There are some factors which make growing sweet potatoes in the North different from in the South. The heat levels, for instance. I know Sandhill Preservation Center (located in IA) lays down black plastic mulch to warm the soil, planting through it. They just lay the plastic, weigh down the edges and plant through it. They get good results this way.
I use plastic mulch, here in Oklahoma, but I have to lay drip irrigation on top of my hill and under the mulch. Then, during the growing season I have to deeply water each row of sweet potatoes (ovenight) once a week. With our heat, plastic mulched sweet potatoes will simply dry out and fail to yield anything unless watered, and that, from under the plastic. However, with the drip irrigation they absolutely thrive and produce like crazy during our 6-8 weeks of triple digit heat.
Here's a photo of last year's rows. The drip tape is under the plastic, which was, in turn under the organic mulch. I got a late start planting and by this time the heat under the bare plastic mulch would literally cook my slips as it chimneyed out the holes through which I planted the slips. In order to prevent this I had to mulch the plastic so the sun wouldn't heat it up so much.
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