I figured out that you actually need to plant all 10 unique plants. It doesn’t specify that in game.
To my knowledge, the gardening plots unlocking is not based on the amount of plants planted, but the amount of plants discovered and seeds collected. Try to get as many seeds and as many varieties as you can and it should unlock that way.
“With Pili recruited, you can unlock additional garden plots by growing your plants to full size.”
I’m having the same problem, I’ve even replanted several times and still the final plot won’t open. I even have the “little helper” on the roof. (Don’t want to say who for spoilers)
I figured out that you actually need to plant all 10 unique plants. It doesn’t specify that in game.
