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Your hobby greenhouse can do a few things really well:

1. It can protect your fragile (and expensive) crops from getting destroyed by those prairie hail storm.
2. It can give you a head start on growing early in the season (harvesting your own lettuce and spinach in April? Uh, Yes Please!!)
3. It gives you space (other than the kitchen window) to grow seedlings for multiple planting throughout the year.
4. It is a hot spot for growing plants that require warmth

Not all plants require warmth. In fact, many of the crops we tend to grow in Alberta gardens are actually cool weather crops. 

Here are some food plants that do best in COOL weather:

1. Peas (I plant mine in April, just as soon as the soil is thawed enough to work it. Soak the seed for a day before you stick them in the earth and you’ll get about a week’s start on them sprouting).
2. Beans (ditto with the peas, you can plant them quite early).
3. Lettuce*
4. Chard
5. Spinach*
6. Carrots
7. Broccoli
8. Cauliflower
9. Cabbage
10. Kohlrabi
11. Brussels sprouts

Numbers 7-11 are all brassicas. Brassicas are a cool weather crop. They are also susceptible to the dreaded cabbage moth. Growing them in the greenhouse won’t save them because bugs can easily fly in through the open door or other open vents that you are using to keep the place from getting too hot. Consider a floating row cover for them, right from the moment you put them in the ground.

Don’t waste your greenhouse space growing cold weather crops. They’ll suffer and underperform in the heat.

Hot Stuff in the Hothouse

You save that hot space for plants that really need it:

1. cucumbers (I’m growing patio snackers like Picolino)
2. tomatoes
3. sweet peppers
4. egg plant
5. herbs (basil, rosemary)
6. melons*

*The Exceptions

Consider planting spinach seeds in your greenhouse late in fall just before everything freezes. And water them in well. In spring, while you are waiting for the snow to melt, the spinach will be one of the first things to sprout and you’ll be harvesting from the greenhouse in April. Kind of like a walk-in cold frame.

Lettuce is a fast growing plant that prefers a bit of cool weather or even shade. Plant your lettuce in containers in your greenhouse in early spring. When it starts getting consistently hot, move the lettuce container outside (unless you’ve already harvested and eaten everything!). You can even get multiple crops of leafy greens from your greenhouse if you plant them up every two weeks starting in late March or early April.

Melons like heat but they also like to take up a lot of room. A single plant can take over your greenhouse! Start your melon seeds early and plant them outside. Choose varieties that have a short maturity so that you increase your odds of getting fruit before the first frost hits in the fall.