The only thing I can think of is to line the beds with heavy-duty landscape cloth. You can grow sweet potatoes in containers, too. Takes less space and gives a pretty good yield. I grew some once in a cheap $3 dollar general laundry hamper lined with a dollar store laundry bag. I know people who grow them in those fabric plant containers. If you're good with sewing, you can make those for yourself out of landscape fabric.

Gal, there is a type of landscape fabric impregnated with little prills of herbicide. It's been a very long time since I've seen this stuff, so don't even remember what it was called, but I want to alert you to the existence of such an item. It is used (or was used) for situations much as you describe. Some kind of googlization should get you there.

Wouldn't the herbicide kill the sweet potato plants?

BTW, did you hill your sweet potatoes? If you do, most of your potatoes will be above the soil line and out of the way of the tree roots.

The sweet potatoes were planted in hills in the raised bed, about 12 inches above the general soil level. The tree roots grew to the top of the raised bed and the organic matter decomposed a bit. I have considered putting some sort of base to the beds, but most material decomposes so quickly it would have to be replaced quite often. Some of the sweet potatoes found little spots between the tree roots and under the edge of the bed.

The idea of that fabric-keep in mind, I'm not advocating for it-was that the tree roots could go wherever they wanted, but when they got to where that stuff was placed, they would be killed off every time they tried to grow into that area. Again, I'm merely reporting the existence of a product of a type that could be useful here. I don't tend to use things like that-fabrics, long-lasting herbicides, etc. in any of the work I do.

I will have to look into both options. :) I grow other plants in the raised beds, but really noticed the amount of roots as I was trying to pry out the sweet potatoes. Double digging the beds after each rotation gets old after a while.

Gal, I deal with something similar each spring. So, our city fathers, in their infinite wisdom, redesigned our main commercial street about ten years ago. It was decided to plant trees in what amounts to concrete vaults-these things go 4 ft. down below grade, so there's nowhere for the roots to spread out to. Then, flowerbeds were decided upon at each end of these tree beds. I might not be making this very clear, but in any case, what has resulted is that each year, we have to try to plant stuff in what amounts to a mass of tree roots-almost no actual soil to work with. It sucks, let me tell you! We just do what must be done, cutting and chopping those roots out of the way. It's not right for the trees, and it's not ideal for the flowers, and so on that we then plant, but it's the best compromise available to us. In fact, because once the flowering stuff is planted, we water, fertilize, etc....the tree roots rapidly regrow back into that area. We have considered installing a barrier but in reality, this should have been done at initial construction, a point which, by the way, I made loudly and often back then!

I know this may sound crazy, but I've seen people dig down a foot and build concrete block beds to keep bamboo in place. Maybe that would work. Maybe if you don't like the idea of above-ground planters, buy one of those plastic barrels, cut it in half lengthwise, put some holes into it for drainage and put your potatoes in that. That way, you could just layer soil and organic matter of them as they grow, and the container wouldn't be so unsightly. Just tossing out idea. I know that's probably more work than you want to do for sweet potatoes.

wisconsitmom,

maybe they can make a separate add on to the beds for the flowers to save some work in the future.

dirty gardener,

I might try to sink the edges of future raised beds. The rock layer varies slightly in height below the soil so there might be some gaps, but it could possibly block more of the roots. I am constantly on the lookout for barrels and large pots that are not to expensive.

Look on craigslist for food-grade barrels. There is usually someone selling them for not much. I got mine for $10 each from a man in Punta Gorda who used to get them from Tropicana. back in the day.