What to Do in the Garden in September (Cool-Climate US)

Plant and transplant

• Plant spring-flowering bulbs from the second week of September through mid-November; Purdue advises starting in late September and allowing 4-6 weeks before the ground freezes.
• Direct-seed fall cover crops in September-October: ryegrass, rye, rapeseed, oats, winter wheat, winter rye; peas and oats sown in September will winter-kill (UMN Extension).
• Plant, transplant, or divide peonies, daylilies, iris, and phlox while they can still root in (Purdue Extension).
• Plant container-grown or balled-and-burlapped trees and shrubs; fall is a good planting window.
• Sow quick crops like radishes, lettuce, and spinach early in the month for fall harvest.

Lawn care

• Finish lawn seeding; the best window runs through late September (UMN lawn care calendar).
• Continue the prime fall windows for fertilizing (through mid-October), aerating, and dethatching.
• Reseed bare spots while soil is still warm.

Harvest and frost protection

• Watch for the season's first frosts: harvest heat-sensitive crops beforehand or cover plants overnight with blankets or newspaper (Purdue Extension).
• Ripen mature green tomatoes indoors at about 55-60 F.
• Harvest winter squash when mature, before a hard frost.
• Dig onions and garlic after the tops fall naturally.
• Dig tender bulbs before frost, and bring houseplants indoors before night temperatures reach about 55 F.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• University of Minnesota Extension — Upper Midwest home garden care schedule — Plant spring bulbs 2nd week of September through mid-November; September cover crop sowings (peas/oats winter-kill; ryegrass, rye, winter wheat overwinter); divide peonies through September
• Purdue University Extension — September garden calendar (Indiana Yard and Garden) — Frost protection and pre-frost harvest, ripening green tomatoes at 55-60 F, spring bulbs from late September with 4-6 weeks before ground freeze, dividing peonies/daylilies/iris/phlox, houseplants in before 55 F nights
• University of Minnesota Extension — Lawn care calendar — Lawn seeding best through late September; fall fertilizing, aerating, dethatching windows

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