grows inside. When it looks big enough, I find that they are ready to pick. I only had success one time with a tomatillo plant (altho' I have another in the backyard right now) and I had no idea when things were ready to harvest. But I found out quickly. I watched the husk part grow and then the tomatillo itself started growing inside the husk. You can feel if it is ready - if the tomatillo itself fills up the husk, and it seems firm, then it is ready to pick. It might take some practice figuring it out. Good luck. RisaG (hoping this years Tomatillo plant doesn't get the weird bugs the last plant got)