Think your growing season is over once summer crops begin to fade? You may still have time for another fresh harvest before winter arrives.
Planting cool-season vegetables such as radishes, spinach, and baby bok choy is a practical way to use the space left by tomatoes, cucumbers, and other summer crops. Some varieties can be harvested as baby leaves or tender roots in just 2 to 6 weeks.
With greenhouse protection from cold winds, early frost, and sudden temperature changes, these late plantings have more time to grow.
This guide covers 10 quick-turnaround vegetables, along with their harvest times, ideal growing conditions, and practical care tips.
10 Best Vegetables for Late-Season Greenhouse Growing
| Vegetable | Approximate First Harvest | Best Harvest Stage |
|---|---|---|
| Microgreens | 7-21 days | Young shoots |
| Radishes | 22-35 days | Small roots and greens |
| Arugula | 20-40 days | Baby or mature leaves |
| Mustard greens | 21-45 days | Baby or mature leaves |
| Leaf lettuce | 25-50 days | Baby or outer leaves |
| Spinach | 30-50 days | Baby or mature leaves |
| Mizuna | 25-45 days | Baby or mature leaves |
| Baby bok choy | 30-45 days | Small heads |
| Salad turnips | 35-50 days | Young roots and greens |
| Green onions | 40-60 days | Young stalks |
1. Radishes
Radishes are a dependable choice for late autumn planting, with some salad varieties ready to harvest in about a month. Both the crisp roots and leafy tops are edible, making them a practical crop when growing time is limited.
- Harvest Time: 22–35 days for fast salad varieties
- Temperature: 50–70°F (10–21°C)
- Humidity: 50–70%; maintain good airflow
- Soil: Loose, well-drained soil with a pH of 6.0–7.0
- Varieties: Cherry Belle, French Breakfast, Champion, Easter Egg
Sow radishes directly and thin the seedlings according to the spacing instructions on the seed packet. Keep the soil evenly moist to prevent split roots, and harvest the radishes as soon as they reach a usable size.
2. Microgreens
Microgreens are the fastest option when only a few weeks remain in the season. They are young vegetable seedlings harvested shortly after their first true leaves appear. Because they grow in shallow trays, they also make productive use of shelves and other small greenhouse spaces.
- Harvest Time: 7–21 days, depending on the crop
- Temperature: 60–75°F (16–24°C)
- Humidity: 40–60%; provide airflow to reduce dampness and mold
- Soil: Sterile, fine-textured, well-drained seed-starting mix with a pH around 6.0–6.5
Start with clean trays and keep the growing mix moist by adding water to the bottom tray. Move seedlings into bright light after they emerge, then harvest them with clean scissors when they reach the recommended size.
3. Arugula
Arugula grows quickly in cool conditions and develops a distinctive peppery flavor. While warmer temperatures can cause it to bolt, autumn weather helps the leaves remain tender and productive for longer.
- Harvest Time: 20–30 days for baby leaves; 35–50 days for mature leaves
- Temperature: 50–65°F (10–18°C)
- Humidity: 50–70%; ventilate during damp weather
- Soil: Fertile, well-drained soil with a pH of 6.0–7.0
- Varieties: Astro, Rocket, Speedy, Sylvetta
4. Mustard Greens
Mustard greens grow quickly and tolerate cool autumn conditions well. Their young leaves add color and a mild peppery flavor to salads, while larger leaves can be cooked. Cooler weather can also make their naturally sharp flavor milder.
- Harvest Time: 21–30 days for baby leaves; 35–45 days for mature leaves
- Temperature: 50–70°F (10–21°C)
- Humidity: 50–70%; provide enough spacing for good airflow
- Soil: Moist, fertile, well-drained soil with a pH of 6.0–7.5
- Varieties: Green Wave, Red Giant, Miz America, Tendergreen
5. Leaf Lettuce
Loose-leaf and miniature lettuces are better suited to a quick autumn harvest than large, slow-forming heads. Baby leaves can be ready within a few weeks, while picking the outer leaves allows the plant's center to continue growing.
- Harvest Time: 25–35 days for baby leaves; 40–55 days for larger leaves or mini heads
- Temperature: 45–65°F (7–18°C)
- Humidity: 50–70%; ventilate to reduce mildew and rot
- Soil: Moist, fertile, well-drained soil with a pH of 6.0–7.0
Lettuce seeds may struggle to germinate when greenhouse soil is still warm. Start them in trays in a cool, shaded area, then transfer the seedlings once temperatures fall.
6. Spinach
Spinach tolerates cold conditions well, making it a reliable choice for the fall season. It grows best in cool weather and can be picked early as tender baby leaves or left to develop into larger plants.
- Harvest Time: 30–40 days for baby leaves; 40–55 days for mature leaves
- Temperature: 45–65°F (7–18°C)
- Humidity: 50–70%; maintain good airflow to limit leaf diseases
- Soil: Fertile, moisture-retentive but well-drained soil with a pH of 6.5–7.5
- Varieties: Bloomsdale Long Standing, Giant Winter, Space, Tyee
7. Mizuna
Mizuna is a Japanese green with feathery leaves and a mild mustard flavor. It grows well in cool conditions and can be planted in containers, raised beds, or small gaps between slower-growing crops.
- Harvest Time: 25–30 days for baby leaves; 40–50 days for mature plants
- Temperature: 50–70°F (10–21°C)
- Humidity: 50–70%; maintain airflow around dense plantings
- Soil: Fertile, consistently moist, well-drained soil with a pH of 6.0–7.5
Sow mizuna directly or transplant young seedlings into available spaces. Pick the outer leaves as needed, or cut the plant above the crown to encourage regrowth. Check young plants regularly for flea beetles.
8. Baby Bok Choy
Baby bok choy provides crisp stems and tender leaves sooner than full-size brassicas. Compact varieties can be ready in about four to six weeks and grow well in the cooler temperatures of autumn.
- Harvest Time: 30–45 days for baby varieties
- Temperature: 50–70°F (10–21°C)
- Humidity: 50–70%; ventilate well and avoid crowding
- Soil: Fertile, well-drained, consistently moist soil with a pH of 6.0–7.5
9. Salad Turnips
Salad turnips are smaller, sweeter, and quicker to mature than traditional storage turnips. Their tender roots can be eaten raw or cooked, while the leafy tops provide an additional harvest.
- Harvest Time: 35–50 days
- Temperature: 50–70°F (10–21°C)
- Humidity: 50–70%; maintain airflow around the foliage
- Soil: Loose, fertile, well-drained soil with a pH of 6.0–7.5
- Varieties: Hakurei, Tokyo Cross, Tokyo Market, White Lady
10. Green Onions
Green onions mature sooner than bulb onions because they are harvested while the stalks are still young. Their narrow shape also makes them easy to grow along bed edges or in small spaces between other crops.
- Harvest Time: 40–60 days from seed; earlier when grown from sets
- Temperature: 55–75°F (13–24°C)
- Humidity: 40–60%; maintain good ventilation to limit foliar disease
- Soil: Loose, fertile, well-drained soil with a pH of 6.0–7.0
Choose bunching varieties or plant onion sets for a faster start. Keep the soil evenly moist and the bed free of weeds, then harvest individual stalks as needed instead of clearing the entire row.
Final Thoughts
Late summer and early autumn do not have to mark the end of your greenhouse harvest. Radishes, spinach, and other quick-growing crops can turn newly available beds into another round of fresh produce.
Ready to extend your growing season? Planta Greenhouses offers durable greenhouse kits designed to protect crops from changing weather and support year-round growing.
Growers can also benefit from nationwide delivery, free off-season storage, and step-by-step installation support for a smoother setup.
Frequently Asked Questions
1. What is the fastest vegetable to grow in autumn?
Microgreens are usually ready first, often within 7 to 21 days. If you want a mature root vegetable, fast salad radishes can be ready in approximately 22 to 35 days.
2. Which vegetables can I harvest within 30 days?
Microgreens, baby arugula, baby mustard greens, mizuna, baby lettuce, and some radish varieties may be ready for harvest within 30 days under favorable conditions.
3. How late can I plant vegetables for a fall harvest?
The latest planting date depends on the crop, available daylight, and conditions inside your greenhouse. To estimate it, check the crop’s days to maturity, add an extra 10 to 14 days for slower autumn growth, and count backward from your desired harvest date.
4. Do vegetables grow more slowly in autumn?
Yes. Shorter days, weaker sunlight, cooler soil, and lower nighttime temperatures slow growth. A greenhouse offers protection, but crops still need enough light and warmth to develop.
5. What can I grow in an unheated greenhouse in the fall?
Spinach, lettuce, mustard greens, salad turnips, scallions, and suitable baby bok choy varieties are strong options. Results depend on local temperatures and the amount of sunlight the greenhouse receives.