Gardeners use two methods to plant tomatoes: planting in a vertical hole and planting in a trench. Both methods are successful because tomato plants will develop roots along any part of their stem that touches the soil. Always remove tomato seedlings from plastic pots. You may plant seedlings pot and all as long as the pot is biodegradable. Completely bury biodegradable pots under the soil as any exposed pots (particularly peat pots) release water back into the air.
When to Plant Outside
Plant tomato seedlings outside when soil temperatures are above 60 degrees F. Wait until your seedlings reach 6 to 10 inches tall before planting outside.
Hardening Off Seedlings
Harden off your tomato seedlings to reduce transplant shock. Transplant shock is just that; the plant goes into shock from the stress caused by the transition of indoor versus outdoor growing conditions. Place your tomato seedlings in a bright spot out of direct sun and wind. Increase the time your seedlings spend outdoors gradually over 1 to 2 weeks.
Dig a hole deep enough to place the tomato seedling's bottom leaves at soil level. Remove leaves from the bottom 1/3 of the seedling to prevent soil-borne diseases from attacking the lower leaves.
Plant "leggy" tomato seedlings (seedlings taller than 10 inches) in a trench. Dig a trench at a 30 degree angle that is long enough to expose only the top 5 to 6 inches of the tomato plant and deep enough to completely cover the stem. Remove the leaves from the portion of the seedling buried in the trench.