Our tomato seedlings grew to be massively tall in the greenhouse this year. There are a few approaches to how to plant such tall creatures! Tomatoes grow roots off their stems (called adventitious roots). We usually try to put the tomato plants in up to half their stem in soil. This may not be possible for you to dig a foot and a half hole! So you can either bury them six inches or more in the ground and make sure you add a stake or trellis right away. Or you can make a horizontal trench at least 6 inches down and bury the root ball and most of the stem, curving the upper part up so that a foot or so of the top of the plant is straight up above soil level. These plants would need a fair bit of water right away to encourage the new root growth. But having all that root surface is sure to make for a tastier tomato!