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Potting up Tomato Seedlings: Tips for Success

Generally speaking tomato seedlings are ready to be planted outside in the garden about 6 weeks following germination. It is a huge benefit to later success of healthy tomato plants if seedlings are “potted up” from their original small cells into slightly larger pots before planting outside. Potting up tomato seedlings is best done around the 4-week stage. Click here for detailed information on how to get seeds to germinate..

4-week heirloom tomatoes still in flat
4-week heirloom tomatoes still in flat
Tomato seedlings at 4 weeks after being potted up
Tomato seedlings at 4 weeks after being potted up

Tips for Potting up Tomato Seedlings:

  1. Although the 4-week timeline for potting up tomato seedlings is a good benchmark, another sign that they are ready is when they have two sets of true leaves (do not count the seed leaves at the bottom).
  2. Handle the seedlings by the leaves rather than the stems. If you tear a leaf, the plant will still grow. If you break the tender stem, the plant is ready for the compost pile.
  3. I grow my initial heirloom tomato plants in 196-cell trays, and when it is time to pot up I take a butter knife and gently pop the plant out of the cell with the soil bundle (aka a plug) intact.  Have a 4″ pot ready with moist potting soil and make a hole with your finger in the center of the soil in the 4″ pot.  Place the seedling plug into this hole and gently press the soil around it to make contact with the roots.
  4. If you grow many seedlings in one large container rather than in cells, you will need to tease apart the roots from each seedling and then place in the hole of the 4″ container.  Alternatively, you can snip off the weaker seedlings at the soil line and leave the strongest plant in the container to take advantage of the nutrients in the remaining soil.  Remember…don’t handle the seedlings by the stem!
  5. Newly potted up heirloom tomato plants may look limp and stressed the first day or two.  Don’t fret and do anything drastic like fertilizing them.  They will recover with a couple days rest in the same environment they were in prior to potting up.  Keep them out of bright sunlight for a couple of days.
  6. When potting the heirloom tomato plants up to larger pots, plant them a bit deeper than they were in the cell or original container.  Additional roots will form along the buried stem and give you a more vigorous plant.  You can cover the seed leaves and plant right up to the lower set of true leaves.
  7. Use the same potting soil that you used to start your seeds…not garden soil.
  8. Water the tomato seedlings in their cells or container well BEFORE you start to pot up.  Moist soil will cling to the roots and protect them from drying out.
  9. Depending on the weather and when you are going to put the heirloom tomato plants in their permanent bed, you may want to pot up a second time.  A good rule of thumb when deciding when to do the second transplant is to wait until the height of the seedling is three times the diameter of its pot (probably around 6-10″ tall).
5-week old pepper plants
5-week old pepper plants

Trouble Shooting Problems

If your heirloom tomato seedlings are getting tall and spindly, it may be related to one of the following:

  • the light source may be too weak or too far away from the growing tip;
  • the room temperature may be too warm (I keep my daytime temperature around 70 degrees and the night temperature around 50 degrees; or
  • you are using too much fertilizer.  Just use potting soil that already has fertilizer in it or use potting soil with compost.  Wait until they are outside before getting more generous with fertilizer.
April 19, 2013 in Forest Lake, MN
April 19, 2013 in Forest Lake, MN

This is the longest, coldest winter we have had here in Minnesota, and while my heirloom tomato plants and pepper seedlings are currently quite happy in their protected environment, they will be hurting soon if we can’t get them hardened off.  It is April 19th and 4″ of new snow on the ground, and still snowing!!  Ugh.  I haven’t given up hope that we will have a long warm summer, but spring is looking pretty doubtful.  As a farmer, you do learn to roll with the punches, but I have to admit this is getting pretty depressing.

3 Comments

  1. Kristen on January 22, 2016 at 2:59 pm

    I realize this is an old post and might no longer warrant a response but I figure I’ll try 😉

    Could you be more specific as to what you initially plant the tomatoes in? How deep are the 196 cell trays? And then 4″ refers to diameter or depth of the larger pots?

    Sorry, I am new to growing from seed and there are so many options in a little confused.

    Thanks!

    • Dorothy Stainbrook on May 5, 2016 at 6:59 pm

      The plug trays come in different sizes but the depth doesn’t vary much. I plant mine in trays that have 128 plugs. I think they’re probably a couple inches deep. The larger 4″ pots — yes, that is the diameter. Thanks for asking and sorry it took so long to reply.

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