I know greenhouses create a warmer temperature for plants to grow, but I was wondering if there’s a best temperature for my greenhouse. I want to make sure my plants have the perfect environment for growth.
What is the best temperature for a greenhouse? The best temperature for a greenhouse is between 80 and 85 degrees Fahrenheit. That’s an easy answer, but the best temperature really depends on which plants you’re growing in your greenhouse. Different plants like different temperatures.
That may leave you wondering where in the world to start. Well, I’ve done the research and I can show you the best temperatures for a number of different plants you might think about growing.
A Guide to Plant Temperatures
The first step to figuring out the best temperature for your greenhouse is to understand that plants are broken up into four groups depending on how they tolerate the temperature.
The four groups are: very tender plants, tender plants, semi hardy, and hardy plants.
Very tender plants: These plants grow best in very warm weather with 8 to 12 hours of sunlight. Any cold temperatures will really affect their growth. These types of plants also have longer growing seasons.
Tender plants: Tender plants can’t tolerate temperatures any lower than 60 degrees Fahrenheit. They need at least eight hours of sun and can’t tolerate any frost. These plants grow best in warmer weather.
Semi-hardy plants: Temperatures for semi-hardy plants should stay above 50 to 55 degrees. Semi-hardy plants can handle light frosts and require at least six hours of sunlight.
These plants are sometimes also known as semi-woody.
Hardy plants: Hardy plants are ones that can tolerate temperatures as low as 45 degrees. These plants can handle harder frosts and some vegetables can even taste a little sweeter after a little frost.
They also require less sunlight, only 3 to 6 hours of direct sun. Some hardy plants actually die if it gets too hot.
These plants are sometimes classified as woody rather than hardy.
Obviously, a greenhouse gives you more freedom as far as what plants you can plant when, but it’s still a good idea to look at the United States Department of Agriculture’s plant hardiness zone map.
This map helps growers figure out which plants grow best where they live. The map is broken into zones based on the average annual minimum winter temperature. Find your location here.
Temperature Growing Guide for Plants
Very Tender: Some plants just love sun; they can’t get enough. These very tender plants include cantaloupe, eggplant, peppers, and tomatoes.
These plants thrive in very warm temperatures and plenty of sunlight. There’s the best temperature for germination and then another best temperature for growing. This chart gives you an idea of the best temperatures for tender plants.
| Plant | Best Germination Temp | Best Night Growing Temp | Best Day Growing Temp |
| Tomatoes | 70-80°F | 60-65°F | 70-80°F |
| Peppers | 75-85°F | 60-70°F | 70-80°F |
| Eggplant | 75-90°F | 65-70°F | 70-80°F |
| Cantaloupe | 75-95°F | 60-70°F | 70-90°F |
There are plenty of other plants that still like the sun, but don’t need it to be quite as warm. These tender plants include begonias, calla lilies, celery, summer squash and cucumber. Check out these temperature guidelines for these plants.
| Plant | Best Germination Temp | Best Night Growing Temp | Best Day Growing Temp |
| Begonias | 72-75°F | 60°F | 70°F |
| Calla Lilies | 65-75°F | >55°F | 60-75°F |
| Celery | 60-70°F | 55-65°F | 65-75°F |
| Summer squash | 75-95°F | 60-70°F | 70-90°F |
| Cucumber | 75-95°F | 60-70°F | 70-90°F |
Then, there are plants who don’t mind cooler temps and might even do well with a little frost. Some semi-hardy plants are geraniums, hydrangeas, beets, carrots and lettuce. Here’s a look at some temperature guidelines for those plants.
| Plant | Best Germination Temp | Best Night Growing Temp | Best Day Growing Temp |
| Geraniums | 75°F | 50-60°F | 65-75°F |
| Hydrangeas | Best from cuttings | 60°F | 70°F |
| Beets | 50-85°F | NA | 60-65°F |
| Carrots | 50-85°F | NA | 40-85°F |
| Lettuce | 65-75°F | 60-70°F | 60-70°F |
Finally, there are the hardy plants that embrace cooler temperatures and frosty weather. Yarrow, columbine, asparagus, broccoli, and peas like a good frosty environment.
Some of the hardy flowers don’t necessarily flower in the winter but won’t die off in cold temperatures. Check out these temperature guidelines.
| Plant | Best Germination Temp | Best Night Growing Temp | Best Day Growing Temp |
| Yarrow | 70-85°F | NA | 55-60°F |
| Columbine | 70°F | 50-60°F | 60-70°F |
| Asparagus | 70-77°F | 60-70°F | 75-85°F |
| Broccoli | 70-80°F | 50-60°F | 60-70°F |
| Peas | 75°F | 55-65°F | 55-65°F |
You’re probably wondering how plants like asparagus that like a growing temperature of 75 to 85 degrees are considered hardy and thrive in frosty weather. There’s certainly no frost happening in that temperature range.
That’s because many of these plants go into a dormant phase and something happens in that cooler weather that affects their growth in a positive way once warmer weather arrives.
Best Time to Plant-Based on Temperature
Even though you can manipulate the temperature in your greenhouse to some degree, there are still good times to plant certain crops.
Winter/Early Spring: This is the best time to plant frost tolerant plants like spinach, lettuce or broccoli. Since these plants can handle lower temperatures and frost, you won’t have to heat your greenhouse as much, if at all.
You can also start the plants in your greenhouse and then move them outdoors when evening lows stay above 30 degrees.
Mid-Spring: Spring is usually considered the official planting season. During this time, you can start planting quite a few more plants. Keep in mind, these plants need at least eight hours of sunlight. Melons, cucumbers and squash are good crops to plant during this time.
Summer/Late Summer: This is the time when sun loving plants should be planted. Eggplant, tomatoes and hot peppers all do great in very warm temperatures. Too warm in not good, though. A ventilation system can help cool things down.
Autumn: Once things start to cool down, it’s time to plant a round of vegetables that like cooler seasons. Kale, snow peas and turnips do well during this time. Another plus is you shouldn’t have to heat the greenhouse since these plants prefer cooler temps.
Controlling Your Greenhouse Temperature
Now that you know the ideal greenhouse temperature and the best times to plant, you may be wondering what you can do when your greenhouse isn’t in the right temperature range.
The first thing to do is buy a thermometer so you can keep an eye on the temperature. I always recommend this one from Amazon because it measures temperature and humidity. It’s always important to monitor both when it comes to your greenhouse’s environment.
Lowering the temperature: The first step to take in lowering the temperature is you need to install a ventilation system. Hopefully you already have one because every greenhouse needs a ventilation system. Even if you live in a wonderfully moderate climate and don’t need to lower the temperature, put a ventilation system in.
A ventilation system helps cool your greenhouse, regulates humidity and circulates air. If you haven’t put in a ventilation system, you might already have problems with your plants.
Other ways to cool your greenhouse include these options:
- Shade cloth
- Misters
- Evaporative cooler
- Portable air conditioning unit
- Placing larger plants that provide shade along the walls of the greenhouse
You can pick the best cooling option for your budget.
Raising the temperature: In cooler weather, you might need to bring up the temperature in your greenhouse. Here are some ways to do that.
- Compost pile (puts off heat)
- Water barrels painted black and filled with water
- Portable heater (don’t place too close to plants, could dry them out)
- Plant coverings
- Heat lamps (make sure you have a fan to circulate the heat)
So, as you can see, the right temperature for your greenhouse depends on exactly what you want to plant. You might plant one type of plant one season and another type another season. The beauty of a greenhouse is you can adjust the temperature to meet your plant’s needs.