disease (most likely blight) then I'd remove them at sight.
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