A correspondent recently wrote in to ask why all the seed packets for melons and squash tell us to plant the seeds in mounds, or hills.
Simple answer: Because the people who write the seed packets don't live in the desert.
For much of the country, excess water can be one of the downfalls of the vegetable garden. Soggy soil results in seeds that rot before they sprout, cold soil with no air, and rotting roots in already growing plants. Their solution – and it's a good one – is to plant on raised ridges, hills, or mounds that allow the soil to warm and drain.
Excess rainfall is not a problem in the Desert Garden, and following the seed-packet advice can cause your plants to fail. There's two reasons:
First, we want our garden planting areas to collect as much rain water as they can, and to hold as much irrigation water as possible; anything that leads water away from your vegetables can result in dry soils and consequent poor plant growth.
The second, and less intuitive, reason has to do with our salty soils and water. If you've ever watered a patch of bare desert soil and then let it dry out in the sun, you may have noticed a white or yellowish-white residue left on the soil surface; if there are clods of dirt in the area, they may have been completely encased in this residue. That residue is made up of salts that have been dissolved out of the soil and have then migrated to the soil surface. As the water evaporates out of the soil, the salts are deposited at the highest point of evaporation. If we were to plant our vegetables on ridges or mounds, the highest point of evaporation would be right where the seedlings come out of the soil, resulting in salt deposition at the tender spot where the roots meet the stem at soil level; if you recall what the Romans did to Carthage, you know that's really bad for our plants.
So here's what we do instead: after you've prepared your soil by digging in plenty of compost, use a rake, or your hands, to form a planting basin. Just pull some soil aside to make little "walls" that surround the area you'll be planting, then plant your seeds or plants in the bottom of that basin. Not only will that allow rain and irrigation water to collect where it's most needed, but it will also result in the salts being deposited on the high edges of the basin, well away from the plants.
I hope this helps,