How to Grow Vegetables in an Unheated Greenhouse This Winter

An unheated greenhouse can extend your growing season long after your beds have slowed down. You can get more out of the growing season without turning on the heater constantly, while protecting leafy greens and herbs from harsh weather. 

Temperatures inside an unheated greenhouse can fall below freezing on cold nights, and plant growth slows considerably. But with the right setup, you can still have plenty to harvest. 

The trick here is choosing the right crops. Spinach, kale, Asian greens, and certain herbs are better suited to greenhouse growing in the winter season than tomatoes and peppers. You'll also need to pay extra attention to watering, daylight exposure, and ventilation as winter progresses.

Can You Grow in an Unheated Greenhouse in Winter?

Cold-hardy vegetables can grow or stay harvestable in an unheated greenhouse during winter, although your results will depend on your greenhouse construction, regional climate, planting date, and sunlight exposure.

A greenhouse gives plants some protection from wind, rain, snow, and rapid temperature changes. It can also capture warmth from the sun during the day. However, without supplemental heat, the temperature inside can drop close to the outdoor temperature overnight. For most gardeners, a realistic goal is to extend the harvest, and protect established crops.

What Vegetables Grow Best in an Unheated Greenhouse?

If you’re wondering how to grow vegetables in an unheated greenhouse, start with those that tolerate cool temperatures and don't need too much sunlight for a useful harvest.

Spinach

Spinach is one of the best vegetables for an unheated greenhouse. It prefers cool conditions and can tolerate frost, making it a good choice for fall and early-winter harvesting.

Plant them early enough for it to establish before daylight becomes scarce. Once the plants are growing well, pick the outer leaves as needed and leave the central growing point intact. Spinach is particularly handy for small greenhouses because you can grow several plants in a shallow bed or container without needing much space.

Lettuce

Lettuce can grow well in a cool greenhouse, especially when you choose varieties suited to fall and winter. Loose-leaf varieties are ideal because you can harvest leaves while the plant keeps producing. They fit well into containers and smaller raised garden beds.

Kale

Kale is another good choice for the cold season. It handles cold weather better than many summer vegetables, and you can keep harvesting leaves instead of pulling up the whole plant.

For a winter harvest, choose a cold-hardy variety and establish it before temperatures drop too far. Remove damaged outer leaves as needed, but avoid stripping the plant bare.

Asian Greens

Bok choy, tatsoi, mizuna, and other Asian greens are useful choices for unheated greenhouse gardening. Many mature relatively quickly and prefer cooler conditions over summer heat. Choose varieties with suitable maturity times and give them enough room for airflow.

Herbs

Parsley is a good candidate for a cool greenhouse, and cilantro can also be useful in fall and mild winter conditions. Herbs are worth planting because a small number of them can provide regular harvests for everyday cooking. Put them somewhere easy to reach, rather than behind larger plants.

What Should You Plant in an Unheated Greenhouse? 

The best planting depends on your winter climate.

If you live in a region with a mild climate: You may still have time to sow spinach, lettuce, radishes, Asian greens, and some herbs.

If you live in a region with freezing temperatures: Prioritize hardy transplants and crops already established in late summer or fall. Garlic may also be planted outdoors where local conditions are appropriate, although it doesn't need greenhouse space per se.

When deep winter is approaching: Concentrate on protecting established crops and preparing your greenhouse rather than starting vegetables. 

Before planting, check your local first frost date, the forecast, and each variety's days to maturity.

When Should You Plant Winter Vegetables?

The best time to establish winter greenhouse crops is late summer to early fall, while temperatures are still favorable with sufficient daylight. 

That gives seedlings time to grow roots and leaves before winter slows them down. It also means you have established plants ready to harvest rather than relying on seeds to germinate in cold conditions For example, spinach and lettuce planted in early fall may provide useful leaves later in the season.

If you're starting in October, focus on quick-growing crops suited to your climate, or consider buying established transplants.

Setting Up Your Unheated Greenhouse 

Inspect the greenhouse and make sure it is ready for the season before winter arrives. Make sure your panels are secure, doors close properly, and drainage works properly. Remove finished crops, clean your surfaces, and make sure you can reach every bed or container easily. 

Vego's Pacific Greenhouse for year-round growing is designed to extend your season, built with an aluminum frame, polycarbonate panels, and roof vents. The 8.5-foot series uses 8 mm double-wall polycarbonate panels, while larger Pacific models use triple-wall polycarbonate roofing and side panels.

Panel construction and ventilation is key to how well a greenhouse performs, but they don't guarantee frost-free nights. Think of the greenhouse as one part of your winter-growing setup, along with crop selection, and suitable planting dates. 

Keeping an Unheated Greenhouse Warmer

You won't be able to control winter temperatures precisely without heating, but a few practical steps can help make better use of the warmth in a greenhouse. Position growing beds where they receive as much direct sunlight as your space allows. Keep greenhouse panels clean, and avoid blocking light with tall plants or shelving.

Water stores heat and releases it gradually as the surroundings cool. Some gardeners use containers filled with water to store a daytime warmth, although this depends on the amount of water, sunlight, and outdoor temperature.

For particularly cold nights, a row cover or low tunnel inside the greenhouse can add another layer of protection for crops. Covering a smaller area may be more practical than trying to keep the entire place warm.

Do You Need Grow Lights?

Not every winter greenhouse needs supplemental lighting, but low daylight can become a limiting factor. Hardy greens may survive and grow slowly with natural light, especially in a greenhouse designed for winter. If your greenhouse receives very little sun, or you're trying to raise seedlings during short winter days, grow lights may help.

Decide on your crop before investing in lighting. If your main goal is to keep garden greens alive and harvest periodically, you may be able to manage with natural light. If you expect fast growth or want to start seedlings, supplemental lighting may be useful.

Use Raised Garden Beds to Organize Crops

A small greenhouse can become crowded, especially when you have seedlings, pots, and covers in the same space. Raised garden beds help you divide the growing area into sections. You can dedicate a bed to spinach and lettuce, another to Asian greens, and use containers for herbs.

Vego's raised garden beds can be used inside a greenhouse to create defined planting zones. You can also use raised garden beds or containers to organize your crops and make watering easier to manage. Just measure the internal space first and ensure you leave room for doors, watering, and access around your beds.

FAQs 

What vegetables grow best in an unheated greenhouse in winter?

Spinach, kale, lettuce, Asian greens, arugula, and some herbs are among the best choices. The right crops depend on your local temperatures, sunlight, planting date, and greenhouse construction.

Can an unheated greenhouse keep plants from freezing?

Not reliably. A greenhouse can buffer plants from cold weather and help extend the growing season, but its interior temperature can still fall below freezing. Row covers or other protection may be needed during cold snaps.

What should I plant in an unheated greenhouse in October?

In regions with enough remaining growing time, consider spinach, lettuce, arugula, radishes, Asian greens, kale, and suitable herbs. In colder areas, established transplants may be more practical than starting new seeds. Check your local frost timing before planting.

How cold is too cold for an unheated greenhouse?

There isn't one temperature that applies to every greenhouse or crop. Cold-tolerant vegetables can cope with lower temperatures than tender summer crops, but a greenhouse's interior can approach outdoor temperatures during a long cold spell. Use a thermometer and match protection to the crops you're growing.

Can tomatoes grow in an unheated greenhouse during winter?

In mild climates, tomatoes may survive longer under cover, but an unheated greenhouse in a cold region generally won't keep them actively producing through winter. Tomatoes need more warmth and light than most winter greens.

Do vegetables grow in winter, or do they just survive?

Both can happen. Hardy crops may continue growing slowly when temperatures and daylight allow, but they may simply remain alive during the coldest, darkest part of winter. Harvesting established plants is often more realistic than expecting rapid new growth.

Do I need grow lights in an unheated greenhouse?

Not always. Cold-tolerant greens may do adequately with natural light, but supplemental lighting can help when winter daylight is very limited or when raising seedlings. Lighting won't replace the need for suitable temperatures.

How often should I water vegetables in a winter greenhouse?

Check the soil before watering. Plants generally use less water in cooler weather, so the growing medium may stay wet for longer than in summer. Avoid both waterlogging and allowing newly established seedlings to dry out.

Should I open greenhouse vents in winter?

Yes, when needed. Sunny winter days can cause excessive heat buildup, while inadequate airflow can encourage condensation. Ventilate when conditions are suitable, then close the greenhouse as temperatures fall.

Can raised beds be used inside an unheated greenhouse?

Yes. Raised beds help organize growing space and make it easier to manage soil, crops, and protective covers. Make sure the greenhouse has enough room for the beds and safe access around them.

Final Thoughts

Having an unheated greenhouse can keep your garden productive for longer, while the key is choosing what to grow carefully. Start with cold-tolerant vegetables, and keep an eye on temperature, moisture, and airflow. Spinach, kale, Asian greens, and herbs are good options to start with. 

Even a few handfuls of fresh garden greens or herbs when the garden has slowed can be a good return for a little planning.