What Not to Grow in a Greenhouse?

While greenhouses can be a gardener’s paradise, it’s important to know “What Not to Grow in a Greenhouse?”. Some plants are better left outside due to their specific needs. This guide will help you identify those plants and choose better alternatives for your greenhouse.

Understanding Greenhouse Conditions 

Greenhouses create a unique environment that supports plant growth through controlled temperature, humidity, and light. However, these conditions can also present challenges for certain types of plants.

The Unique Environment of a Greenhouse

Greenhouses are designed to trap heat and humidity, creating a warm, moist environment. This is ideal for many tropical and temperate plants but can be problematic for others that require different conditions. The ability to control these environmental factors makes greenhouses versatile, but not all plants can thrive in such settings.

Common Challenges Faced in Greenhouses 

Issues such as overheating, high humidity, and limited air circulation can pose significant challenges. Understanding these challenges is key to selecting the right plants for your greenhouse. Over the years, I’ve noticed that some plants struggle significantly more than others, which is why choosing the right species is crucial for a successful greenhouse garden.

 

 

Plants Not Suitable for Greenhouses 

❌Plants with High Space Requirements 

• Giant Trees and Shrubs 

Large trees and shrubs, such as oak or maple, are impractical for greenhouses due to their mature size. These plants require extensive root systems and canopy space, which greenhouses typically cannot provide. My attempt to grow a small citrus tree in my greenhouse was initially successful, but as it grew, it became clear that it needed more space than the greenhouse could offer. These plants are best grown outdoors where they have ample space to spread their roots and canopy.

• Large Vining Plants (with caution)

Vining plants like pumpkins and melons can quickly overtake a greenhouse. Managing their growth requires extensive space and support structures. If you choose to grow these, use vertical trellising systems to maximize space efficiency. However, even with trellises, these plants can become too cumbersome. I once grew a variety of squash in my greenhouse, and while the vertical growth was initially manageable, the sheer volume of the vines eventually overwhelmed the space.

❌Plants with Specific Temperature or Humidity Needs

• Plants Requiring Cold Stratification

Some plants, such as tulips and apples, need a period of cold stratification to germinate. Greenhouses, with their warm environments, cannot provide these conditions. My experience trying to germinate tulip bulbs in a greenhouse setting was disappointing due to the lack of cold periods. Using cold frames or sowing seeds outdoors during the appropriate season is a better approach.

• Plants Thriving in Cooler Temperatures 

Cool-season crops like lettuce, spinach, and broccoli may struggle in the consistently warm temperatures of a greenhouse. I found that my attempts to grow lettuce in a greenhouse during summer resulted in bolting and poor leaf quality. Growing these plants outdoors during cooler seasons or in a dedicated cooler greenhouse section is recommended.

❌Plants with High Water or Airflow Needs 

• Plants Prone to Fungal Diseases 

High humidity and stagnant air in greenhouses can promote fungal diseases, which are problematic for plants like potatoes and tomatoes. Growing these plants outdoors with proper spacing can help reduce disease incidence. Alternatively, select disease-resistant varieties for greenhouse cultivation. My tomato plants often developed blight in the greenhouse until I improved airflow and switched to more resistant varieties.

• Plants Requiring Dry Soil 

Cacti and succulents prefer dry soil conditions, which are difficult to maintain in a greenhouse with regular watering practices. Using pots with excellent drainage and watering less frequently can help accommodate these plants. My cacti collection struggled with rot when I first introduced them to the greenhouse environment, but switching to well-drained pots and reducing watering frequency improved their health significantly.

❌Plants with Low Light Requirements 

• Shade-Loving Plants 

Greenhouses maximize sunlight exposure, which is unsuitable for shade-loving plants like ferns and hostas. These plants thrive in shaded outdoor areas or can be grown in shaded sections of the greenhouse using shade cloth. I attempted to grow hostas in my greenhouse, but the intense light caused them to wilt. Moving them to a shaded outdoor spot resulted in much healthier growth.

❌Plants with High Pollination Requirements 

• Wind-Pollinated Plants

Plants like corn rely on wind for pollination, which is limited in a greenhouse environment. Hand pollination or choosing self-pollinating varieties can improve success rates. My trial with growing corn in the greenhouse was less productive due to inadequate pollination, highlighting the importance of choosing the right methods for such plants.

❌Aggressive or Invasive Plants

• Plants Prone to Spreading 

Aggressive plants such as mint can quickly take over a greenhouse. Growing them in contained pots or dedicated outdoor gardens helps manage their spread. I once planted mint in the greenhouse soil, and it quickly spread beyond control, overshadowing other plants. Containing it in pots has since been a better approach.

 

Additional Considerations for Greenhouse Plant Selection 

✅Soil Compatibility 

Different plants have varying soil requirements, and not all soil types are ideal for greenhouse conditions. For example, plants that prefer sandy soil may not thrive in the more compact and moisture-retentive soil commonly found in greenhouses. Ensuring the soil type matches the plant’s needs is crucial. My personal experience with growing rosemary, which prefers well-drained sandy soil, highlighted the need for soil amendments to achieve optimal growth in the greenhouse.

✅Nutrient Needs 

Greenhouse plants can have different nutrient requirements compared to their outdoor counterparts. Some plants may require more frequent fertilization or specific nutrient balances to thrive. For instance, leafy greens like spinach and lettuce may need additional nitrogen to support vigorous growth. Monitoring and adjusting nutrient levels based on plant needs is essential for maintaining healthy greenhouse plants.

✅Pest and Disease Resistance 

Selecting plants that are resistant to common greenhouse pests and diseases can save time and effort in managing these issues. For example, choosing tomato varieties resistant to blight and mildew can reduce the need for chemical treatments and improve overall plant health. In my greenhouse, I’ve had success with blight-resistant tomato varieties, which significantly reduced disease incidence and improved yields.

 

Common Issues with Unsuitable Greenhouse Plants 

Overheating 

Plants not suited for greenhouse conditions can suffer from overheating, leading to stress and poor growth. My broccoli plants, which prefer cooler conditions, wilted and turned bitter in the warm greenhouse environment.

Disease and Pest Proliferation 

High humidity and stagnant air can lead to increased disease and pest issues, affecting plant health. I found that plants like potatoes and tomatoes, prone to blight and mildew, struggled without proper airflow and humidity control.

Inadequate Growth and Poor Yields 

Plants that struggle with greenhouse conditions may exhibit inadequate growth and lower yields, making them less productive. For instance, my spinach crop yielded significantly less in the greenhouse compared to outdoor cultivation due to the consistently high temperatures.

 

Tips for Optimizing Plant Growth in Greenhouses 

Selecting the Right Plants 

Choose plants that thrive in controlled environments and match the specific conditions of your greenhouse. Research and select varieties known for their compatibility with greenhouse settings. Over time, I have found that tomatoes, peppers, and herbs like basil and oregano thrive exceptionally well in my greenhouse.

Managing Temperature and Humidity

Use ventilation, heating, and cooling systems to maintain optimal temperature and humidity levels. Installing automatic vents and fans has made a significant difference in controlling the climate inside my greenhouse.

Regular Maintenance and Monitoring

Regularly inspect your plants and greenhouse conditions to address any issues promptly. Keeping a journal of plant performance and environmental conditions helps in making informed adjustments. My routine includes checking for signs of disease, adjusting watering schedules, and ensuring proper airflow.

 

FAQs About What Not to Grow in a Greenhouse?

1. Can I grow large trees and shrubs in a greenhouse?
   • Large trees and shrubs like oaks and maples are impractical for greenhouses due to their mature size and extensive root systems. These plants are best grown outdoors where they have ample space to develop fully.
2. Why do some plants prone to fungal diseases struggle in greenhouses?
   • Greenhouses often have high humidity and limited air circulation, which can promote fungal diseases. Plants like tomatoes and potatoes can develop blight and mildew more easily in these conditions unless proper ventilation and disease-resistant varieties are used.
3. Are there plants that require too much space to be grown in a greenhouse?
   • Yes, vining plants like pumpkins and melons can quickly overtake a greenhouse with their extensive growth habits. While vertical trellising can help manage their size, these plants are often better suited for outdoor cultivation where they have more room to spread out.
4. How do temperature-sensitive plants fare in greenhouses?
   • Plants that thrive in cooler temperatures, such as lettuce, spinach, and broccoli, may struggle in the consistently warm environment of a greenhouse. These plants are better grown outdoors during cooler seasons or in a section of the greenhouse that can maintain lower temperatures.
5. Can I grow cacti and succulents in a greenhouse?
   • Cacti and succulents prefer dry soil conditions, which can be challenging to maintain in a greenhouse with regular watering practices. These plants can be grown in greenhouses with excellent drainage and less frequent watering.
6. What about shade-loving plants?
   • Shade-loving plants like ferns and hostas are not well-suited for greenhouses designed to maximize sunlight exposure. These plants thrive in shaded outdoor areas or in specially designated shaded sections of the greenhouse using shade cloth.
7. Why are wind-pollinated plants difficult to grow in a greenhouse?
   • Plants like corn rely on wind for pollination, which is limited in a greenhouse environment. Hand pollination or choosing self-pollinating varieties can improve success rates for these plants.
8. Are there any aggressive or invasive plants to avoid in a greenhouse?
   • Aggressive plants such as mint can quickly spread and dominate a greenhouse, making it hard to manage other plants. Growing these plants in contained pots or outdoor gardens helps control their spread.
9. How can I prevent disease and pest proliferation in a greenhouse?
   • Proper ventilation, disease-resistant plant varieties, and regular monitoring are key to preventing disease and pest issues in a greenhouse. Maintaining a clean environment and using organic pest control methods can also help.

 

Conclusion for What Not to Grow in a Greenhouse?

By understanding “What Not to Grow in a Greenhouse?” you can avoid unnecessary challenges and create an optimal growing environment. Using this knowledge, you can select plants that will do well, ensuring your gardening efforts are more gratifying and pleasant.