From Bonnie Plants who grow to sell:
Each Bonnie tomato label urges you to plant tomatoes deep, so that a full 2/3 of the plant is underground. That means that if you buy a 10-inch tall plant, all but the top 3 to 4 inches is buried. Why? Because the plant will have a better, stronger root system. Better roots mean better tomatoes.
We know, we know. This goes against everything you've ever heard about "don't plant too deep or you'll kill the plant." Tomatoes break that rule because they actually have the ability to sprout additional roots along the buried stem. These extra roots strengthen the plant so that it can support more fruit and is better able to survive hot weather. (This applies whether you're growing in the ground, in a raised bed, or in a container.)
Here's how to plant your tomato deep in the ground:
Start with great soil. If you're growing in the ground, improve the texture and nutrition of your native soil with compost or Miracle-Gro® Garden Soil for Vegetables and Herbs. When growing tomatoes in containers, fill pots with a premium quality potting mix like Miracle-Gro® Potting Mix, which is lighter and fluffier than in-ground soil. Raised beds need soil that's heavier than potting mix but lighter than in-ground soil; Miracle-Gro® Raised Bed Soil is a good choice.
Dig a hole that is about 2/3 the height of the plant, including the root ball.
Remove the plant from its container (or, if it's in a biodegradable pot, simply remove the label and very bottom of the pot) and place it in the hole.
If you prefer, you can instead lay the plant on its side in trench, provided that it is at least 5 or 6 inches deep when buried and that the ground beneath it isn't hard as a brick. To do this, angle the plant so that the growing tip is above ground.
Fill in the hole or trench with some of the soil you removed. Only the top few inches of the plant will be exposed.
Label the plant to help you remember which variety you're growing.
Water well. Throughout the growing season, continue to water thoroughly whenever the top inch of soil is dry.
Make sure your tomatoes get just the right kind and amount of nutrition by feeding with a top quality plant food like Miracle-Gro® Shake 'n Feed® Tomato, Fruit & Vegetable Plant Food, which contains calcium to help ward off blossom end rot.
Now comes the fun part. Within a couple of months or so, your plants with super roots will delight you with a bountiful harvest of lovely fruit!
I do not plant my tomatoes quite as deep as they do , but I have found after years of doing this that those planted deeper do better over all, especially in dry weather.
Removing as many lower stems as possible has always made keeping the lower plant leaf free to avoid splashed soil disease easier, but I do use Serenade or Sonata regularly.
If there is plant disease in your soil, douse the soil with a bio-fungicide when you plant.
Sadly I have had plants from bucks-up greenhouses that were already diseased, so to not use a fungicide is shooting craps at best.
Never plant in a biodegradable pot as they do not rot away and restrict the roots.
I have pulled plants, when I used to do that, in late summer, and found most of the pot still there so the root could only get out through cracks.
Number 3 compost will work best plus even though the bagged stuff is generally free of disease, to treat with bio-fungicide still is a good idea.
I bought a bag of raised bed soil, that was bad news for anything I planted in it, so you can never be too careful.
