Most people don't realize that the loofah sponge in their shower is actually a plant — and not just any plant. Luffa is a gourd, in the same family as cucumbers and squash. It grows on a vine, produces fruit, and when you let that fruit dry down completely, what's left inside is a natural fibrous sponge.
Growing luffa does require a long warm season, so if you're in a cooler climate you'll want to start your seeds indoors early. They love full sun, need something sturdy to climb, and are pretty low maintenance once they get going.
When the skin turns brown and papery and the gourd feels light, it's ready. Peel the skin back, shake out the seeds, rinse it well, and let it dry completely in the sun. That's it. You've got a sponge that's completely natural and fully compostable when it's done.
Save your seeds every year and you'll never have to buy them again.