What Plants to Overwinter in a Greenhouse: A Comprehensive Guide

Imagine stepping into a lush, green oasis in the dead of winter. Outside, snowdrifts pile high, and the wind howls, but inside your greenhouse, vibrant blossoms perfume the air, and leafy greens thrive. Overwintering plants in a greenhouse is more than just a way to extend the growing season; it's a chance to nurture your beloved plants through the harsh months, ensuring they return stronger and more productive in the spring. But which plants are best suited for this sheltered existence? This comprehensive guide will explore the ideal candidates, covering everything from tender perennials to vulnerable edibles, and provide the knowledge you need to create your own winter sanctuary.

Understanding Overwintering in a Greenhouse

Before we dive into specific plant selections, let's clarify what overwintering truly means in the context of a greenhouse. It's not simply about keeping plants alive; it's about providing an environment where they can rest, conserve energy, and ideally, experience minimal stress. This often involves a combination of temperature control, light management, and careful watering practices.

What Overwintering Achieves

**Protection from Frost:The most obvious benefit. Greenhouses shield plants from damaging frosts and freezing temperatures, which can be fatal to many species.
**Extending the Growing Season:While some plants go dormant, others can continue to produce, albeit at a slower rate, providing fresh harvests even in winter.
**Early Spring Head Start:Overwintered plants are already established when spring arrives, giving them a significant advantage over those started from seed.
**Preserving Tender Perennials:Many plants that are considered annuals in colder climates are actually tender perennials that can live for years if protected from frost.
**Conserving Rare or Prized Plants:If you have a particularly special or hard-to-find plant, overwintering it in a greenhouse is a great way to ensure its survival.

Greenhouse Considerations for Overwintering

**Temperature Control:Maintaining the correct temperature is crucial. This may require supplemental heating, ventilation, or a combination of both. Knowing the specific temperature requirements for the plants you intend to overwinter is essential.
**Light Availability:Winter days are shorter and often overcast. Consider supplemental lighting to ensure plants receive enough light for photosynthesis, even in dormancy.
**Humidity Management:Greenhouses can trap moisture, leading to fungal diseases. Proper ventilation and careful watering practices are key to preventing problems.
**Pest and Disease Control:Regularly inspect plants for signs of pests or diseases and take appropriate action. Prevention is always better than cure.
**Watering Strategy:Reduce watering frequency during the winter months, as plants generally require less water when they are not actively growing. Overwatering can lead to root rot.

Top Plant Candidates for Greenhouse Overwintering

Now, let's get to the heart of the matter: which plants thrive in a greenhouse environment during the winter months? We'll break it down into categories, considering their specific needs and potential benefits.

Tender Perennials

These are plants that are perennial in warmer climates but cannot survive freezing temperatures. Overwintering them in a greenhouse allows you to enjoy them year after year.

**Geraniums (Pelargonium spp.):These popular bedding plants are easy to overwinter. Cut them back hard in the fall, and keep them relatively dry. They'll be ready to bloom again in the spring.
**Fuchsias:With their delicate, dangling flowers, fuchsias add a touch of elegance to any garden. They require similar treatment to geraniums: cut back and keep cool and dry.
**Begonias:Many types of begonias, including tuberous and rhizomatous varieties, can be successfully overwintered. Lift the tubers or rhizomes in the fall and store them in a cool, dry place until spring.
**Dahlias:Similar to begonias, dahlia tubers need to be lifted and stored in a frost-free environment.
**Lemon Verbena (Aloysia citriodora):This fragrant herb is a tender perennial that can be overwintered in a greenhouse. It may lose its leaves, but it will regrow in the spring.
**Rosemary (Salvia rosmarinus):While rosemary is relatively hardy, it can struggle in very cold climates. Overwintering it in a greenhouse provides extra protection.
**Lavender (Lavandula spp.):Similar to rosemary, lavender benefits from the added warmth and protection of a greenhouse in colder regions.
**Citrus Trees (Citrus spp.):Dwarf citrus trees like lemons, limes, and oranges can be grown in containers and overwintered in a greenhouse, providing you with fresh fruit even in winter.

Vegetables

While a greenhouse won't allow you to grow a full summer vegetable garden in the winter, it can be used to extend the season for certain crops and to get a head start on spring planting.

**Leafy Greens:Spinach, kale, lettuce, and arugula can continue to produce in a greenhouse throughout the winter, especially with supplemental lighting.
**Herbs:Many herbs, such as parsley, chives, oregano, and thyme, can be overwintered in a greenhouse, providing fresh flavor for your winter meals.
**Tomatoes (Solanum lycopersicum):With adequate light and warmth, you can grow tomatoes in a greenhouse during the winter, although production will be slower. Choose determinate or compact varieties.
**Peppers (Capsicum spp.):Similar to tomatoes, peppers can be overwintered in a greenhouse, but they require plenty of light and warmth.
**Strawberries (Fragaria × ananassa):Everbearing strawberry varieties can produce fruit in a greenhouse throughout the winter.
**Garlic and Onions:While you won't get bulbs in the winter, you can plant garlic and onions in the fall and harvest the greens throughout the colder months.

Ornamental Plants

Beyond flowers and edibles, many ornamental plants benefit from greenhouse overwintering, protecting their delicate foliage or ensuring they retain their shape and vigor.

**Succulents and Cacti:These desert natives are highly susceptible to frost. A greenhouse provides the dry, bright conditions they need to thrive.
**Tropical Plants:Plants like hibiscus, bougainvillea, and mandevilla need warm temperatures to survive. A greenhouse allows you to enjoy their vibrant colors year-round.
**Tender Bulbs:Plants like gladiolus, cannas, and calla lilies can be lifted in the fall and stored in a greenhouse (or other frost-free location) to protect their bulbs from freezing.
**Japanese Maples (Acer palmatum):While relatively hardy, young or grafted Japanese maples benefit from the protection of a greenhouse in very cold climates.
**Camellias:These beautiful flowering shrubs bloom in late winter and early spring. Overwintering them in a greenhouse protects their delicate blooms from frost.
**Figs (Ficus carica):Fig trees can be grown in containers and overwintered in a greenhouse to protect them from harsh winter weather.

Considerations for Specific Plants

It’s important to remember that not all plants have the same overwintering needs. Here's a closer look at some specific factors to consider:

**Light Requirements:Plants that are actively growing, even slowly, will need more light than those that are dormant. Supplement with grow lights if necessary.
**Temperature Preferences:Research the ideal temperature range for each plant species. Some plants prefer cool temperatures (40-50°F), while others need warmer conditions (60-70°F).
**Watering Needs:Reduce watering frequency, but don't let plants dry out completely. Check the soil moisture regularly and water only when the top inch or two feels dry to the touch.
**Ventilation:Proper ventilation is crucial to prevent fungal diseases. Open vents on mild days to allow for air circulation.
**Pest Control:Regularly inspect plants for pests and take action promptly. Common greenhouse pests include aphids, spider mites, and whiteflies. Use organic pest control methods whenever possible.

Preparing Plants for Overwintering

The success of overwintering depends not only on the greenhouse environment, but also on how well you prepare your plants for the transition.

Timing is Key

**Before the First Frost:Bring plants into the greenhouse *beforethe first frost is predicted. This gives them time to adjust to the change in environment.
**Gradual Acclimation:If possible, gradually acclimate plants to the greenhouse environment by placing them outdoors during the day and bringing them in at night for a week or two before the final move.

Pruning and Cleaning

**Remove Dead or Diseased Foliage:Prune away any dead, damaged, or diseased leaves and stems. This helps to prevent the spread of disease.
**Clean the Plants:Gently wash the plants with a mild soap solution to remove any pests or dirt.
**Cut Back Growth:Depending on the plant species, you may need to cut back the foliage to encourage dormancy. Refer to specific instructions for each plant.

Potting and Soil

**Repot if Necessary:If plants are root-bound, repot them into larger containers with fresh potting mix.
**Use Well-Draining Soil:Ensure that the potting mix is well-draining to prevent root rot.
**Top Dress with Compost:Add a layer of compost to the top of the soil to provide nutrients.

Maintaining Your Greenhouse Oasis Through Winter

Once your plants are safely ensconced in the greenhouse, the real work begins: maintaining the optimal environment for their survival and well-being.

Temperature Management

**Monitor Temperatures Regularly:Use a thermometer to monitor the temperature inside the greenhouse. Check it daily, especially during periods of extreme weather.
**Heating Systems:Depending on your climate and the temperature requirements of your plants, you may need to use a heater to maintain a consistent temperature. Options include electric heaters, propane heaters, and gas heaters.
**Ventilation:Open vents on mild days to allow for air circulation and prevent overheating.
**Insulation:Consider insulating the greenhouse to conserve heat. Options include bubble wrap, insulated blankets, and double-paned glazing.

Light Management

**Assess Natural Light:Observe how much natural light your greenhouse receives during the winter months.
**Supplemental Lighting:If necessary, provide supplemental lighting to ensure plants receive enough light for photosynthesis. LED grow lights are an energy-efficient option.
**Light Duration:Adjust the duration of supplemental lighting based on the needs of your plants. Generally, 10-12 hours of light per day is sufficient for most plants.

Watering and Humidity

**Water Sparingly:Water only when the top inch or two of soil feels dry to the touch. Overwatering can lead to root rot.
**Water in the Morning:Water in the morning so that the foliage has time to dry before nightfall.
**Improve Air Circulation:Proper ventilation helps to reduce humidity levels.
**Consider a Dehumidifier:If humidity levels are consistently high, consider using a dehumidifier.

Pest and Disease Control

**Regular Inspections:Regularly inspect plants for signs of pests and diseases.
**Organic Pest Control:Use organic pest control methods whenever possible, such as insecticidal soap, neem oil, and beneficial insects.
**Remove Infested Plants:If a plant becomes heavily infested, remove it from the greenhouse to prevent the spread of pests to other plants.
**Good Sanitation Practices:Maintain good sanitation practices in the greenhouse, such as removing dead leaves and debris.

Troubleshooting Common Overwintering Problems

Even with the best planning and care, problems can sometimes arise during overwintering. Here are some common issues and how to address them:

**Yellowing Leaves:This can be caused by overwatering, underwatering, nutrient deficiencies, or pests. Check the soil moisture, fertilize if necessary, and inspect for pests.
**Leaf Drop:This can be caused by sudden changes in temperature, insufficient light, or dry air. Maintain consistent temperatures, provide supplemental lighting, and increase humidity levels.
**Mold or Mildew:This is caused by high humidity and poor air circulation. Improve ventilation, reduce watering, and apply a fungicide if necessary.
**Pest Infestations:Control pests with organic methods, such as insecticidal soap, neem oil, and beneficial insects.
**Stunted Growth:This can be caused by insufficient light, low temperatures, or nutrient deficiencies. Provide supplemental lighting, maintain appropriate temperatures, and fertilize if necessary.

Spring Awakening: Transitioning Plants Back Outdoors

As spring approaches, it's time to start thinking about transitioning your overwintered plants back outdoors. This process should be done gradually to avoid shocking the plants.

Hardening Off

**Gradual Exposure:Gradually expose plants to outdoor conditions over a period of one to two weeks.
**Start with a Few Hours:Begin by placing plants outdoors in a sheltered location for a few hours each day, gradually increasing the amount of time they spend outdoors.
**Protect from Frost:Bring plants back indoors at night if there is a risk of frost.
**Monitor Weather Conditions:Pay attention to the weather forecast and protect plants from strong winds, heavy rain, and extreme temperatures.

Repotting and Fertilizing

**Repot if Necessary:If plants have outgrown their containers, repot them into larger pots with fresh potting mix.
**Fertilize:Fertilize plants with a balanced fertilizer to provide them with the nutrients they need to resume active growth.

Pest and Disease Control

**Inspect for Pests:Before moving plants outdoors permanently, inspect them carefully for pests and diseases.
**Treat as Needed:Treat any infestations or diseases before moving the plants outdoors.

Beyond the Basics: Advanced Overwintering Techniques

For the dedicated gardener looking to take overwintering to the next level, consider these advanced techniques:

**Heated Propagation Mats:These mats provide gentle warmth to the soil, promoting root growth and improving germination rates for seeds and cuttings. [externalLink insert] They are particularly useful for starting seeds early in the spring.
**Cold Frames:Cold frames are miniature greenhouses that can be used to extend the growing season for cool-season crops. They are a simple and effective way to protect plants from frost.
**Hydroponics:Growing plants hydroponically in a greenhouse allows you to control the nutrient supply and optimize growing conditions. This can result in faster growth and higher yields.
**Aquaponics:Aquaponics is a system that combines aquaculture (raising fish) with hydroponics (growing plants without soil). The fish waste provides nutrients for the plants, and the plants filter the water for the fish.
**Integrated Pest Management (IPM):IPM is a comprehensive approach to pest control that emphasizes prevention and uses a variety of methods, including cultural practices, biological controls, and chemical controls, only when necessary.

Conclusion: A Winter Sanctuary for Your Garden

Overwintering plants in a greenhouse is a rewarding experience that allows you to extend the growing season, preserve tender perennials, and get a head start on spring planting. By understanding the specific needs of your plants and providing them with the appropriate environment, you can create a winter sanctuary where they can thrive, even in the midst of the coldest months. So, embrace the challenge, experiment with different techniques, and enjoy the satisfaction of nurturing your garden through the winter. When spring arrives, you'll be rewarded with healthy, vigorous plants that are ready to burst into bloom.