I hedge on the last frost date. It is supposed to be 1 June around here but I try to get the potatoes in by 1 May if the weather looks like it will hold up (and not be too wet - they rot if it is too wet when you put them in). I dense plant and always have a full hedge by the end of June. The main reason to plant early is to avoid potato beetles. By the time they show up, my plants are past the pollination stage and they just seem to ignore them. As summer moves on, they die off and look like nothing. We just ignore them after that. Sometime between late September and early November we use a fork to carefully turn the bed and strong young eyes to spot the potatoes. We always get enough from that one bed to last us into the early spring - and we still have a lot left right now. The purple potatoes are my favorites, but I've had great luck with intermixing red and large yellow or white in the same bed - along with sunflowers. If the sunflowers don't beat the potatoes to a certain height, they won't make it. Otherwise they are beautiful. The birds eat most of the seeds, but that just means more fertilizer for the potatoes.
Oh, and as with other nightshades (tomatoes, peppers, eggplant, etc.), potatoes like to be grown in the same place each year. Don't bother with crop rotation.
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