Plant Selection – What Thrives and What Doesn’t in a Greenhouse
Selecting the right plants for your greenhouse is key to maximizing productivity and ensuring healthy, thriving crops. While many plants benefit from the controlled environment of a greenhouse, others struggle due to factors like heat, humidity, and space constraints. In this guide, we’ll explore the best plants to grow in a greenhouse, those that may require extra attention, and those that are better suited for outdoor cultivation. We’ll also touch on pest management considerations to keep your greenhouse thriving.
Cool-Weather Staples and Early Starter Plants
Greenhouses are fantastic for jump-starting the growing season by allowing cool-weather crops to mature weeks earlier than they would in an outdoor garden. These plants tolerate lower temperatures and thrive in the fluctuating spring conditions of an unheated greenhouse.
Best Cool-Weather Crops for a Greenhouse:
- Lettuce – Grows quickly and can be harvested as baby greens or full heads.
- Spinach – Prefers cool temperatures; avoid letting it bolt by keeping temperatures moderate.
- Kale – Hardy and cold-tolerant, kale grows well in early spring and fall.
- Radishes – Fast-growing and ideal for staggered planting to ensure continuous harvests.
- Peas – Can be started much earlier in a greenhouse, producing pods weeks before outdoor crops.
Tips for Getting a Head Start for Plants in Early Spring:
- Use Row Covers or Cold Frames Inside the Greenhouse – This adds an extra layer of insulation for particularly cold nights.
- Plant in Raised Beds – Soil in raised beds warms faster, helping plants establish quickly.
- Select Fast-Maturing Varieties – Look for early-maturing types of lettuce, radishes, and peas to make the most of your greenhouse space.
- Ventilate Properly – Even cool-weather crops can suffer if the daytime temperatures climb too high in early spring.
Warm-Weather Favorites
The real power of a greenhouse is in heat-loving plants that would otherwise struggle in Alberta’s short summer. A greenhouse allows these crops to develop earlier, produce longer, and often yield better quality fruit than outdoor-grown counterparts.
Best Warm-Weather Plants for a Greenhouse:
- Tomatoes – Thrive in the stable warmth of a greenhouse and produce abundant fruit with proper pruning and support.
- Cucumbers – Benefit from the controlled heat but require good airflow to prevent mildew.
- Peppers – Whether bell peppers or hot varieties, they develop best in consistently warm conditions.
- Eggplants – Need high temperatures and long growing seasons, making them a prime greenhouse candidate.
Key Factors for Growing Warm-Weather Crops:
- Daytime Heat Buildup – These plants thrive on daytime temperatures between 25–30°C (77–86°F). Properly positioning your greenhouse to maximize sun exposure is crucial.
- Nighttime Temperature Maintenance – Avoid temperature drops below 10°C (50°F) by adding thermal mass (e.g., water barrels, rocks) or using row covers.
- Pollination Assistance – Because a greenhouse lacks natural wind or insect activity, hand-pollinating tomatoes, cucumbers, and peppers can increase fruit production.
- Good Ventilation – Warm-season crops require airflow to reduce humidity and prevent diseases like powdery mildew.
Herbs and Specialty Crops
Greenhouses are ideal for herb cultivation, as they provide consistent temperatures and protection from the elements. Some herbs grow year-round in controlled environments, while others need more warmth to thrive.
Best Greenhouse Herbs:
- Basil – Loves heat and humidity; grows well in pots or raised beds.
- Parsley – Can tolerate cooler conditions but flourishes in greenhouse settings.
- Thyme – Prefers drier conditions, making good airflow essential.
- Microgreens – These small, nutrient-packed greens grow exceptionally well indoors, offering a continuous harvest.
Year-Round Herb Options in a Controlled Climate:
- Winter: Microgreens, parsley, chives (cool-weather herbs)
- Spring & Summer: Basil, rosemary, oregano, mint (heat-loving herbs)
- Fall: Cilantro, dill, sage (transitioning crops that tolerate moderate temperatures)
Best Growing Practices for Greenhouse Herbs:
- Use Containers for Portability – Herbs like basil and rosemary benefit from being moved to different spots depending on light and temperature needs.
- Avoid Overwatering – Many herbs prefer well-draining soil; too much moisture leads to root rot.
- Prune Regularly – Frequent trimming encourages bushy, healthy growth and prevents herbs from flowering too soon.
Plants That Struggle in Greenhouses
While greenhouses provide optimal growing conditions for many crops, some plants are better suited for outdoor gardens due to their soil, space, or temperature needs.
Root Vegetables That Prefer Cooler Soil:
- Carrots
- Potatoes
- Beets
These crops generally prefer cooler soil and develop best outdoors. However, if grown in deep raised beds, they can flourish in a greenhouse if temperatures are carefully managed.
Crops Sensitive to High Humidity or Extreme Heat:
- Brassicas (broccoli, cauliflower, cabbage) – These prefer cool, outdoor conditions and can struggle in a humid greenhouse.
- Lettuce in Summer – Bolts quickly in excessive heat.
Plants That Overtake Space:
- Large Squash Varieties (Pumpkins, Zucchini, Winter Squash) – These grow aggressively, need pollination, and can quickly take over valuable greenhouse space.
- Corn – Requires wind for pollination, making greenhouse growth difficult.
Best Alternatives for Large or Difficult Crops:
- Instead of zucchini or pumpkins, grow compact bush varieties in containers.
- Instead of potatoes, try small-rooted radishes or turnips.
Pest Management Considerations with Edible Plants
One of the downsides of a greenhouse is that pests can thrive in the warm, enclosed environment. Without natural predators or rain to wash them away, infestations can spread rapidly if not monitored.
Common Greenhouse Pests:
- Aphids – Thrive in humid conditions and quickly multiply.
- Whiteflies – Feed on plant sap and weaken crops.
- Spider Mites – Prefer dry, hot conditions and are difficult to eradicate.
- Fungus Gnats – Often a sign of overwatering; their larvae damage plant roots.
Organic vs. Synthetic Controls:
Method | Pros | Cons |
---|---|---|
Beneficial Insects (Ladybugs, Lacewings) | Chemical-free, environmentally friendly | Need to maintain a suitable habitat for them |
Neem Oil | Effective against soft-bodied pests, organic | Must be applied regularly |
Sticky Traps | Good for monitoring pest populations | Won’t eliminate an infestation |
Chemical Pesticides | Fast-acting, highly effective | Can disrupt the greenhouse ecosystem and kill beneficial insects |
Best Pest Prevention Tips:
- Regularly Inspect Plants – Catch infestations early before they spread.
- Encourage Beneficial Insects – Introduce ladybugs or predatory mites to keep populations in check.
- Improve Air Circulation – Many pests (such as spider mites) thrive in stagnant air; use fans to disrupt them.
Choosing the right plants for your greenhouse is essential for maximum productivity and minimal maintenance.
✅ Best for Greenhouses: Cool-season crops (lettuce, radishes, peas), heat-loving vegetables (tomatoes, cucumbers, peppers), and herbs.
❌ Better Outdoors: Large, sprawling crops (squash, corn), root vegetables needing cool soil, and plants sensitive to high humidity.
With careful planning and proper pest management, you can enjoy extended harvests and higher yields in your greenhouse—even in Alberta’s challenging Zone 3 climate.
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- Introduction to Greenhouses
- Greenhouse Materials
- Greenhouse Beds
- Selecting Greenhouse Plants