I've grown sweet potatoes three times here in Northern Michigan, I'm zone 5. The first year the Georgia Jets I planted did wonderfully. The last two years I've planted Beauregard, Nancy Hall, Georgia Jet and another white skinned sweet potato. My yield has been pretty much zero, in spite of watering, mulching, building up that high "hill" that Steele tells me SPs like. I've given them up, I'll just have to buy them, the slips cost about $20 by the time I pay shipping and SPs are less than 50 cents a pound here in season. Definitely not cost effective, as well as frustrating... I never thought of the night time temps, although I did cover the ground around the plants with black plastic to keep in heat, with no apparent improvement. It's usually in the 50s here at night, even in July and August, and some nights dip into the 40s. Annie

You need a soil test to be certain what's going on with the ground itself. No amount of guessing will help. Tell them what you're trying to grow and they can offer corrections tailor made for specific crops, usually. Generally, you need softer soil so the tubers will be able to expand. Incorporate all the organic matter you can find. Compost is supreme, but chipped leaves are great, manures are good, and if you absolutely have to, try peat moss. Someone suggested sand to me, but I believe that red clay plus sand = concrete and I don't want it to be worse than it is already. I plant about 300 slips per year. I do the same thing every year. I get varying results and still don't know why. Last year, I had a decent crop, but not any huge tubers like in the past. So this March, I hauled in fresh horse manure and dumped it and tilled it in. When I was ready to plant in May when the slips arrived, that had mostly rotted. I use a turn plow to make hilled rows. It digs deep. I go both ways to make one row so it's uniform on both sides. Works for me. Then I'll go back and hand plant 18 to 24 inches apart and firm the soil around them on top of the ridge. I make a depression around them to help catch water. Otherwise, it runs off, not in. Red clay forms a crust in a heart beat. I'll water a few times as is, then I'll go back and poke holes in the hill 6 or 8 inches deep and water again into the holes so water gets into the hill. Usually I have to water 4 to 6 times, once every other day, before it rains enough and the plants look like they will survive on their own. I may go 3 days between if it's overcast and not so hot. Watering is done by hand. I'm not near a spigot. Used to be 2 gallon cans filled from a rubbermaid container and spritz each plant. Now I have a 300 gallon tank on a trailer and a hose. I still walk the rows, but I can give each plant more water at a time without fear of running out so soon. 300 plants means about 500 row feet. last year, I dumped a quart of fish emulsion, a bottle of NOG, and a small bottle of chelated iron (needed magnesium or manganese, I forget) and just for good measure, a handful of 20 mule team borax for boron. They need boron. I added 200 gallons of water. Nice fragrant cocktail. I watered both the sweets and the tomatoes and pepper transplants with it. I had the best pepper production I've ever had. Sweets, not so much. This year, I added fresh manure in March and watered again with the cocktail. I dug sample sweets early last week that were the size of gallon milk jugs. Literally. I'm waiting to dig the rest another week or two. I usually dig them right at the September/October change. I've had rabbit issues this year from what I can tell looking at the patch. But the vines are thick and heavy, the leaves are huge. I'm looking forward to the big dig in another week or two. Next year, I should be using rain water. That's the plan anyway. See if this works. If not, click the link below. This is from two random plants near the end of the rows. Smaller regular size tubers are in the bucket... Here is a link that might be useful:

Sweet!!!

"I may put a 2 foot bamboo stick next to each plant when I plant them, as a marker." That's a good idea for many plants, especially those that need to be dug and those that vine. I try to do that with all my squash and cucumbers. It helps a lot when hand watering. Watering vines far from the roots doesn't do much good.