What to Plant in September in Pennsylvania

September is arguably the most productive month in the Pennsylvania garden. Summer crops are reaching peak harvest, fall crops planted in July and August are sizing up, and three major tasks all open simultaneously: lawn overseeding, garlic bed preparation, and one last round of cool-season direct sowing before the frost window closes.

Most gardeners are in wind-down mode by September. The gardeners who know better are just getting started on a second season that runs straight through November.

September in Pennsylvania: Two Seasons at Once

September occupies a unique position in the PA garden calendar. For the first two weeks you’re still in full summer-harvest mode — tomatoes, peppers, and winter squash need constant attention. At the same time, the fall planting window has opened, and missing it means no cool-season vegetables in October and November.

What to Direct Sow in September

September opens the best cool-season sowing window of the fall. Soil is still warm (above 50°F), which accelerates germination — and the cooling air temperatures are exactly what lettuce, spinach, and arugula need to stay tender and productive rather than bolting.

Spinach

Spinach is the most cold-hardy leafy green in the PA fall garden. It germinates in soil as cool as 40°F, grows slowly through October frosts, and can overwinter under a cold frame or low tunnel to provide very early spring harvest in March or April. Sow in early September for fall harvest, or mid-September for overwintering.

Sow seeds ½ inch deep, 2 inches apart in rows. Thin to 4–6 inches. According to University of Maryland Extension, spinach planted in September and protected with a row cover or cold frame can survive Pennsylvania winters and resume growth in early spring — one of the most productive low-effort strategies in fall gardening.

Lettuce

Lettuce matures in 45–65 days and tolerates light frost. Sow in early September for harvest in mid-to-late October in zones 6a–7a. Northern zones (5a–5b) should sow by September 5–10 and cover with row cover once temperatures drop consistently below 35°F. Leaf lettuce varieties — Black Seeded Simpson, Red Sails, Oak Leaf — are faster and more forgiving than head types.

Direct sow ⅛ inch deep. Cut-and-come-again harvesting works well for fall plantings where you want as many leaves as possible before frost ends production.

Arugula and Mustard Greens

Arugula is the fastest fall green — baby leaves in 21–30 days, full harvest in 40 days. It handles light frost easily and actually improves in flavor when temperatures drop below 50°F. The peppery bite that some find harsh in summer becomes pleasantly spicy in fall. Sow through the end of September in all PA zones; zone 7a can sow into October.

Mustard greens (30–40 days) are similarly cold-tolerant and produce a high volume of nutritious leaves per square foot. Both crops are excellent for raised beds or containers where you want a quick fall return from recently cleared summer beds.

Radishes

Radishes mature in just 25–30 days, making them the most flexible crop in September. Sow through mid-September in zones 5a–5b, and through late September in zones 6a–7a. Successive sowings every two weeks produce a continuous radish harvest well into October. Don’t overlook daikon radishes (50–60 days) for zones 6a–7a — a late-September daikon sowing produces a November harvest and the roots act as natural tillage in compacted soil.

Kale and Chard

Kale planted in September delivers the best-tasting kale of the year. Frost sweetens kale dramatically — cold temperatures convert stored starches to sugars, transforming a sometimes-bitter leaf into something genuinely sweet and tender. Lacinato (dinosaur kale) and Red Russian kale are the most cold-hardy; standard curly kale is also fine. Kale planted now will produce through November and can survive mild Pennsylvania winters with minimal protection.

Peas — Zones 6b–7a Only

A fall pea planting is ambitious but rewarding in the right zones. Bush sugar snap peas (60 days) planted by September 5–10 in zone 7a will produce in early November. Zone 6b can attempt it with the first week of September only. Inoculate seeds with rhizobium bacteria and choose the fastest-maturing variety available — Oregon Sugar Pod II (60 days) is a reliable choice for tight fall windows.

Transplant Fall Brassicas Outdoors

If you started broccoli, cauliflower, cabbage, or Brussels sprouts indoors in July, early September is transplant time. These seedlings have been growing for 5–7 weeks and should have 4–5 true leaves. Don’t wait — every week of delay means a week less growing time before hard frost.

Harden off seedlings that have been indoors by setting them outside in a shaded, sheltered spot for 3–5 days before full-sun transplanting. Brassicas are prone to transplant shock when moved directly from indoor conditions to full outdoor exposure. Water well at transplant and again 2 days later. Space broccoli and cauliflower 18 inches apart, cabbage 12–18 inches depending on head size.

Floating row cover over newly transplanted fall brassicas serves two purposes: it adds frost protection AND excludes cabbage white butterflies, which are still active in September and will lay eggs on unprotected plants.

Get Ready for Garlic Planting

Garlic gets planted in Pennsylvania between mid-October and early November, but September is when you confirm your seed garlic has arrived and prepare the bed. Hardneck varieties — Music, German Extra Hardy, Chesnok Red, Georgian Crystal — are the right choice for PA winters. Softneck types are bred for milder climates and often winter-kill or produce poorly in zones 5–6.

The Penn State Extension garlic guide recommends amending beds with compost and ensuring good drainage before planting — garlic rots in waterlogged soil. Use September to work 2–3 inches of compost into your planned garlic bed so it integrates before planting season. This is also the time to test soil pH and add lime if needed; garlic prefers 6.0–7.0.

Peak Harvest Month: What to Pick in September

While fall planting is underway, September is simultaneously the biggest harvest month of the year for many PA crops.

Tomatoes and Peppers: Final Push

As night temperatures drop into the 50s, tomato and pepper fruit set slows and eventually stops. The plants are putting all remaining energy into ripening fruit already on the vine. Remove any blossoms that open after September 1 in zones 5a–6a — they won’t have time to produce ripe fruit before frost. This redirects plant energy to ripening existing fruit faster.

When frost is forecast: pick all tomatoes showing any color (even a hint of orange) and ripen indoors on the counter — not in the refrigerator. Cold temperatures destroy tomato flavor compounds. A slightly underripe tomato ripened on a kitchen counter at 65–70°F tastes far better than a refrigerated one.

Winter Squash, Pumpkins, and Potatoes

Winter squash (butternut, acorn, delicata, Hubbard) is ready when the rind resists a fingernail scratch and the stem has dried and corked over. Cure in a warm, ventilated spot (75–85°F) for 10–14 days after harvest to harden the skin and convert starches to sugars before long-term storage. Cured winter squash stores for months.

Potatoes should be harvested by mid-to-late September at the latest in most PA zones. Once tops die back, potatoes can be attacked by soil-borne diseases if left in the ground too long. Cure potatoes at 50–60°F with high humidity for 1–2 weeks before storing in a cool (40°F), dark location. Research from UMass Extension shows that proper curing heals skin cuts and dramatically extends storage life through winter.

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September Lawn: The Prime Seeding Window

September 1–20 is the best time to seed a lawn in Pennsylvania — by a wide margin. Soil is warm from summer (perfect for germination), air temperatures are cooling (reduces stress on new seedlings), and fall rains are typically more reliable than spring. This is also the window when weed pressure drops sharply as crabgrass and other summer annuals begin to die off.

Overseeding vs. Full Renovation

If your lawn has thin spots but reasonable turf density, overseed: mow short (2–2.5 inches), dethatch if thatch exceeds ½ inch, core aerate, then apply seed at 3–4 lbs per 1,000 sq ft. If more than 50% of the lawn area is bare or weedy, consider full renovation: kill existing vegetation, loosen the surface, grade, then seed at 5–7 lbs per 1,000 sq ft.

Pennsylvania’s best lawn grasses for overseeding: tall fescue (best all-around — deep roots, good drought tolerance, disease resistant), Kentucky bluegrass (beautiful but needs supplemental irrigation), and perennial ryegrass (fast germination, excellent for mixing with bluegrass or fescue). A tall fescue/Kentucky bluegrass mix is the standard recommendation for PA lawns.

The Lawn Seeding Deadline

New grass seedlings need a minimum of 6 weeks of growth before the first killing frost to develop enough root mass to survive winter. That puts the absolute latest seeding date around October 1–5 for zone 5a, October 10–15 for zone 6a, and October 20 for zone 7a. Earlier is better — September 1–15 is ideal in all PA zones.

September Garden Maintenance

Clean Up Diseased Material Now

As summer crops start to decline, remove diseased foliage aggressively. Late blight on tomatoes, powdery mildew on squash, and fungal issues on other plants get worse in September’s cooler, damper conditions. Don’t compost diseased material — bag and discard it. Clearing plant debris in fall dramatically reduces fungal disease spore loads that overwinter in the soil for next year.

Soil Amendment and Cover Crops

As summer beds clear out, September is an excellent time to apply compost and plant a cover crop. Winter rye, crimson clover, and hairy vetch all establish well when seeded in September. These crops protect bare soil from compaction and erosion, fix nitrogen (legumes), add organic matter when turned in spring, and suppress early weed germination.

Winter rye (not ryegrass) is the most cold-hardy option and the easiest to establish — broadcast seed at 2–3 lbs per 1,000 sq ft over a cleared bed, rake lightly, and water. It establishes in 7–10 days, grows actively until hard frost, resumes in March, and is ready to till under 3–4 weeks before spring planting.

September Planting Calendar at a Glance

Season planning: Check our month-by-month Pennsylvania planting guide to keep your garden producing all year. Browse all Pennsylvania vegetable guides.

Frequently Asked Questions About Planting in September in Pennsylvania