If you find yourself with spindly, leggy tomato plants that don't look as strong and hearty as you would like, it may be the way you are putting the tomato plant in the ground that causes this problem. By simply planting them as deep as you possibly can your plant will thrive. There are two methods for deep planting that you can choose from.
The Science Behind Deep Planting
Burying your tomato plants deep into the soil helps them grow better because tomatoes form roots all along any buried portion of the stem—if you look closely you will see tiny bumps, which are the roots before they develop. These are called adventitious roots, meaning these roots form on the upper part of the plant—the stem, leaves, branches (but just the stem for the tomato plant)—instead of the main root ball. More roots equate to your plants having the increased ability to take in water and nutrients, which means you will end up with a healthier plant that is less susceptible to drought.
The Deep Hole Method
The most basic way to plant the tomato seedling is to dig a hole deep enough that when the plant is placed in the hole, the soil line reaches just below the top-most set of leaves. However, if it is too difficult to dig this deeply, you can also bury the tomato plants up to just below their bottom-most set of healthy leaves. The advantage is that there may be more water farther down in the soil and thus the roots will reach the water immediately and more easily.
Simply dig the hole as deep as you need it to be to accommodate the rootball and most of the stem. Loosen the roots a little and then drop the seedling in the hole and gently backfill with soil.
The Trench Method
The second technique is digging a trench and laying the seedling on its side. There are two advantages of doing it this way: it is easier than digging a deep hole, and the soil is warmer toward the top which may cause the plant to grow quickly from the start. This method may work best with a 6- to 10-inch seedling.
Dig a six-inch-deep trench that is the length of the rootball plus the length of how much of the stem you want to bury. So if your seedling is eight inches tall and you want to leave two inches of the stem above the soil, then you need to dig a trench that is six inches long. You can also make the trench at an angle, so the rootball sits deeper into the ground than the top of the plant, but this is completely optional.
Loosen the rootball as usual and lay the plant down in the trench. Backfill with soil, making sure to leave a few sets of leaves uncovered (the amount of stem you decided to expose). You can stake the top of the plant up straight, rather than leaving it on the ground, but you need to be very gentle—losing a few leaves is ok, but snapping the stem will kill the plant. If you don't stake it, don't worry that your tomato plant will end up growing horizontal on the ground—the sun will attract the stem and cause the plant to stand upright eventually.
Deep Planting Tips
Unlike when you place fresh cut flowers in water, you do not need to remove the leaves that are submerged, in this case under the soil. Taking off the leaves will risk snapping the stem. You also want to be careful when inserting the cage or stake into the garden, especially when using the trench method—make sure you don't place it on top of the rootball, remembering it is to the side and not straight down below. And of course, right after planting, no matter which method you used, make sure to water well.
Just a little heads up—because deep planting causes the roots to grow stronger, you may have a more difficult time pulling out the plants at the end of the season. But the healthy, more productive tomato plant will be worth it!