When I first started growing tomatoes, I was a little worried about what my harvests would be like: would I be able to get decent yields by planting them outside in the PNW? Or would it be worth investing in a greenhouse to grow them?

In my first summer, I decided to plant all my tomatoes outdoors. Despite my best efforts, the harvest was underwhelming. This experience pushed me to consider investing in a high tunnel.

Like many fellow gardeners, I was on a quest to discover the best way to grow a bountiful harvest of juicy, delicious tomatoes. After my first summer’s disappointing outdoor harvest, I decided to buy a greenhouse the following summer. The promise of extended harvests and better yields, especially for my prized tomato plants, seemed worth the investment. And after experimenting with both approaches, I’ve gathered valuable insights about what to expect from each method

In this article, I’ll share my personal experiences and compare the nuances of growing tomatoes in a high tunnel versus a traditional garden. I’ll tell you the benefits and challenges of each method from what I’ve learned.

If you’re a gardener in the PNW or a similar climate, my hope is that you’ll have a clearer idea of which method would work best for you based on my experiences and lessons.

Temperature Regulation

• Greenhouse-grown tomatoes generally enjoy ideal temperatures for ripening and growth, but there are exceptions.
• A shade cloth and venting system can help reduce the temperature of a greenhouse during heatwaves.
• Raised bed gardens have the advantage of better soil structure and less compaction, so plants can thrive and access water more easily.

After my first summer’s disappointing outdoor harvest, I invested in a polytunnel, and it’s been super helpful, especially with our cool nights.

Oregon State Extension says tomatoes love temps between 65-85°F. In Portland and Seattle, our nights usually dip below 60, so the greenhouse keeps them in their happy zone for around 8 extra hours a day.

Plus, the high tunel lets me plant my seedlings about a month earlier than I could outside. Sure, I could keep them inside in pots, but they really thrive when they’re able to settle into their planted location during April and May before the growing season officially begins.

But here’s the kicker... with climate change, we're seeing more heatwaves in the PNW. Our summers are normally pretty pleasant, but lately, Seattle and Portland are getting hotter than usual.

For example, The Seattle Times reported that in early July 2024, Seattle's temps averaged 86°F, which is 12 degrees higher than our norm.

This means while greenhouses are great for tomatoes in normal weather, they can get too hot on those extra warm days. When temps go above 85°F, tomatoes stop ripening. And working in a sweltering greenhouse isn't fun either!

I can prop the door open and roll up the sides of my Quonset-style polytunnel to cool it down some, but the plastic cover still makes it hotter inside than out.

For example, during the heat dome of 2021, my tomato plants really suffered and took around a month to recover. The harvest wasn’t that great, which was quite discouraging.

So, it’s a bit of a trade-off. The greenhouse helps ripening when the temperatures are average, but can hinder it when it gets super hot.

Currently, my high tunnel-ripened tomatoes consistently produce bigger, earlier, and longer-lasting harvests compared to my outdoor plants. Even the last couple of years, my outdoor tomatoes have just barely finished ripening before temperatures get too cool for their liking.

Even though the PNW has a long stretch of the year with frost-free days, WSU recommends that with our cool temperatures, we should focus on fast-maturing tomato varieties.

With the season-extending capabilities of my greenhouse, I’m able to not worry too much about the days to maturity of the tomato varieties that I select each year. Being able to grow tomatoes that wouldn’t thrive outdoors here gives me a huge advantage, especially since I focus on growing heirloom paste varieties that typically need a growing season of 80 or more days.

Humidity Control

• During most growing seasons, air circulation outdoors helps regular humidity compared to greenhouse tomatoes
• While costly, setting up fans in the polytunnel can help remedy this

Here in the PNW, our summers are typically quite dry, with decent breezes moving through during the evenings. This gives outdoor tomatoes an advantage - the dry air quality and improved circulation helps to prevent fungal issues and disease.

Inside a closed greenhouse, though, there’s far less air circulation going on. Additionally, a greenhouse is like putting a humidity dome over a seed starting tray - great for germination, but not so great for disease susceptible tomato plants.

I can manage humidity levels within the high tunnel if I roll up the sides, but there’s still higher levels of humidity inside the structure.

I also trellis my tomatoes inside of the greenhouse, which helps the plant’s leaves to get some air flow around them and manages plants that would otherwise spread out across the ground.

Overall,  my outdoor-grown tomatoes face fewer disease issues, thanks to better air circulation and lower humidity levels.

Water Management

• The covered environment of a polytunnel means that there can be no overhead water source if drip irrigation is in place.
• You can create a covered area over your outdoor tomatoes to remedy this issue

One of the major perks of greenhouse irrigation systems is their consistency; they ensure my tomatoes get just the right amount of water, minimizing waste.

In the garden, natural rainfall can cause wet leaves and disease issues.

Typically, though, summers in the PNW are nearly free from rainfall. This means I don’t have to worry about overhead rainfall for most of the growing season.

However, my outdoor plants do suffer some when September rolls around and the rain returns. Since that’s when my outdoor tomatoes start to really ripen, the fruit can get damaged from staying wet during all the rainfall.

Growth Rate

• Greenhouse grown tomatoes mature faster and grow bigger compared to outdoor plants in the PNW

All these controlled conditions contribute to a faster and more reliable growth rate for my greenhouse tomatoes. I’ve noticed they often grow quicker and healthier compared to those in my garden.

Notes on Tomato Pollination in a Greenhouse

Many people worry about pollination for their polytunnel grown tomatoes.

Honestly, I haven’t worried too much about this because I always grow a variety of flowers in the greenhouse, and leave a few broccoli plants from my spring crop stay to provide a diverse source of pollen and nectar all summer long.

Because of this, I often see a variety of bees, wasps, and butterflies in my high tunnel.

If my polytunnel was strictly food crops, then I probably would have to worry more about hand pollinating my tomatoes. This is especially true when you consider how the wind that naturally helps tomatoes pollinate outdoors is nearly absent in a greenhouse.

Should You Grow Tomatoes in a Greenhouse or Garden in the PNW? My Takeaways

If you’re a gardener in the PNW or a similar climate, my hope is that you’ll have a clearer idea of which method would work best for you based on my experiences and lessons. My decision to invest in a greenhouse after that first underwhelming summer has paid off, giving me extended harvests and healthier plants. I hope my journey helps you make the best choice for your garden.

‍

Read more on Tomatoes + Greenhouses:

‍Mistakes I Made When Growing Tomatoes in a Greenhouse (and How I Fixed them)

Top Picks for Eco-Friendly Greenhouse Kits for Your Backyard

‍

‍

‍

‍