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
it works really well. i did it several times. the cardboard will get soaked over time until it nearly decomposes by itself. you will need to be careful when lifting the pot to plant it into the garden, but the cardboard wont be a potent barrier by then anymore. the rest will be eaten by earthworms etc. who really love cardboard.