I used to exclusively use the paper towel method for germinating seeds. Now I am trying soil blocks (I found some seeds germinate better if I don't cover them).

And I was wondering when should I take the cube out of my "incubator", transplant them and start exposing them to light.

With the paper towel method, as soon as I see the radicle, I move them to a little 330c pot, cover them just enough, and leave them under lights/sun. But with the soil blocks, I am not sure what would be better.

Should I wait until cotyledons show up (or the seed completely "emerge"), or should I do it as soon as I see the radicle digging into the cube (covering with soil before doing it to protect the roots)?

  • Usually, seeds sown in soil blocks are allowed to start growing as seedlings until they have 2 or 3 sets of true leaves - depending on the type of seedling, at that stage they may be moved into pots, or may be left a bit longer to form more roots. Light exposure is necessary, but more info please - do you mean a heated container by 'incubator', is that in daylight or artificial light or in the dark, what kind of seeds, how many seeds did you put to a block of soil, what sort of soil block...
    – Bamboo
    Aug 28, 2019 at 10:27
  • 3/4 inches blocks, with mostly tomatoes, eggplants and peppers. The "incubator" is a closed (so in the dark) styrofoam box with a heater, a temperature sensor, and some electronics that control the on/off of the heater to keep temperature at whatever I set it (currently at 28C).
    – csamx
    Aug 28, 2019 at 14:14

1 Answer 1


The trouble is, you can't tell when the seeds germinate in the soil blocks until green shows above the soil surface; in order for green growth to happen, light is essential, or any growth will not be green. If you can tell they've germinated, then they need to be moved out of the dark immediately, but they also need to be kept warm, otherwise the temperature difference might kill them, so when you know they've germinated, proceed as you do for the tissue method. Because they're in soil blocks, you can wait till each seedling has a pair of true leaves, then pop the soil block into a small pot with potting soil.

  • Hmm. Seeds germinate independent of light. For vegetable gardening seedling, it is often recommended to put a paper above them, to keep them dark (and so less problems drying the pots, and less snails). The energy is given by seed, not by cotyledons. [It is the temperature which tell seed to germinate]. OTOH seedling direct in the field doesn't allow us to put light blockers, and seed germinate anyway. Aug 29, 2019 at 8:49
  • 1
    I've clarified the answer a bit...
    – Bamboo
    Aug 29, 2019 at 9:59
  • Ok, thank you! I had some difficulty on your first sentence on previous version Aug 29, 2019 at 10:04
  • when I reread it after your comment, I could see why... it was not clear.
    – Bamboo
    Aug 29, 2019 at 10:06

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