What Not To Grow In A Greenhouse

Growing plants in a greenhouse is a fantastic way to extend your season and protect your crops. But knowing what not to grow in a greenhouse is just as important as knowing what to thrive inside one. Making the wrong choices can lead to wasted effort, poor harvests, and even encourage pests. Let’s look at some common greenhouse mistakes and how to avoid them, so you can make the most of your protected space.

It might seem like a greenhouse is the perfect place for any plant. However, some plants simply don’t benefit from the unique conditions. In fact, putting them inside can cause more problems than it solves. The controlled environment can work against plants that need specific triggers to grow or set fruit properly.

Plants That Need Wind Pollination

Many plants rely on wind to move pollen from male to female flowers. In the still, enclosed air of a greenhouse, this process often fails completely. You might get beautiful plants with lots of flowers, but little to no fruit.

Corn: Each kernel needs to be pollinated by wind-blown pollen. Without strong air movement, you’ll get ears with lots of missing kernels.
Wheat & Other Grains: These are classic wind-pollinated crops that are better suited to open fields.
Many Trees: If you’re trying to fruit trees like pecans or walnuts in pots, they will likely struggle without ample wind.

Plants That Require Chill Hours or Dormancy

Some plants need a period of cold winter dormancy to reset their growth cycle. A greenhouse that stays too warm in winter prevents this crucial rest period. The plant will become weak and fail to produce.

Blueberries: Most varieties need hundreds of hours below 45°F. A warm greenhouse in winter won’t provide this.
Lilacs: These shrubs require a good chill to set their spectacular spring blooms.
Peonies: Their roots need a cold period to develop the energy for those large flowers.
Apples and Pears: While dwarf trees can be grown in large containers, they must experience winter chill to fruit reliably.

Extremely Large or Spreading Plants

Space is a premium in any greenhouse. Plants that vine aggressively or grow very large can quickly take over, shading out everything else and becoming a managment nightmare.

Pumpkins and Winter Squash: These plants can cover dozens of square feet. Their large leaves create too much shade and humidity.
Field Corn: It grows very tall and will quickly outgrow the height of most hobby greenhouses.
Artichokes: They become huge, spiky plants that demand a lot of room for multiple seasons.
Raspberries & Blackberries: Their spreading canes are thorny and difficult to control in a confined space.

Plants That Attract Too Many Pests

A greenhouse’s warm, humid environment is a paradise for certain pests. Growing plants that are especially susceptible can create a persistent infestation that’s hard to eliminate and can spread to other crops.

Fava Beans: They are highly attractive to black aphids, which can swarm in a greenhouse.
Some Cucumber Varieties: Older varieties can be magnets for whiteflies and spider mites, though many newer hybrids have better resistance.

Better Alternatives for Your Greenhouse

Instead of those problematic plants, focus on crops that truly benefit from protection. This includes heat-lovers, tender perennials, and plants with long seasons.

Tomatoes, Peppers, and Eggplants: These classics produce earlier and for much longer under glass.
Cucumbers and Melons (on trellises): Use vertical space to manage vines and improve air circulation.
Lettuce and Salad Greens: Grow them in the cooler shoulder seasons for a continuous harvest.
Basil and Other Tender Herbs: They will thrive long after outdoor herbs have faded.
Starter Plants: The best use of all! Start all your summer vegetables and flowers from seed.

Common Greenhouse Mistakes to Avoid

Choosing the wrong plants is just one pitfall. Here are other critical errors that can undermine your greenhouse success.

1. Poor Ventilation and Airflow

Stagnant air encourages fungal diseases like powdery mildew and botrytis. It also lets heat build up to dangerous levels on sunny days, even in winter.

Always install roof vents that open automatically or manually.
Use a circulating fan to keep air moving gently around plants.
Don’t be afraid to open the door on mild days.

2. Ignoring Temperature Extremes

Greenhouses can overheat rapidly and also get much colder than you’d expect at night. You need to manage both.

Use shade cloth in the peak of summer to prevent scorching.
Invest in a heater for winter if you’re keeping tender plants, or use thermal mass like water barrels to buffer temperatures.
A simple max-min thermometer is your most important tool.

3. Forgetting About Humidity Control

High humidity leads to disease, while low humidity stresses plants and encourages spider mites.

Water the floor (damping down) on hot days to cool the air, but do it in the morning so things dry by evening.
Ensure your greenhouse has adequate ventilation, as mentioned above, to reduce humidity.
Space plants properly to allow air to move between them.

4. Skipping Pest Prevention

Pests that get inside a greenhouse face no natural predators and can multiply explosively. Prevention is everything.

Check every plant thoroughly before bringing it inside.
Use insect netting on vents and doors to keep bugs out.
Introduce beneficial insects like ladybugs or predatory mites early, before a problem starts.
Keep the area around your greenhouse clear of weeds and debris where pests can hide.

5. Overcrowding Your Plants

It’s tempting to fit in as many plants as possible, but this creates competition for light and water. It also drastically reduces airflow, creating a perfect microclimate for disease.

Follow spacing guidelines on seed packets, and remember plants will often grow larger in the protected greenhouse environment then they do outside. Give them room to breath.

Steps for a Successful Greenhouse Season

Plan Your Layout: Decide what you’ll grow based on season and space. Group plants with similar water and temperature needs together.
Prepare the Soil: Use fresh, high-quality potting mix for containers. If planting in ground beds, amend the soil with plenty of compost.
Implement Climate Control: Set up your fans, vents, heater, and shade cloth before you need them. Don’t wait for a crisis.
Plant with Care: Sow seeds or transplant seedlings, ensuring proper spacing from the start.
Monitor Daily: Check temperature, humidity, and look for early signs of pests or disease every single day. Catching issues early is key.
Water Wisely: Water the soil, not the leaves, and do it in the morning. Overwatering is a common cause of root rot in greenhouses.
Harvest Regularly: Picking fruits like tomatoes and cucumbers encourages the plant to produce more.

FAQ: Greenhouse Growing Questions

What plants should you avoid in a small greenhouse?

Avoid large, sprawling plants like pumpkins, tall corn, and fruit trees. Also, be cautious with plants that need insect pollination unless you’re willing to hand-pollinate them yourself.

Is it bad to grow mint in a greenhouse?

It can be. Mint is incredibly invasive. If planted in a greenhouse bed, its roots will spread everywhere and it’s roots can be hard to remove. Always grow mint in its own separate container, even in the greenhouse.

Can you grow root vegetables in a greenhouse?

Yes, but it’s often not the best use of space. Root veggies like carrots and potatoes don’t need the heat protection as much as tomatoes do. They are better grown outdoors, but can be excellent for early spring or late fall harvests in the greenhouse.

How do you keep a greenhouse from getting too hot?

Use automatic vent openers, install exhaust fans, apply whitewash or shade cloth to the roof, and make sure you have good air circulation inside with fans. Watering the floor can also provide cooling.

By understanding what not to grow in a greenhouse and steering clear of these common errors, you set yourself up for a much more productive and enjoyable gardening experience. Your greenhouse should be a place of abundance, not frustration. Focus on the plants that truly revel in the protected environment, manage your climate carefully, and you’ll be rewarded with a fantastic harvest.