You can do that, but it's better to plant deep (or in a trench) because roots will grow along the stem to create a stronger plant. Your choice, and if you do as you suggest and need more plants, you can always root the tops in water - it's very easy :).

Thanks Labradors! I didn't know that about rooting the tops in water. Great idea!

Catherinet, yes! Tomatoes love to grow. They are almost a continuous root system looking for a place to call home. But personally I would trench the leggy part of the plant, even if you cut part of the plant off, the bottom of the stem is probably still kind of weak and long. Then later when it's put out good growth, if you want more plants then trim off a branch or sucker and stick it in water.

The one you stuck in water - watch for root growth, once you get roots a couple of inches long you will want to pot it with good potting mix. There's not enough nutrients in water to keep it growing indefinitely. Then after a few days for it to regain its equilibrium, start hardening it off like any other seedling, taking a couple of weeks. At that point it should be ready to go back out into your garden. Good gardening to you! :>)

Thanks again ediej!