Zones:

September is the month the calendar and the thermometer finally agree. In most parts of the country, the heat is beginning to break, and the days are noticeably shorter — two signals the garden has been waiting for. In cold winter zones, September is a sprint to the finish: a few seeds can still go in the ground, but the window is closing fast, and the emphasis shifts to transplanting and harvesting. For mild-winter zones, September is prime planting season — the single best month to get cool-season crops in the ground for fall. And for warm-winter zones, September is the real beginning of the gardening year: heat is easing, transplants are going out, and the prime growing season is just getting started.

Cold Winter Zones (3–7): The direct-sow window is nearly closed, but it is not gone. Radishes are the last reliable bet — fast enough to mature before October's frost if you sow them in the first week of September. Brassica transplants started in July and August are the main event now: get them in the ground while there are still 6–8 weeks of growing weather ahead—if you have not ordered yet, order today.

Mild Winter Zones (8–9a): September is your month. Cool-season crops of every kind — leafy greens, root vegetables, brassicas, alliums — go in now. August indoor starts are hardening off and moving outside. Short-day onion transplants are going in. The fall garden is assembling itself in real time, and the seed orders that support it need to happen now.

Warm Winter Zones (9b–12): The turning point has arrived. September is when the fall and winter garden takes physical shape in warm-winter zones. Brassica transplants started in August and will go outside as temperatures ease. Direct sowing of cool-season crops is broadly open. The fall season that defines gardening here — the one that runs from October through March — depends on seeds ordered and planted this month.