Greenhouses offer a controlled environment where plants can thrive, regardless of external conditions. However, knowing the ideal time to start your plants in a greenhouse is crucial for maximizing growth, yield, and overall success. Whether you're a beginner or a seasoned gardener, understanding the factors influencing plant growth within a greenhouse can jump start your gardening endeavors and give your young seedlings a head start.
When to Sow Seeds in a Greenhouse
If you want to transplant your seedlings into the garden in spring, sow seeds in the greenhouse about six to eight weeks before your area's last frost date. Begin hardening off plants two weeks before transition to the garden, in early spring or after the last frost date in your climatic zone.
Last Frost Date
A frost date is the average date of the last light freeze in spring or the first light freeze in fall. In most of the United States this is late March or Early April.
The classification of freeze temperatures is based on their effect on plants:
- Light freeze: 29° to 32°F (-1.7° to 0°C)—tender plants are killed.
- Moderate freeze: 25° to 28°F (-3.9° to -2.2°C)—widely destructive to most vegetation.
- Severe freeze (also called a killing freeze): 24°F (-4.4°C) and colder—heavy damage to most garden plants.
Please be aware that frost dates serve as estimations based on historical climate patterns and are subject to variability. There is a 30% likelihood of frost occurring after the spring frost date or before the fall frost date, indicating the possibility of frost before or after the specified dates.
When to Start Plants
- Annuals and Vegetables: For annual flowers and vegetables, starting seeds indoors 6-8 weeks before the last frost date is generally recommended. This allows ample time for seedlings to establish before transplanting outdoors to garden beds.
- Perennials: Perennial plants typically require a longer growing season to establish roots before winter. Starting perennial seeds indoors 8-10 weeks before the last frost date ensures robust growth and increased chances of survival.
- Tropical Plants: Tropical plants thrive in warm, humid conditions. Starting tropical plant seeds indoors 10-12 weeks before the last frost date allows sufficient time for germination and establishment before transplanting into the greenhouse.
Important Factors to Consider
- Climate Zone: Understanding your climate zone is fundamental. This knowledge helps determine the timing for starting plants in a greenhouse. Referencing the USDA Plant Hardiness Zone Map or similar resources can guide you in understanding the specific needs of plants based on your location.
- Plant Variety: Different plants have varying requirements regarding temperature, light, and moisture. cool-season crops, like spinach or broccoli thrive in cooler temperatures, while others, like pumpkins require warmer conditions. Researching the specific needs of each plant variety ensures optimal growth.
- Frost Dates: Knowing the average last frost date in your area is essential for planning when to start plants in a greenhouse. Starting plants too early can expose them to frost damage, while delaying planting may lead to a shorter growing season. Utilizing frost dates as a guideline helps mitigate risks associated with temperature fluctuations.
- Growing Medium and Containers: The quality of the growing medium and containers used also influences plant growth. Ensure that the soil is well-draining and nutrient-rich to support healthy root development. Additionally, choosing the appropriate container size for each plant ensures adequate space for growth.
- Light Exposure: Proper light exposure is critical for photosynthesis and overall plant health. While greenhouses offer protection from harsh sunlight, it's essential to monitor light levels and provide supplemental lighting if necessary, especially during the early stages of growth.
- Ventilation and Humidity: Maintaining proper ventilation and humidity levels within the greenhouse promotes airflow and prevents fungal diseases. Consider a greenhouse with a fan for additional ventilation. Monitoring humidity levels helps prevent issues such as damping off, a common problem among seedlings.
Hardening off Greenhouse Plants
Plants that have been indoors (even those bought early from a local greenhouse) need to be acclimated to the fluctuating temperatures, wind and sunlight of life outdoors. This is called hardening them off.
Gradually acclimatize tender or indoor-raised plants to outdoor conditions to toughen them up and prevent a check in growth. Place plants outside in a sheltered spot during the day then bring indoors at night for a couple of weeks.
Here is an example of a hardening off schedule for the first 4 days:
Day 1: Set your seedlings outside in dappled or partial sun for 2 to 3 hours.
Day 2: Set your seedlings outside in partial to full sun for 3 hours.
Day 3: Set your seedlings outside in full sun for 4 hours.
Day 4: Set your seedlings outside in full sun for 5 to 6 hours.
Once outdoor temperatures remain around 50 degrees Fahrenheit through the night, and if the seedlings are tolerating the hardening process well, you can start to leave them outdoors overnight. This should take place 7-10 days into the hardening process.
Transplanting Outdoors
Transplant young plants outside once the nighttime temperatures are consistently above 55 degrees. Alternatively, you can keep warm-season crops growing in your greenhouse all summer long and into the fall. If the weather is mild and the plants are hardy enough, they can be transplanted when they are a few inches tall and have at least two sets of true leaves. It's important to avoid transplanting seedlings too early, as the colder temperatures can lead to shock and stunted growth.
Your Greenhouse In The Summer
To optimize your summer greenhouse, consider transitioning to crops capable of thriving in high temperatures while yielding abundant harvests.
Options such as tomatoes, peppers, eggplants, beans, and other heat-tolerant plants, cultivated in soil from the beginning of spring, are ideal selections for your summer greenhouse environment. With sufficient soil moisture, these plants will sustain continuous productivity through the late summer months within your greenhouse.
Your Greenhouse in the Winter
If you live in a cold climate you can use your greenhouse through the winter months to continue to grow food. You can make use of the autumn sun and warm temperatures inside the greenhouse to extend the growing season into the winter.
A winter greenhouse is a specially designed structure that allows gardeners to extend their growing season into the colder months. Unlike standard greenhouses, which are often used to start seedlings in early spring, winter greenhouses are insulated and engineered to capture and retain heat, even during cold weather to give the ideal environment for a variety of plants to grow in the late winter. Leafy greens, Brussels sprouts and cold-hardy vegetables can do well in a greenhouse even during harsh weather conditions.
Starting plants in a greenhouse requires careful planning and consideration of various factors such as climate, plant variety, and environmental conditions. By understanding the specific needs of each plant and referencing frost dates, gardeners can effectively time their planting to maximize success. Whether you're growing vegetables, flowers, or tropical plants, a well-timed start in a greenhouse sets the stage for a bountiful and rewarding gardening season.
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Frequently Asked Questions
You can put seedlings in an unheated garden greenhouse when they are still tiny, and are in their seed starting trays or pots. Generally speaking seedlings can be transplanted when they have just two sets of leaves upwards. The time frame for this is usually when they are 2-4 weeks old.
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. Research the specific plant you want to grow to specific temperature requirements.
Open all doors and vents on sunny days. These can be left open at night if the temperature remains high, usually in the late summer.
Many ornamental flowers can be started around 8-10 weeks before the average last frost date. Cold sensitive plants, which require warmer temperatures to grow properly, should be started a little later, around 6-8 weeks before the average last frost date.
South-facing is best but in cases where that is not possible, a southwest or southeast facing greenhouse is doable. Southeast is preferable to southwest because it will get the morning sun, which helps heat up the greenhouse after it's cooled down overnight.
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