As soon as the roots had grown through the plug – about 9 days after planting the seed – I transferred the little plant to its final destination, a huge self-watering pot by Lechuza. The top layer is not the super soil from love_to_grow, as it is very rich in nutrients and probably too "hot" for young plants. I used some standard soil made for growing herbs, which contains few nutrients. Before planting the plug, I sprinkled "Living Organics" from Green Buzz Nutrients to promote root growth.
The plant looks quite lost in its huge pot! But hopefully, that this will change soon.