I would say anywhere upward of 12" higher than the surroundings. Then, try to direct any run off water away from the area.

seysonn, the area is at the base of a long rolling pasture, and there is typically no evidence of runoff... simply the movement of large quantities of water at/below surface level. We have an engineered septic due to the poor perc qualities of our plot. The ground is extremely densely grassed, and there is never any standing water. However, the ground is almost always spongey, and can get soft to the point that your foot sinks into it during rainy periods. Do you think 1 foot of bed depth will permit any wicking of moisture up into the root zone of those plants? The melons might reach down for it, but isn't the corn fairly shallow rooted? It would be annoying to have to water out in that area given how naturally moist the ground is!

So your are trying to balance the issue of overly soggy native soil that can't be drained apparently against the need to provide any supplemental water for the location and the water needs of 2 very different crops? That is quite a complex situation and I'm not sure it is even possible.

If the only issue was the depth of a raised bed over soggy soil - the goal being to allow for proper drainage and moisture for the crop growing there then I'd agree that 12" would be great. People solve that problem that way often.

But we can't have our cake and eat it too. :)

(a) Build a 6" deep bed and grow shallow rooted crops. there and let the soggy soil provide the water. Things like onions, cauliflower, cabbages, lettuce, etc.

(b) or build an 18" deep bed, grow whatever you want in it but water it normally and disregard the ground water.

(c) trench in vented drain pipe to drain the area and build an 8-10" bed, grow whatever you want in it and provide it with needed water.

(d) treat it as a bog garden area. No raised bed just raised ridges alternating with furrows and plant edible bog gardening plants there, things like celery, cranberries, rhubarb, onions, etc. Make the raised ridges high enough and you can grow corn there and let the boggy ground provide the water.

Dave

Melons taste better when grown in dryish conditions. I grew (primarily greens, which like water and are shallow rooted) in similar spot for almost ten years, and for me 8 inches was successful.

This thread made me think about perched water table. If I understand it correctly, the perched water table theory will cause the water of the soggy soil under the beds to go up toward the surface of your raised beds. I assume that the height the water will go depend on your soil texture. The finer your soil texture, the higher the water will go nearer to the bed's surface. 12 inches should be enough to stay away from the soggy ground water for most kinds of sandy soil. If your soil is heavier then just build higher beds.

I've had success in a similar situation by planting in 6-8 inches of municipal compost over my somewhat soggy garden soil. I only have to water occasionally in August and the only overwatering problems I've noticed are enthusiastic growth of weeds and unfortunately tomato foliage diseases. I also have not had any success with mulching - too water retentive - other than with Kraft paper.

My corn does wonderfully, although I grow an open-pollinated variety. I've only really tried melons in this situation once and I grew a variety seemingly not noted for being super-sweet (Amish muskmelon). Nice flavor but could have been sweeter, and they had a tendency to split.

So maybe the corn in a shallower bed and the melons in a deeper one?

~emmers