When to Plant Tomatoes in an Unheated Greenhouse

When to Plant Tomatoes in an Unheated Greenhouse

It is crucial to know the correct time of year to plant your tomato crops. If you have an unheated greenhouse you will need to transplant your tomato seedlings at the appropriate time in order to get the strongest, healthiest tomatoes.

A long successful growing season is dependent on planting your tomatoes at the perfect time. Sowing your tomato seeds indoors in the middle of March will help to ensure your tomatoes thrive when you transplant them. Close to the end of April, you should transplant your tomato seedlings into your unheated greenhouse. Of course, these dates will vary a lot depending on where you live.

Where to Sow Tomato Seeds

Obviously, a greenhouse (even if it is unheated) will significantly extend your growing season. However, if it is freezing outside your unheated greenhouse will not be warm enough to sustain your tomato plants. Tomato plants require a temperature of 10 degrees Celsius or higher for proper growth. A lower temperature will damage your plants, especially if they are immature. A general rule of thumb is that you cannot sow tomatoes if it is frosty outside, even inside your unheated greenhouse. wait until the last expected frost dates in your particular geographical area before you consider sowing tomato plants inside an unheated greenhouse.

Learn more about growing tomatoes in a greenhouse here.

Start By Sowing Indoors

If you want to get five weeks ahead in your planting schedule sow your tomato seeds inside. By planting your tomato seeds inside and allowing them to grow for five weeks you can extend your growing season significantly.

Waiting until the last expected frost date means you may not sow your tomato seeds until April! If you plant them indoors you can start sowing in February. What a difference!

It is important to remember that tomato seeds take 5-6 weeks to become strong seedlings that are ready for transplanting.

Our recommendation is to take a look at your geographical zone and sow your tomato seeds indoors 5 or 6 weeks before the last expected frost date. For many, that means sowing your plants in late February and transplanting them into your unheated greenhouse at the beginning of April.

Can't I just sow the seeds in my greenhouse?

If you decide to wait and sow your tomato seeds directly in your unheated greenhouse you will need to be sure there is no chance of a frost happening. This may put you five or six weeks behind schedule.

The higher the temperature when you sow your tomato seeds the better chance they have of being healthy and strong. A temperature of around 10°C means there probably won’t be any more frost but it is still a little low for ideal germination. Tomatoes love the heat and the perfect germination temperature is between 21-27°C.

rustic tomatoes

At lower temperatures your tomatoes will likely germinate in your greenhouse, but it will take much longer for them to grow into strong and healthy seedlings. You probably don’t want this delay if you can help it, which you can!

You can either wait to plant in your unheated greenhouse or you can heat your greenhouse to the ideal temperature for your tomato plants.

Tomato Grow Bags or Tomato Pots? See our post here.

When to Sow Tomato Seeds

To ensure your tomato seeds grow into healthy plants you should sow your tomato seeds indoors five to six weeks before the last expected frost in your particular area. After growing your tomatoes for five to six weeks you are safe to transplant them into your unheated greenhouse without worrying about damage to your plants.

What happens if I get my timing wrong?

Planting your tomato seeds at the correct time is a crucial factor in determining whether or not your plants will produce healthy delicious tomatoes or not. There are several environmental factors to consider.

Planting Too Early

If you plant your tomato seedlings in an unheated greenhouse during January or early February may result in tomato plants that are leggy (too tall with thin stems) and unhealthy. During these months the temperature is too cold and the light is insufficient.

Even later in the season, your plants will not grow big and healthy. You will end up getting fewer tomatoes and you will see that the leaves on your plants are pale.

This problem can be fixed if you install artificial lights and heaters in your greenhouse.

Planting Too Late

Planting your tomato plants late will not result in unhealthy plants; it will just shorten your growing season. If you are growing tomatoes for profit, you will see a significant decrease. The earlier you can start growing the more tomatoes you will produce and therefore the more profit you will make.

plum tomatoes on vine

How Can I Economically Heat My Greenhouse?

There are a few ways to increase the heat in your greenhouse without using gas or electricity.

The first way to increase the heat to your greenhouse is to add thermal mass. The most common way to do this is to use water barrels. By stacking drums of water in a greenhouse the grower can store daytime heat and release it at night. Placing the water barrels in areas where there is a lot of sunlight will allow the thermal mass to absorb the heat from the sun during daylight hours and radiate it back into the greenhouse at night.

A second way to add heat to your greenhouse is to add decaying compost. As the compost decays it releases heat and helps to regulate the greenhouse temperature.

Finally, you can install solar heaters to warm up your greenhouse.

How do Farmers Plant Tomatoes?

Tomato plants require three to four months of warmth to grow into strong healthy plants and produce a good tomato crop. Take this into consideration when planning your most ideal time to plant.

Remember tomato plants need to stay warm day and night. Consistent night time temperatures of between 12.7°C  to  23.8°C are necessary to set the tomatoes.  Leaving it too late in the year may expose your plants to temperatures that are consistently higher than this range. This is not favourable for your plants because it causes the plants to stop producing a necessary growth hormone.

tomatoes in hands

When temperatures rise above 23.8°C the fruits will not ripen as quickly and they will stop growing altogether if the temperature increases past 35°C . Therefore, planting either too late or too early does not favour tomato production in an unheated greenhouse.

Don’t forget that your greenhouse requires sufficient ventilation, especially during those hot summer months.

Grow Lights

Utilizing grow lights in an unheated greenhouse can help your plants thrive. Especially if you plant early, providing extra light will be necessary because the sun goes down too early in February to provide sufficient light for your tomato plants.

Using a grow light allows you to plant early and harvest your tomatoes early. If you do or don’t have a grow light in your unheated greenhouse it will help you to determine the perfect time to plant tomato seedlings.

If a farmer does not have grow lights in their greenhouse they must delay planting until the light is sufficient enough to allow tomato plants to succeed. However, grow lights can allow a grower to plant early, allowing the tomatoes to ripen early and perhaps increasing their profits.


For any agricultural endeavour, a farmer must know the correct time in which to carry it out. This is true of planting and growing anything, not just tomatoes. Having knowledge about the perfect time to plant and well as how to plant is crucial.

Planting your tomatoes at the correct time will increase productivity which will increase sales. Timing should never be taken lightly.

Taking the time and energy to accurately plan when to plant your tomatoes will result in a good healthy yield. Farmers with unheated greenhouses have many more environmental factors to consider when compared with farmers that have heated greenhouses.

Possible damage to your plants from cold, frost or low light intensity makes timing even more crucial. If you start planting too soon and your tomato seedlings are very likely to be damaged by environmental conditions. Introducing growing lights can help you avoid some of these concerns.

Tomato plants that are planted at the most appropriate time will always do better than those that were planted too early or too late.

It is very common for tomato farmers with minimal knowledge to experience planting peer pressure. This is when a farmer decides to plant just because they see other farmers planting. In this situation, the farmer may have different greenhouse conditions than the others. It is important to consider whether you have heat, grow lights and the temperature of your area. It is not important to watch what other farmers are doing.

You can find more useful guides on our blog, and you can visit our home page for more information on the greenhouses we sell.

matt garner Author: Matt Garner
I'm an amateur gardener based in Birmingham in England, utilising my 30 years experience to help others learn all about gardening for South West Greenhouses. My specialist expertise are with assembling and dismantling greenhouses of all shapes and sizes. I've spent countless years growing fruit and vegetables at Walsall Road Allotments, and I was also a proud member of the Balsall & District Horticultural Society for many years. Linkedin | Twitter