Pippi, I tried finding an answer to your question. First, I have to ask you a question. Do the tubers have roots or any growth? Toni

I have 3 JMs in containers and I have a stepping stone under each one. Has worked so far, going on 5 years. JM roots aren't real agressive so that one must have been in its location a long time. I think JM's look best in short but fat containers (also helps keep them from blowing over if the wind gets up). I just use potting soil and add a little compost to it. I use green moss as a "mulch" on top of the the soil and the wrens just love that. They steal the moss to line their nests.

Use the brand you prefer. Some bags will say "for seed sowing" or something similar on them. Stay away from bags of soil that say weed-free as they may contain chemicals to prevent germination of weed seeds in that soil. Those chemicals will also prevent the germination of sown seeds. You may reuse potting soil or seed sowing mix after it has been sterilized. Put the sowing medium into a flat and dowse it thoroughly with boiling water. Allow to cool to room temperature before sowing your seeds.

Any type at all will work. Mixes with soil for potting, or "soilless mixes" all work fine. Avoid any soil mixes that say "weed-free" because they can contain an "anti-germinant". This is a chemical which is mixed into the soil to prevent any weed seeds the soil might contain from germinating. Unfortunately it also keeps ALL seeds from germinating. What's MY favorite? I like the cheapest one I can find, lol! I do a LOT of Winter Sowing...it's fun and is very addictive. So I look for the biggest and largest bag of soil I can find. It's a very economical choice to get the biggest bag because it costs less per pound or quart. Plus, with more soil, I can sow more flats! Trudi