I didn't know sunflowers basically face east and I planted them in a location where they're all standing with their backs to us, smiling cheerfully at our neighbors when they're out on their deck.
I learned that pennyroyal does NOT make a good ground cover for a small area beside a border full of lovely pebbles... I have been out there every few days with my florist's shears snipping and snipping and snipping. And snipping and snipping and snipping. We paid waaaaay too much money for those pebbles to lose them to the pennyroyal. Now I have to dig it all up and plant it somewhere else.
I learned that when a seed packet gives directions for planting it's probably a good idea to follow those directions rather than thinking I know better. Not one poppy came up, only one lavender plant made it out of 3 or 4 packets of seeds, I've been secretly hoping I would see some pyrethrum daisies, but I think I'm gonna have to give up the ghost. I will have to plant them all over again next spring, and this time when they say cover them with plastic wrap I'm going to do that!
I learned that you can't plant seeds and expect to see flowers or pick veggies in a few days, not even a few weeks. It's been over 2 months now and I'm still waiting. When that wise man said "nature never hurries" he wasn't kidding!
I could go on but it's probably best if I stop right here.