The Best Vegetables to Grow in a Greenhouse (And what NOT to Plant in a Greenhouse)

A greenhouse is a wonderful asset to the zone 3 backyard garden, or to any northern garden. Benefits of a backyard greenhouse include:

• warmer growing conditions

• protection from wind

• protection from hail

• extended growing season

Maybe your house came with a small backyard greenhouse or maybe you have been able to build one yourself. Either way, once you have one, the next step is determining the best vegetables to grow in a greenhouse in Canada, or whatever climate you are in.

A photo of my small, unheated backyard greenhouse near Calgary Alberta

The Best Vegetables to Grow in an Unheated Greenhouse

When deciding what vegetables to grow in a backyard greenhouse, look for fruit and vegetables that 1) require a longer growing season 2) require plenty of heat.

In zone 2 - 6 gardening climates, cool nights can quickly slow down heat-loving fruit and vegetables. With our short growing season, this slowdown in growth may result in vegetables and fruit not reaching maturity before the first frost.

Fruit and vegetables that you want to grow in a greenhouse if you have one include:

• Basil (this heat loving herb is a wonderful companion plant for tomatoes!)

• Cucumbers

• Peppers

• Tomatoes

• Squash (only if you have plenty of space for it to spread)

• Melons

• Eggplant

• Cucamelons (less common but fun to grow)

Note that corn is a heat-loving plant but not recommended for greenhouses as it is too tall and requires block planting and wind for pollination. I go more into detail on that in my blog post on planting corn.

Cucumbers growing up a trellis inside my unheated backyard greenhouse

What Not to Grow in a Greenhouse

Not all vegetables grow better in a greenhouse. As a zone 3 gardener, it is great to realize that many of the common vegetables grown in northern gardens actually thrive in cooler weather! There are so many cool-loving veggies that suffer if planted in a hot greenhouse. The heat can stunt growth and cause plants to bolt. Plants that I would not grow in a greenhouse over summer include:

• Beets

• Lettuce

• Spinach

• Carrots

• Brassicas (cabbage, broccoli, cauliflower)

• Onions

• Garlic

• Potatoes

• Zucchini (it grows so well outdoors there is no need for a greenhouse)

Perennials like strawberries and asparagus would not overwinter well in a greenhouse due to the continual heating and thawing throughout the winter.

Growing in the early and late seasons is a whole different story and the above lists are not applicable for that. I cover this topic here in this Guide to Growing Cold Hardy Plants Under Cover - Unheated Greenhouse Growing.

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Tips for Growing Vegetables in a Greenhouse

Greenhouse Ventilation

Some sort of ventilation is important in your greenhouse to prevent humidity running too high. High humidity along with heat creates an ideal environment for bacteria and disease. Diseases like powdery mildew can be an issue inside greenhouses. I have solar fans from Amazon that work great for ventilating my small 10 foot x 11 foot greenhouse.

Greenhouse Pollination

Pollinating tomatoes in a greenhouse has to be more purposeful than tomatoes grown outdoors. Tomato blossoms are pollinated by the wind when outdoors. The wind causes the plant to move, shaking the pollen and allowing it to fall on nearby blossoms. Inside a greenhouse you can shake your tomato plants, run your hands over the blossoms, or use a paintbrush or Q-tip.

Pollinating cucumbers and squash in a greenhouse can happen naturally if you have plenty of bees around. The only way to ensure pollination is by doing it by hand. Male blossoms will only have a plant stem beneath the blossom, where female blossoms will have what looks like a baby fruit. Run a Q-tip or paintbrush around the inside of the male blossom and then do the same around the inside of the female blossoms to ensure pollination of cucumbers and squash.

In the photos below, the female cucumber blossom is clearly shown on the right, and a male blossom in the left photo as the top blossom.

The male blossom is on top in this first photo, with a clear female blossom shown on the right photo

Female cucumber blossom

Greenhouse Temperature Control

I love my min/max thermometer to track temperature highs and lows inside my greenhouse. This way I know when my greenhouse is staying consistently above freezing, how many degrees warmer it stays at night than the outdoor temperatures and can track if it is getting too hot. If a greenhouse is too hot it may cause tomato blossom drop. I haven't had an issue with that since installing my ventilation fans, thankfully.

Greenhouse Irrigation

If you are able to irrigate your greenhouse, consider drip irrigation or something near the soil that doesn't cause unnecessary moisture on the leaves. As with high humidity, water on the leaves of tomatoes, cucumbers and squash can easily lead to powdery mildew or other diseases. Tomatoes especially are prone to disease, so protecting them any way possible just makes sense.

Check out my blog posts on growing tomatoes, cucumbers, and peppers for more tips on growing these finicky plants.

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