For most tropical houseplants, the minimum recommended temperature is around 55°F, although it depends on the specific plant variety. Some tropicals and subtropicals can handle slightly cooler temperatures by slowing their growth or going dormant. But in general, tropicals will not survive extended freezing conditions below 32°F.
When overwintering tender landscape plants and bulbs in an unheated greenhouse, their cold tolerance also varies widely depending on the plant. Checking the plant’s hardiness zone helps determine its minimum temperature needs.
It’s also useful to know if a plant is root hardy — meaning the top foliage may die back when it gets cold, but the roots or bulbs will survive underground. Root-hardy bulbs can be stored dormant over the winter and replanted in the spring.
While a greenhouse without heat may not be ideal for the most tender plants, it can still allow gardeners in colder climates to get a head start on spring plantings. An unheated greenhouse also enables overwintering plants that might not otherwise thrive in that hardiness zone.
Monitoring Conditions
Keep an eye on overnight lows and make sure the greenhouse insulation is adequate. Thermometers and heating mats can help monitor and regulate temperatures.
On cold nights, heat sources like radiant heaters, heating cables, or small space heaters may be needed. Make sure heat sources are rated as safe for greenhouse use. Avoid bringing in propane or kerosene heaters that can produce carbon monoxide.
Ensure proper ventilation is maintained even while attempting to preserve heat. Proper airflow prevents fungal diseases. Vents, exhaust fans, and cracked doors can improve air circulation.
Place sensors throughout the greenhouse to track for excessive humidity. Ideal relative humidity levels typically range between 40% and 60% for most greenhouse plants. Dehumidifiers or exhaust fans help lower humidity.
Protecting Plants
Group plants together based on similar cold tolerance. This allows you to focus your heating efforts on the most vulnerable ones. Tropicals sensitive to anything under 40°F should be grouped separately from subtropicals that can handle temperatures in the mid-30s.
Provide insulation around the roots by heavily mulching the soil surface with leaves, straw, or other organic matter. Container plants can be moved together and wrapped in insulation.
Cover plants overnight with breathable fabric row covers or plant blankets. Avoid plastic sheeting that traps moisture. Remove covers during the day to allow light and air circulation.
Prune deciduous plants to remove dead leaves and branches to prevent disease and reduce the surface area that loses heat. However, don’t prune evergreens like citrus.
With extreme cold snaps, some gardeners will immerse potted plants in buckets of warm water overnight. This acts as a heat sink, releasing warmth back into the roots and soil.
Overwintering Tender Bulbs & Perennials
The key to overwintering root-hardy bulbs like canna, dahlia, and gladiolus is keeping the dormant tubers or rhizomes above freezing but cool enough to maintain dormancy. Store them packed in peat moss, vermiculite, or sawdust at around 40°F.
Seed trays with perlite, sand, or wood shavings also work for bulb storage. Make sure the bulbs are dry when storing and check them periodically for rotting. Discard any that show signs of mold, shriveling, or pests.
Other tender perennials like hibiscus, mandevilla, plumbago, and brugmansia can be overwintered in a cool greenhouse. Prune them back and reduce watering to match their dormant state. They’ll regrow the following season.
Growing Cool Weather Crops
While tropicals struggle in the colder months, an unheated greenhouse allows you to grow cool-weather vegetable crops even in winter. Spinach, kale, lettuce, carrots, and radishes are some options.
Start seeds or transplant cold-hardy seedlings into the greenhouse soil beds. You can also use containers, raised beds, or hydroponic setups. Just keep the soil temperature above 40°F for good germination. Row covers, low tunnels, and cloches inside the greenhouse provide extra frost protection.
Ventilate the greenhouse during the day, but keep as much heat as possible at night. Monitoring conditions and maintaining airflow prevent issues with mold or damping off disease. You can harvest fresh greens all season long.
So, Is an Unheated Greenhouse Worthwhile?
While heating a greenhouse can enable you to grow the widest variety of plants year-round, an unheated greenhouse has its advantages, too. It provides frost protection and shelter from wind and extreme weather. This allows gardeners in colder climates to extend their growing seasons on both ends for an earlier spring start and a later fall finish.
For overwintering tender plants, a cool greenhouse can preserve root-hardy bulbs and perennials that might not survive unprotected outdoors, saving the expense of replacing them each year. An unheated greenhouse also offers opportunities for growing cool-weather vegetable crops through frigid winter months when gardening would otherwise not be possible.
FAQs About Overwintering Plants in an Unheated Greenhouse
How cold is too cold for an unheated greenhouse?
It depends on the plants. Tropicals and subtropicals can’t survive freezing temps below 32°F. An unheated greenhouse can maintain temperatures warmer than outside ambient air, protecting plants down to around 20°F. Monitoring conditions is important.
What's the ideal temperature range for an unheated greenhouse?
Ideally, aim for lows above 40°F at night and highs around 60°F to 75°F during the day. Providing supplemental heat overnight when temps dip near freezing helps tender plants withstand short cold snaps.
Should you ventilate an unheated greenhouse in winter?
Yes, maintaining proper ventilation is crucial, even in cold weather. Adequate airflow prevents mold, fungal diseases, and excess humidity. Balance retaining heat while allowing some air exchange.
Can I grow vegetables in an unheated greenhouse in winter?
Yes. Many cool-season vegetable crops like spinach, kale, carrots, and radishes can be grown in cold greenhouse conditions. Start seeds or transplant cold-hardy seedlings and use row covers for added warmth at night when temperatures drop.
What are good heat sources for an unheated greenhouse?
Small portable space heaters, radiant heaters, under-bench heating cables, and heating mats are common options. Take precautions – make sure heat sources are greenhouse rated and ventilated properly. Avoid bringing in unvented propane or kerosene heaters.
