What Plants to Start in September as Seedlings in Arizona

Not everything in September has to start from a tiny seed in the ground. Around San Tan Valley I also buy or transplant sturdy seedlings so the cool-season garden gets a head start. Brassicas and leafy greens settle in faster as transplants, and garlic goes in as cloves rather than seed.

The key is matching plant type to our Zone 9b calendar. September gives transplants time to root before true winter chill arrives, while still beating the worst of summer. I look for stocky seedlings with healthy roots and no signs of heat stress on the leaves.

Best seedlings and starts for September

• Broccoli, cabbage, cauliflower, and other brassicas from nursery six-packs or home trays.

• Lettuce mixes and Swiss chard for steady leafy harvests.

• Onion transplants for fall planting into winter growth.

• Garlic cloves planted now for a late-spring harvest.

• Culinary herbs such as parsley, cilantro starts, and hardy perennial herbs if you need fillers.

Harden off and protect from residual heat

Even nursery plants need a soft landing. I harden seedlings outdoors for several days, increasing sun exposure gradually. For the first week or two in the bed, light shade cloth helps while roots catch up. Water deeply at planting, then keep soil evenly moist without drowning the crown.

Space plants for mature size and mulch around them once they are settled. Afternoon shade from a nearby fence or temporary cloth can make the difference between a transplant that stalls and one that takes off. Watch for cabbage loopers as brassicas grow, and stay consistent with watering through warm September afternoons.

Starting seedlings in September is how I bridge the gap between summer survival mode and a productive cool-season garden. Choose solid transplants, ease them into the sun, and give them shade and water until they look settled. That care pays off all winter long.