If you find yourself with spindly, leggy tomato plants that don't look as strong and hearty as you would like, it may be how you are putting the tomato plant in the ground that causes this problem. By planting them as deep as you possibly can your plant will thrive. There are two methods for deep planting to 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—instead of the main root ball. With tomato plants, this type of root develops only along the stem. More roots means your plants have increased ability to take in water and nutrients, resulting in a healthier plant that is less susceptible to drought, disease and attack by tomato pests.
The Deep Hole Method
The most basic way to plant the tomato seedling is to dig a hole deep enough so that the entire plant is buried up to the top-most set of leaves. However, if your plants have already reached a pretty good height it may be difficult to dig this deeply. In this case 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. You can test fit the seedling in the hole before dropping it in. 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 to dig a trench and lay the seedling on its side. This method requires a little more finesse but there are some advantages to trenching. It is easier to dig one long shallow hole than it is to dig a deep hole, and the soil is warmer toward ground level which can encourage the plant to grow quickly from the start. This method works 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 stem you want to bury. 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 angle the trench so the rootball sits deeper into the ground than the top of the plant. This might make it easier to train the exposed part of the plant to grow straight up.
Loosen the rootball as usual and lay the plant down in the trench. Backfill with soil, making sure to leave some stem with a few sets of leaves uncovered. 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. It is always a good idea to get your tomato stakes in early. Tomatoes are vulnerable to many soil borne diseases so you want to keep the exposed leaves up off the soil as much as possible.
Deep Planting Tips
Although it isn't absolutely necessary, it is a good idea to remove leaves that you plan to bury. Do this very carefully, gently using your fingers to pinch them off as close to the stem as possible. It is easy to damage and break the stems, especially with small seedlings. You also want to be careful when adding a cage or stake, 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. No matter which method you used, make sure to water right after planting.