Instead of buying those biodegradable seedling pots from the hardware store, make your own by starting the seeds in an egg shell. Granny Miller has a how-to on starting seedling indoors, including a guide on turning an egg shell into a pot for seedlings.
To make the pot, she cracked the egg high so there was plenty of space to plant, poked a nail through the bottom to let water out, and added dirt and seeds. Then she stored the eggs in a clear plastic eggshell holder.
When it’s time to plant, all you have to do is pop the eggshell in the ground. It will break down on its own. The eggshells will also enrich the soil in the process.
ETA: Here is a comment from reader Olive:
This is a cute idea going all over Pinterest but when you put these in the ground you MUST crack the egg enough that the roots can grow thru. Egg shells take forever to decompose and the poor little plant will die before that ever happens.
She’s right. Eggs are always the last thing to break down in my compost, so make sure there are a few cracks in the egg before putting the plant in the ground.