Plant Now, Harvest Later: Best Vegetables to Sow in September for Winter & Spring

Aug 31

As the heat of summer fades and cooler nights set in, many gardeners assume the season is winding down. But in reality, September is one of the most important months for greenhouse growers, especially those focused on year-round harvests.

With the right cold-hardy crops, smart timing, and minimal protection, your greenhouse can keep producing fresh, nutrient-dense vegetables deep into winter — and set the stage for an early spring bounty.

This is your September planting guide for greenhouse growers, with tips on crops, varieties, succession schedules, and environmental strategies for real results in fall and beyond.

Why September Matters

For unheated and passively heated greenhouses, the key to winter harvests is timing. Crops don’t grow much once daylight drops below 10 hours (roughly mid-November in northern states), so the goal is to establish hardy crops in September while the soil is still warm and light is still plentiful.

These crops will mature slowly through fall, then pause their growth in deep winter — but stay alive, fresh, and harvestable all season long.

Think of your greenhouse as a living pantry: stocked in September, opened throughout winter.

Best Vegetables to Sow in September

Here are the most reliable cold-hardy crops for fall planting in an unheated greenhouse or high tunnel:

1. Spinach

Varieties: Bloomsdale, Space, Giant Winter
Why: Excellent cold tolerance, sweetens in winter, regrows after harvest
Timing: Sow mid to late September for winter harvest

2. Kale

Varieties: Winterbor, Dwarf Siberian, Red Russian
Why: Durable, nutritious, survives down to 20°F or lower under cover
Timing: Transplant early September or direct seed by mid-month

3. Bok Choy & Asian Greens

Varieties: Tatsoi, Mizuna, Komatsuna, Joi Choi
Why: Fast-growing, ideal for cut-and-come-again harvests
Timing: Sow every 1–2 weeks through late September for staggered harvests

4. Chard

Varieties: Fordhook Giant, Rainbow, Rhubarb
Why: Withstands light frost, productive into early winter
Timing: Transplant starts in early September for stronger roots

5. Arugula & Mustard Greens

Why: Fast, spicy, highly cold-tolerant, great for winter salads
Timing: Succession sow through September every 10–14 days

6. Lettuce (Cold-Hardy Types)

Varieties: Winter Density, Arctic King, Rouge d’Hiver
Why: Great flavor in cool temps, stores well in soil
Timing: Sow early to mid-September for heads, or late-month for baby leaf

7. Turnips & Radishes

Why: Quick-growing roots, edible greens, store well in soil
Timing: Sow early September for full-size roots by late October

Succession Planting Tips

To stretch your harvest and avoid gaps:

Sow greens every 10–14 days through the end of the month
Start transplants early indoors or under shade to beat early-September heat
Leave space for staggered planting, especially in raised beds or modular systems

By staggering your planting, you'll get multiple harvest waves and can keep picking fresh leaves even as growth slows down.

Environmental Tips for Fall & Winter Greenhouse Growing

Cold-hardy crops can survive tough conditions, but they’ll do even better with a few simple techniques:

Ventilate on warm days: Prevent humidity buildup, especially in fall
Use low tunnels inside your greenhouse: Add row cover or frost cloth for extra insulation
Water earlier in the day: Keep roots moist but avoid soggy soil at night
Mulch heavily: Helps retain warmth and regulate moisture

For growers in Zones 3–6, adding a second layer of protection (like floating row cover) can make the difference between survival and thriving.

A September Greenhouse Plan in Action

Here’s a sample planting schedule for a 12×24 greenhouse in Zone 6:

WeekTaskSept 1–7Transplant kale and chard, direct seed spinach and arugulaSept 8–14Sow bok choy, lettuce, and more spinachSept 15–21Plant turnips and radishes, sow mustard greensSept 22–30Final round of arugula and Asian greens, mulch beds

This approach sets up a steady fall harvest, overwintered leafy greens, and a head start on spring — all without the need for electric heat.

Final Thoughts: September Sets the Stage

If you want a productive winter harvest, September is the month to make it happen. With the right mix of hardy vegetables, staggered sowing, and simple protection, your greenhouse can keep feeding you long after the rest of the garden has gone quiet.

At Sunbow, we build high tunnels and greenhouses made for growers like you — real people growing real food in all four seasons.