For well-rooted and developed slips ready for transplant start 8-12 weeks before plant-out date in your zone depending on how many slips you want. Check out the discussion below on the question. It has some alternate starting suggestions

Remember you can get at least 2 viable slips for transplanting from each slip the potato grows.

Dave

Here is a link that might be useful: Sweet Potato slip growing?

To start in water, begin about 2 to 3 months before they will be transplanted. Submerge one-third of tuber in a half-filled glass of water. Leave at 75F in a sunny place. When the sporuts are 6 inches long, pull them off the potato and set them in water or damp sand until they develop roots. Set plants into the garden two weeks after your last frost date when night temps stay above 60F.

Here's another method (again start 2 to 3 months before setting them out into the garden): Cut the sweet potatoes in half lengthwise. Wedge them cut side down in a few inches of moist potting soil or sand in a shallow tray (aluminum baking tray will work). Tuck the tray inside a plastic bag and place the tray in a warm room, 75F is optimal. The halves will sprout; allow the sprouts to grow to about 6 inches tall--with four or more leaf joints. Now you can "unplug" them from the starter seed potato: pull or snip them away from the starter about a half inch above the tuber but below the roots--so you have a well-rooted slip. Plant them outdoors two weeks after your last frost date as already suggested. You can also snap the shoots off the starter potato and place them in water to root.

Before you set the slips in the garden, dip the roots in a pan of muddy water. This will protect and coat the roots and prevent them from drying out--and it will ensure the root's direct contact with the soil once planted.

Here is a link that might be useful: HarvestToTable.com

I'm so glad Weedlady asked this! I've been trying to figure out the timing, too.

How do you harden off the slips? I plan on rooting mine in water in individual tubes. Would I just take tubes and all outside and harden off as I would any potted seedling?

Thanks,
Bellatrix

Here in Indiana I find that March 10th is the best all around date for starting them in jars of water. Last year the garage was colder than normal in February and the mother potatoes did not sprout a bit and they were late getting started. Still, many of the the harvested tubers were too large. So, they don't have to be particularly early.

Planting the whole potato outdoors in April tends to make them a bit late here though the rooted shoots are super in early June. This method with a bit of heat [cold frame] is probably the best.

My harvest was dismal last year because groundhogs and deer ate all the foliage. I replanted many of them in fall, in the hopes that they would return this year. What are my chances of success? Can sweet potato slips survive underground though the winter and come up on time?

Two-three months prior--that is exactly what I needed to know! Many thanks, GW pals Digdirt, Stephen, and Wayne!

Wild_forager, since they are a plant of the tropics, I do not believe your sweet potatoes will survive the winter inground and return next spring. I think that, like me, you'll have to plug in the toothpicks to a new potato or two and start some new slips in a couple of months!

Bella_trix, yes, the rooted slips should be treated as tomato & pepper transplants. Best to transplant on a warm, cloudy, drizzly day. If that's not feasible, puddle them in well, then keep them well shaded from sun AND wind for a week or until you see new growth. Keep tending them even if they seem to have died. Often they just take a while to recover & grow. I find it best to water them (and all seedlings & transplnts)with seaweed emulsion. (I always have used Maxi-crop, but there are others.)
Thanks again, everybody. Now I think I will go plant a couple of avocado seeds I have just to have something new & growing to tend until I can start propagating in earnest!
CK

You may start earlier by burrying the tuber horizontally in the pot kept inside the house due to cold weather.
Pic of the slips in a pot.

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Chaman I always start my roots that way. Don't have any real documented proof, but I believe that they are less prone to have rotting problems when started in potting medium (vs. water) and intact (instead of cut). "Horizontal" put more of the root in contact with moisture and thus, likely to grow sprouts. That's just my reasoning. But there are people who have done the toothpick and water approach for MANY YEARS and they get sweet potatoes!

Glenn Drowns, at Sandhill Preservation Center, has been doing quite a bit with sweet potatoes. He has the most extensive collection of varieties, available for sale, that I know of. But he, being in a Northern environment, has also been doing a lot to learn about the plant's requirements. Probably the #1 thing I've heard from him, over the years is that the sweet potato requires HEAT to grow and prosper. Glenn often counsels folk not to plant their starts until the nights and soil are warm. Starting slips in the garden early does not give better results than starting late but with plenty of warmth. Last year he started some varieties towards the end of July and still got good results. I started my last planting here on July 18 and found that two out of three varieties did not plump up quite as well I would have liked. My theory on this is that here in OK we have a number of weeks in the summer, when temps are so high, and we get no precipitation, that a lot of plants just sit there, rather than growing. Still, I would not hesitate to plant any sweet potato (here) anytime in June or early July.

Sweet potato vines are incredibly resilient! I do not know why I do lose a slip or two almost every year when planting. But most of the time I put them out and water them (from a can), mainly for my own sake, not for theirs.

When we lived in the high desert, in Hidalgo, Mexico; we helped a private school with agronomy. We used flood irrigation, as it rained infrequently. Anyway, I once saw a student harvest sweet potatoes and drop a vine in an unused garden plot. That plot received no irrigation or rain for four months. The vine was crisp and brown. When someone irrigated that plot the vine grew sprouts and rooted again! This was a native variety so folks, don't try this one at home, unless you just want to satisfy your curiosity.

In my opinion, sweet potatoes are one of the most neglected and most useful crops a person can grow, and, I believe Northern gardeners really should try them. If one has a really short season or cool nights, then go with earlier varieties and use black plastic mulch.

George
Tahlequah, OK