My opinion, I don't know but.... I thought about doing this earlier, I did once grow slips in water but at the wrong time so I didn't try to plant them.
About three weeks ago I decided to have a go and bought some.
Looked on the net, saw the water method, and saw the peat method ,I chose the latter. I have just had a poke in the earth and the one I dug up is covered in roots about 3 inches long.
I bought a little bag at Tesco's which is all they had, tubers were small, when I looked in my fridge I found 2 large ones I had bought for the kitchen the previous week and forgotten about. I planted them all,it will be interesting to see if there is a difference.
Anyway they are coming up.
Plastic clear tub of barely damp potting soil, tuber buried about 4 inches down on their sides, clear plastic lid on and they went in the greenhouse.
XX Jeannine