Tuberous Begonias from Seed
Preparation of seed starting media
The seeds are very small and can easily fall between cracks of particles such as perlite and long fibered sphagnum moss. You can buy special fine grained propagation mixes, or you can sift a regular propagation media so only the finer particles are left. Normally I discourage this practice as the larger particles provide for better aeration, but we are allowed to make an exception in this case.
Pre-moisten the growing media in a small plastic bag before adding to containers and then water once really well to thoroughly settle the media in the pot. If need be, you can smoothe the surface of the mix with a ruler to ensure it is completely level, but try to avoid squeezing too much air out of the media. Aeration has already been compromised by the removal of larger particles - don't make it worse.
I do recommend that you familiarize yourself with the quality potting and watering techniques before starting. These have been detailed in 3 previous articles.
Light and Temperatures
Begonias should be surfaced sown (see below) as light is required for germination. Germination do not require a lot of light, and keeping the lights on in the room is usually sufficient, but I prefer to place the container under fluorescent grow lights to ensure that the young seedlings receive sufficient light in the early stages as they germinate. This prevents stretching.
Germinate the seeds at soil temperatures of approximately 70 - 80 F. Most reference materials seem to indicate 65-75 F, but seeds germinate faster above 75 F. I would avoid much higher temperatures than that.
Sowing the Small Seeds
The seeds are very small and fragile and can easily be crushed. To avoid stretching of the young seedlings, it is ideal if the seedlings are spaced at least 1/4" apart or wider. If you use celled flats (72, 128, 144, or smaller) then one seed per cell is fine.
In reality, however, it can be extremely difficult to sow seeds individually. When you buy seeds, they typically come in a small glycerin envelope with a few specs of dust at the bottom (no kidding). Trying to sow these seeds typically result in some seeds getting lost in the air on their way to the growing media, and the rest is bunching up on top of each other resulting in very few transplantable seedlings.
To avoid this, you can fill the envelope with a
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