If you live in an area that freezes, you will need to harvest your tomatillos prior to the first hard frost of the year. This will vary by geographical area. In cold weather zones, the growing season can be very short and it may be necessary to start tomatillo plants indoors in early spring to have sufficient time for the plants to grow and for fruit to develop. Another option is to purchase started plants once the last frost date is past. I live in USDA zone 5, and planted tomatillos from seed in the middle of March, growing them in peat pots indoors until our last frost date was past. Our last frost date is in late May, so the plants had a significant start prior to planting. We have harvested several batches of ripe tomatillos from our plants and have many more in development. Fortunately, this particular September has had no frost dates for us, so I will continue to harvest until the killing frost arrives. I would watch your forecast and wait to harvest until you see a frost warning posted.