September is the best planting month of the year in mild-winter zones. The worst of the heat is behind you, soil temperatures are beginning to drop, and cool-season crops have exactly the conditions they want. The August indoor starts are ready to harden off and go outside. Short-day onion transplants are going in. And direct sowing opens up broadly — greens, roots, brassicas, and alliums all have a genuine window. If you only plant once in the fall, plant now.
Indoors (fresh harvests):
- Microgreens: peas, broccoli, sunflowers, arugula
- Sprouts: alfalfa, broccoli, radish, mung
- Herbs & greens under lights: chives, parsley, mint
Transplants to move outdoors (harden first): Broccoli, cauliflower, cabbage, Brussels sprouts, kale, and kohlrabi started in August are ready to harden off and go outside once daytime highs drop consistently below 90°F. Pansy, viola, and parsley transplants also go out this month.
Short-day onion transplants: Short-day bulbing onions started indoors in August can begin going out in September. These need a long in-ground season to develop full bulbs — every week of establishment now pays off at harvest.
Crops that can be directly sown outdoors in September: Spinach, kale, Swiss chard, collards (early September); lettuce, pak choi, kohlrabi, turnip, cilantro, dill, corn salad (mid-September); arugula, mustard, radish (through late September). Carrots and bunching onions can go in early September as well. For spinach sown while the soil is still warm, pre-germinate seeds indoors in a cool spot and sow before the radicle fully develops.
Cover crops: Oats, field peas, crimson clover — good choices for beds coming out of summer production.
Tip: An Olympia, WA zone 8 is very different from a Jackson, MS zone 8. Consider checking with your state Extension Service office for more specific recommendations and guidelines.
Use your USDA zone as a guide. Microclimates matter — coastal/urban spots run warmer; high-elevation spots run colder.
