I cant deal with any specifics of your climate, as I've never been to your beautiful corner of the country. However, I think some of the general principles of growing warm-season crops in cooler climates will apply:

Anything you can do to raise the heat units around the plant will help. Black plastic mulch, poly hoop tunnels, and warm microclimates all may help. Gardeners in places like Alaska, Scotland, Ireland, Iceland, Yukon, etc., all have grown warm-weather crops like peppers, eggplant, to

Did you try posting this in the Northwestern forum? I'm growing regular potatoes but not sweet potatoes. HOwever, I'm sure someone there has. Just a thought.

NJ is a bit warmer, nevertheless when we lived there I found that laying down black plastic over the ridged earth, before planting the slips, made a great difference. The vines with black plastic thrived and produced more sweet potatoes.

George
Tahlequah, OK

Sue, I'm north of you in Island county and tried sweet potatoes for the first time last year. I think I got the slips from Territorial. Even with black mulch cloth to prewarm the ground the results were dismal. But since it's the first time I've tried them it may have just been inexperience. But I think that daytime highs in the 60s and nite temps just about at 50 the weather here isn't conducive. But if someone really wanted to put the effor into them with black mulch and a hoop house they could do it. You have much warmer summer temps than we do here so I'd give it a try. Tom

Sue
We live very close to each other,I live in University Place. Dont grow Sweet Potatoes but have very good luck with Black Potatoes which I had read also requires heat but even in our unusually cool Spring we have a large crop coming on this year.
Jim

I live in Eastern Wahington Ephrata/Soap Lake, has anybody had expierience growing sweet potatoes in this area?? seems like a good area for them hot and long growing season. I have plenty of water.

I have not grown them myself but I was researching the matter. On Mother Earth News I found an article from a gardener who grows them in Canada:

Grow Sweet Potatoes Even in the North
http://www.motherearthnews.com/organic-gardening/growing-sweet-potatoes-zm0z11zsto.aspx

The author said the trick in the northern regions is to use clear plastic to warm up the soil before planting.

On another link, a blogger from Seattle related his experience in trying to grow them--he was not very successful.

http://www.goodfoodworld.com/2011/01/growing-sweet-potatoes-in-seattle-is-it-possible/

However he did relate in the article that you can ALSO eat the leaves and they are quite nutritious. He said he planted his in black pots but the year he did it for--Seattle had a particularly cold summer. My guess is that it would be worth a try but warm the soil first.

A gardener here on Olympic Peninsula and close to the water grew them. He likes to experiment with things at least once. I took 4-Hers on field trip to his garden in October and he saved his peanuts and sweet potatoes to harvest when they came. The sweet potatoes were only a little smaller than those in store and about 6 to a plant. They were planted on the south side of a stone/ concrete foundation of his greenhouse outdoors.

We are outside of Leavenworth in the foothills of the Cascade mountains. Our climate is a bit different but also challenging due to our cold night temperatures well into the summer.

I've been growing sweet potatoes for several years now with great success. First, don't order i.e. pay for slips. Buy a couple of sweet potatoes at the grocery store in January. Lay them on their sides in a fairly shallow pan and fill it with water so the potatoes are about half in the water and put it in front of a window, by the kitchen sink. Keep the water level up; and every week or so change it for fresh. When the potato has sprouted and they're about 3" tall, carefully pick them off of the potato and put in water until they root. All of this takes until about mid-April. I then put my rooted slips into 4" pots when I'm starting my other seeds. Our frost danger isn't over until the end of May and by that time, the plants are vigorous and ready to go out.

This is probably the jump-start that makes growing these possible. When I tried putting little slips in the ground there wasn't enough time for the potatoes to mature and I harvested lots of little pencil-sized ones. Now, I get enough for both holiday dinners for our large family from starts off of two potatoes.

PS Even if you get "pencils" they are still very edible ~ scrub, chop and saute in butter; add brown sugar for sweet or try salt, pepper and sliced onion for savory.