Seedling pot made from an empty toilet paper roll. After germination, it can be buried directly in the garden bed as is; the roots will grow through the softened cardboard, and soil organisms will then consume it. Make sure to only use toilet paper rolls made from unchlorinated and unprinted cardboard.
edit: please check if recycling has a problem with chemicals or heavy metals in your country
not in germany, there are not a "lot" of bad chemicals in recycling paper, since the limit values are very strict. however i don't know how other countries handle this. i will put an edit in the original post.