Potting up is the process of re-planting tomato seedlings that are not yet ready for the garden into larger pots. Potting up (also called potting on) allows plants to continue their steady growth until they are strong enough to withstand outdoor growing conditions.
Potting up allows for the development of strong roots and because a tomato plant can be re-potted up to its uppermost leaves, it also prevents plants from growing too leggy and accidentally snapping when moved.
Tomato seed are commonly started indoors 6 to 8 weeks before they are transplanted into the garden.
Tomato seedlings are usually potted up about 10 days after they have sprouted—potted up from a seed starting tray into a small pot, and again in another 10 days when they are about 6 to 8 inches tall—from a small pot, 4-inch container, or quart container. At potting up time, multiple plants in a container should be thinned to the strongest single plant for growing on.
Plants in quart containers can be potted up a second or third time into half-gallon or gallon containers if air and soil temperatures in the garden are not yet warm enough for tomato plants. (Tomato plants grow best when the soil and air temperatures are 65°F or greater.)
Steps for Potting Up
1. Water the plants in advance and allow the soil to drain.
2. Place soil mix in the bottom of the new pot; tap the pot a couple of times to settle the soil mix.
3. Snap off any leaves along the stem except for the top two sets. Simply start at soil level and work your way up the stem using your thumb and forefinger to pinch or snap away any small branches or leaves. Tomatoes grow rapidly; pinching early on will allow the plant to direct its energy to root development and away from top growth.
4. Re-pot (transplant) the tomato to the larger container that will hold the plant from the bottom roots all the way to just below the topmost leaves. (A quart milk carton will work well if you are potting up from a 4-inch pot.)
5. Fill the new container with soil mix and firm the soil in so that the root clump and stem make good contact with the soil mix. Give the soil a gentle watering so that the soil mix is moist but not soggy.
6. Place the plant back in a bright window or greenhouse. Shelter the plant from direct sun or a day or two. As plant grow in breadth, move the pots apart so that there is good air circulation and little opportunity for disease to set in.
New roots will develop along the now buried stem where the leaves once grew.