Jump-Starting Tomato Plants

Jump-Starting Tomato Plants

Mid-March is the time to start tomato plants from seed if you’re growing them inside under lights. Some of our customers do this,while others want to jump-start their tomatoes by buying seedlings later in April and then growing them on indoors until it’s time to plant them outside in late-May. Here are some tips for starting your tomatoes inside, and growing them on whether you’ve grown them from seed or purchased seedlings.

  1.  Start your tomato seeds in new, fresh potting soil or seed-starting mix. Espoma and Coast of Maine are the two brand names I’d recommend, and both make good organic seed-starting and regular growing mixes. (This year I used Coast of Maine’s Stonington Blend. It is their premium potting mix and I’ve had great success with it.)
  2. Get the potting soil or seed starting mix wet before you fill the containers you’re starting in. Know that the smaller the containers/pots/cells, the sooner you’ll need to transplant your seedlings into larger pots. After planting the seeds, cover with a clear plastic lid or food wrap. Check every day after 7 days for germination and remove the plastic when the seedlings start poking up. It typically takes about 7 to 12 days for tomatoes to germinate.
  3. When starting tomato plants inside in late-March or early April, plan to have them under lights unless you have a greenhouse or a south-facing slider where the sun isn’t blocked by nearby trees or buildings. More light is better, although the plants do need from 10 to 12 hours of darkness each day as well.
  4. Keep young plants about 3″ from the lights. Once the plants get two sets of true leaves (see photo below) set up a fan on a timer so that the plants get blown for a couple of hours a day. This helps create stronger plants because you are mimicking the natural conditions of wind outside.

    This photo shows a group of tomato plant that have seed leaves, one large set of true leaves, and another newer set of true leaves growing at the top of each seedling.

    5. Don’t start to fertilize plants too early indoors – you will create taller, weaker plants. Wait until the young tomatoes have at least two or three sets of true leaves, and fertilize mildly. If you’ve used a good potting soil you can even wait until early May to start fertilizing.

    6. If you buy tomato seedlings in April, transplant immediately to 6″ pots. (Pots that are 6″ in diameter across the top.) Do not block the drainage holes and do not put rocks, shards or other materials in the pots; fill the pots with planting mix only.

    See how close the plants are to the lights? Make a way to raise the lights as the plants grow, keeping them 3″ above the plants. Lights should be on for 12 to 14 hours a day.

    7. If you have plants in a sunny window, rotate them every few days so that they don’t lean in one direction.

    8. Let the surface of the soil dry out a bit before watering, but don’t let the entire root ball dry. Soil that is too wet will grow algae on the top and can lead to root rot and “damping off” fungus as well.

    These tomatoes were started in small pots. Once they have 3 or 4 sets of true leaves I will transplant them into 6″ pots. Remember that with every plant, what goes on below the soil surface will be reflected up above…so a tomato that has enough root room to stretch out in the pot will create a stronger, larger plant up above.


    Introduce your indoor-grown tomatoes to the “real world” gradually. Put them out in a shady spot at first, waiting for a stretch of mild days. Look at long-range forecasts and don’t put tomatoes outside until the night-time temperatures will be reliably above 50 degrees.

    Don’t have the space for lights or enough sun to start tomatoes indoors? Not to worry…beginning in mid-May we will have a greenhouse filled with tomato plants of all sizes. We typically get in hybrid and heirloom plants in 4″ and 6″ pots that are garden-ready. Some varieties sell out quickly, however, so call if you’ve got your heart set on a particular type of tomato.

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