September in Zone 5 is the month of the great turn: summer crops wind down, fall crops take over, and garlic goes into the ground for next year. The average first frost of Oct 1–15 is only weeks away, so September work is about harvesting well and setting up the garden for winter.
What September means in Zone 5
Zone 5 winters are genuinely cold, with average annual minimum temperatures of -20 to -10°F, and the growing season is short and precious. Every week counts, so May gardeners work fast once the last frost passes. The frost-free season lasts about five months, from early May into early October. The average last spring frost falls in the Apr 15–May 1 range and the average first autumn frost in the Oct 1–15 range — USDA zone averages, not exact dates, so check your local extension service before planting tender crops outside. Our planting calendar by ZIP turns them into a month-by-month plan.
With a frost-free season that ends in early October, Zone 5 September is a race against the calendar. Tomatoes and peppers are finishing, potatoes and winter squash are ready to dig, and the fall greens sown in August are growing fast in the cooling days. September is also planting month for garlic — the one crop you plant now and harvest next July — and for cover crops that will feed the soil all winter.
September task list
- Dig potatoes and harvest winter squash and pumpkins; cure squash in a warm, dry place for 10–14 days. 2. Plant garlic in mid-to-late September. 3. Sow cover crops on empty beds. 4. Pick green tomatoes before the first frost and ripen them indoors. 5. Save seeds from open-pollinated beans, tomatoes and squash. 6. Clean up diseased plants — do not compost them.
What to start indoors now
Very little is started indoors in September, but there are two exceptions. Start lettuce and spinach in trays indoors for transplanting into a cold frame, where they will grow through October. And start herbs like parsley, chives and mint in pots for the kitchen windowsill — they will give you fresh herbs all winter indoors. Everything else waits for spring. Use clean pots and fresh seed-starting mix, give seedlings strong light, and keep the soil moist but never waterlogged. Work out exact sowing dates with the seed-starting calculator.
What to direct-sow outdoors
Direct sow cover crops now: winter rye, crimson clover or field peas sown in September will cover bare beds through winter, suppress weeds and add organic matter when dug in next spring. Sow spinach (40–50 days) and radishes (25–40 days) one last time in early September under row cover for a late-October harvest. And plant garlic — the month's most important sowing — in mid-to-late September, 6 inches apart and 2–3 inches deep, with the pointed end up. Prepare the bed first — fork it over, remove weeds, rake it level — then sow thinly, water gently, and thin seedlings to the spacing in the table below.
What to transplant now
Transplant any remaining fall lettuce and spinach seedlings into the cold frame or under row cover in early September. This is the last transplanting of the year in Zone 5 — anything set out after mid-September will not have time to establish before cold weather. Move tender potted herbs indoors before the first frost, and bring in any houseplants that spent the summer outside. Harden seedlings off for a week first, plant on a cloudy day or in the evening, water them in well, and mulch around — but not against — the stems.
What to harvest this month
September harvests are heavy and satisfying. Dig potatoes (70–120 days) once the tops die back, and harvest winter squash (90–110 days) and pumpkins (90–120 days) when the skins are hard enough that a fingernail does not dent them. Pick the last tomatoes — green ones will ripen indoors in a paper bag with a banana. Harvest onions, beets and carrots, and keep picking fall lettuce, spinach and kale as they grow. Harvest in the morning, when plants are cool and crisp, and pick little and often — regular picking keeps most vegetables producing.
Frost protection in Zone 5
The average first frost of Oct 1–15 is imminent, so September is the month to have frost cloth and row cover ready. A light frost will kill basil, tomatoes and peppers but only sweetens kale, Brussels sprouts and carrots — many Zone 5 gardeners leave root crops in the ground under mulch well into October. Cover tender crops on frosty nights to stretch the harvest, and harvest everything frost-tender before a forecast hard freeze. When a cold night is forecast, water the soil in the afternoon, cover tender plants with frost cloth before sunset, and remove the covers in the morning. The Harvest Date Calculator can help you time plantings around your frost dates.
Planting data for September in Zone 5
All figures below are broad published ranges from standard horticultural references, not exact figures — they vary by variety, climate and soil.
| Crop | Action this month | Days to maturity | Notes |
|---|---|---|---|
| Garlic | Plant now | 180–240 | Plant mid–late Sep; harvest next July |
| Potatoes | Harvest | 70–120 | Dig once tops die back; cure before storing |
| Winter squash | Harvest + cure | 90–110 | Cure 10–14 days in warm, dry place |
| Spinach | Last sowing under cover | 40–50 | Row cover extends harvest into October |
| Kale | Harvest | 50–65 | Frost sweetens the leaves |
| Cover crops | Sow on empty beds | — | Winter rye or clover feeds soil all winter |
Browse the full planting calendar hub for every Zone 5 month, and see the neighbouring months in this zone:
Frequently asked questions
When do I plant garlic in Zone 5?
Mid-to-late September, a few weeks before the ground freezes. Plant cloves 6 inches apart and 2–3 inches deep, pointed end up, and mulch heavily. Garlic needs 180–240 days and is harvested the following July.
What should I harvest in September in Zone 5?
Potatoes, winter squash, pumpkins, beets, carrots, onions, the last tomatoes and peppers, and fall greens like kale, spinach and lettuce. Cure squash for 10–14 days before storing.
Can I still plant anything in September in Zone 5?
Yes — garlic, cover crops, and a last sowing of spinach and radishes under row cover in early September. Tender summer crops are done; September planting is all about fall and next spring.
When is the first frost in Zone 5?
The average first autumn frost falls between Oct 1 and Oct 15, but these are zone averages — check your local extension service and the forecast. A light frost sweetens kale and carrots but kills tomatoes and basil.
How do I ripen green tomatoes indoors?
Pick them before the first frost, put them in a paper bag with a banana or apple, and keep the bag at room temperature. Check every few days and use the tomatoes as they ripen.