When Can Tomatoes Go In An Unheated Greenhouse

Growing tomatoes is one of the first and best challenges a gardener faces. Aside from gardeners who don’t eat tomatoes, they’re one of the most popular vegetables and one of the first ones people want to grow on their own. 

But, tomatoes offer many problems to growers of all experience levels. Having a greenhouse helps, but if your greenhouse is unheated, you still need to ask at least one big question before planting. 

When can tomatoes go into an unheated greenhouse? 

Tomatoes are relatively temperature sensitive, which means you need to make sure the air and ground are warm enough before planting. Plant too early, and your tomatoes may struggle and die. Most gardeners recommend planting outside or in unheated greenhouses in the northern hemisphere toward the end of April, but that’s only a guideline.

It’s better to judge off temperature. Tomatoes do best when it’s consistently about 55 degrees Fahrenheit or 12 degrees Celcius during the day and never dips below freezing at night. Of course, there are a lot of different factors when it comes to planting in a greenhouse. Here’s what you need to know. 

When Can I Put Tomatoes In An Unheated Greenhouse

If you have an unheated greenhouse, you get a lot of the benefits of a heated greenhouse but without the added expense and maintenance. However, heated greenhouses usually have a longer growing season than unheated ones because they have less temperature variability. 

When it comes to tomatoes, cold weather is a natural killer. It’s important to ensure your greenhouse is ready to support your tomatoes before planting, especially if you’re planting in the ground inside your greenhouse. 

You also need to remember that both air temperature and soil temperature matter. Air temperature is more variable than soil temperature, which means it may be warm enough outside several days before the soil in your greenhouse is ready for planting. 

For good measure of soil temp, wait until the soil you’re planting in feels loose and soft in your hands without a thermometer. It should feel slightly cool and moist but not cold, and it should be soft enough that you can pick up a handful without tools. 

In most places in the Northern hemisphere, your greenhouse will meet those conditions no later than late April. You might even be able to plant sooner depending on how well insulated your greenhouse is and how much direct sun exposure it gets. 

In the best greenhouses, you might be able to plant as early as 6-8 weeks before the last frost date in your area. However, you should leave a thermometer in the greenhouse overnight and check soil conditions before planting to ensure it doesn’t get too cold. 

How Cold Can Tomatoes Tolerate In A Greenhouse? 

Being in a greenhouse doesn’t make tomatoes any more cold-tolerant. The main benefit of a greenhouse is that they hold more heat than the outside air and stay warm longer than the surrounding environment. 

Your tomatoes will be happiest if the daytime temp inside your greenhouse is between 55-80 degrees Fahrenheit and never drops below 40 degrees at night. However, as long as your greenhouse doesn’t drop below 35 degrees Fahrenheit they should be fine. 

The less time spent below 40 degrees, the better for your tomatoes. 

Should I Bring My Tomato Plants Inside At Night?

If you’re container planting your tomatoes, you might be tempted to move them outside for more sunshine before you can plant them full time. 

In that case, it’s probably a good idea to bring your tomatoes in as soon as the temperature starts to dip in the late afternoon. 

However, if you’re planting outside or in an unheated greenhouse and it’s warm enough overnight, there’s no reason to move the plants. Moving your tomatoes too often can damage the leaves and stems and stop them from flowering. 

So, if you want to give your tomatoes a little extra light through the day, you can move them in and out. Make sure you bring your tomatoes inside before it dips below 40 degrees each night. 

As a tip, try not to move your tomatoes too much once they’ve gotten too big. Move them all you want until the plants are about 6-8 inches tall. After that, you should minimize how often you’re moving them around. 

How Do I Keep My Greenhouse Warm At Night?

If you have an unheated greenhouse but still want to use it year-round, or at least longer than you otherwise could, there are a few options to help keep the greenhouse warm overnight. 

One of the easiest and most effective methods is to add efficiently heated thermal mass. This can be things like lining the floor of your greenhouse with paving stones that will heat up during the day and then slowly release heat overnight, keeping things warmer. 

Raised beds can also work as additional thermal mass as long as they’re separate from the ground since the soil will heat up more when it’s lifted above the ground. 

Another option is covering the greenhouse or covering the plants inside the greenhouse. Usually, covering the whole greenhouse is a little more effective for holding in the heat, but it can be harder to do. Plant row covers can still help hold in some extra heat without the added cost or effort it takes to pull a tarp or canvas cover over your whole greenhouse. 

The last option we’re going to cover in this article is compost. Compost naturally produces some of its heat, which can easily be used to help heat your greenhouse. 

You’ll need to manage the compost temperature, including cooling it off by adding slow-decomposing ingredients or more water if it gets too warm. But, if you can manage that, having a small compost pile in your greenhouse makes it more accessible to add to your plants; it provides heat and can help your compost turn into high-quality black gold a lot faster. 

How Much Earlier Can You Plant In A Greenhouse?

Planting in a greenhouse means typically you can plant a few weeks earlier, including you can start seeds a little earlier or move sprouted plants outside a little sooner. 

There’s no limit to when you can plant in a heated greenhouse. Instead, your limits are based on how much heat you’re willing to add to the greenhouse and how much light your plants get at different times. 

Even an unheated greenhouse can extend your growing season by several weeks at both ends. 

We would expect an unheated greenhouse to extend your growing season by 4-6 weeks at the beginning of the year, but eight weeks is also relatively common. 

The trick is to pay attention to the temperature inside and outside the greenhouse. Once your greenhouse is consistently above freezing temperatures, even overnight, you can usually start planting. 

However, if you start your seeds inside 4-8 weeks earlier, you’ll be able to maximize your growing season. 

When Should I Start My Greenhouse Tomatoes? 

Greenhouses are usually ready for tomatoes when they hover around 40 degrees for the lowest temperature. You can start a little earlier if you want to, but tomato growth is slower between 35-40 degrees and stops entirely under 35. Under 32 degrees and your plants are risking damage or death. 

Most greenhouses manage consistent temperatures in the right range between March and April. 

However, you can start tomato seeds inside even earlier to make the most of your growing season. Move the tomatoes out into your greenhouse when they’re 3-6 inches tall, OR your greenhouse reaches the right temperature, whichever happens last. 

Things To Consider

Growing in a greenhouse helps with a lot of tomato growth. However, you’ll still need to make sure your tomatoes are getting plenty of water and that you’re keeping pests under control. 

An airtight greenhouse can help reduce how much water your plants need, condensation will help keep the soil moist, but tomatoes don’t do well if they get too dry. If temperatures get too hot, they may also need extra water (or an open window to let in the breeze). 

Anything much above 80 degrees and your tomatoes will need a little extra support to keep them healthy and alive. 

Growing in a greenhouse also doesn’t change that your tomatoes will need plenty of compost and fertilizer to stay healthy. If you’re growing in containers or raised beds, you’ll need to give your tomatoes more compost and fertilizer to make up for their isolated growing condition. 

It’s also important to remember that the same rules about companion plants and plants that don’t play well together apply inside a greenhouse as apply outside. Tomatoes might do well planted with cucumbers or basil, but they don’t play nice with other nightshades, brassica vegetables, or strawberries.