I've never had to start sprouts/slips by placing SPs in water, it can become a stinky mess with nasty slime/mold rotting away at the potato. Periodically changing the water may help to hold down the growing mess. When I dig up mine in the fall I'll wash them off and store them inside on a multi-rack for future use. By about this time (Xmas) many will start sprouting slips without the need of water or anything else. By Spring they're loaded down with slips and I'll carefully carve off the slips and insert/plant them into old used cell packs with a mixture of seed starter and garden soil. Within 3 weeks they're ready to transplant outside. Plus I can still use the SPs that the slips were taken from. No mess, no fuss.

May be late information, but according to the U of M Extension, you can plant potatoes anywhere from April 15th to June 1st. Of course, everything is late this year. The garden store I go to didn't even get potatoes until recently. I planted mine two days ago. See attached link for more. Here is a link that might be useful: Planting the Vegetable Garden

I think you should go ahead a plant them. They should be all right. You can always mulch them if we get an unexpected freeze or heavy frost. You want to get them all settled and started before the summer heat starts.

I add sour cream (or drained kefir curd) and cream cheese to make freezer mashed potatoes. I think this mixture will last in the freezer longer and not be as "wet" after it is thawed. I normally use Yukon Gold potatoes, since that's what I grow. I'll use the larger potatoes for Twice Baked, and most of the rest of the crop gets used for Freezer Mashed Potatoes and boiled herbed potatoes, which I freeze and then pan fry. It's difficult to keep Yukon Gold potatoes for very long in storage after they are harvested, so I freeze the majority of my small crop. Our son gave us 40-pounds of blue potatoes from his garden and I've been converting them for the freezer, too. I've also been using them to make microwave potato chips ;-). Here's a bit of information about the 6 types of potatoes and how to use them, and why one variety will work better for some applications that other varieties. As plllog pointed out, it's also about what potato you use. http://www.all-about-potatoes.com/types-of-potatoes.html -Grainlady