I have a small avocado tree in a pot, & basil I've rooted in water. Both are in a sunny window & seem to be thriving. Can I plant the basil on with the avocado?
It would be much better to pot up the Basil separately, in a small pot - inserting it into the pot with the avocado might damage the avocado roots, and both plants will be competing for root room. The avocado is likely to win that competition, because the Basil plant will be a rooted cutting, and you may lose the Basil, or at the very least, it won't grow on properly into a good sized, useful herb plant. Use new potting soil to pot up the Basil.