When my seed potatoes arrived in mid-March, I followed directions and placed them in a sunny but not-too-hot spot so that they could sprout. I went with the windowsill in the guestroom.
After about three weeks, the seed potatoes all had some nice green sprouts growing out of their eyes. My instructions said I should wait until the sprouts were 1 inch long. But, after another couple of weeks, that didn't seem to be happening, so I just went ahead and planted the seed potatoes as they were.
Because my garden space is limited, I decided to plant my German Butterballs in special Potato Planter Bags. They're basically big, green plastic bags with drainage holes at the bottom and sturdy handles. The bags come with fairly elaborate instructions about planting your potatoes in stages. Basically, as the green stalks grow, you add in more soil and compost. So the top of the soil slowly rises up as your potatoes grow.
Here's how the potatoes look today after two weeks in the bags. The leaves and stems seem fairly healthy, as far as I can tell. I was worried that planting them before they had a 1-inch sprout would mess them up, but that doesn't seem to be the case. I put a lot of compost in the bags, so I'm sure that's helping.
It'll soon be time to add the next layer of soil and compost. And then I'll just have to be patient. According to Seed Savers, these guys need 100 to 120 days to reach maturity.