When to Plant Tomatoes in Unheated Greenhouse

Time factor cannot be set aside in agriculture, in fact, time is agriculture itself and they work hand in hand to ensure maximum production. Knowing the accurate time of the year to plant tomatoes will help determine the appropriate time for you to transplant your seedlings in your unheated greenhouse and will help you produce the best tomato crops.

If you want to have a long and successful growing season for your greenhouse tomatoes. You should generally sow tomato seeds indoors around the middle of March and then towards the end of April transplant the seedlings into your unheated greenhouse. However, these dates highly depend on your geographical region, keep on reading, there is a lot to cover on this topic.

You might be thinking when should you plant tomatoes in your greenhouse, will the perfect time to start tomatoes depends on the region where you want to grow tomatoes along with the sun light which that region gets. Most of the time after the fall of the frost in the mid February is the perfect time to grow tomatoes

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Where should you SOW your Tomato seeds?

We already know that an unheated greenhouse is great for extending the growing season. However, during late winter when it’s freezing outside the temperature inside the greenhouse will be just above freezing especially when its cloudy during the days.

But, Tomato seeds require temperature higher than 10°C (50°F) for germination and proper seedling growth. Furthermore, a temperature of 4°C(39°F) or less, will seriously damage your tomato plants. You should never let your greenhouse temperature go below 10°C for a long period of time especially when your tomato plants are young.

So, You cannot sow tomatoes inside your unheated greenhouse when its frost outside. You can only sow tomatoes in your unheated greenhouse after the last expected frost date is passed in your geographical zone.

Then, where should you sow your tomato seeds ?

You should Sow your Tomato seeds Indoors!

I know it sounds too much, but believe me, by doing so you will be 5 weeks ahead of your planting schedule and it will extend your growing season by 5 weeks. let me explain how.

As we have discussed above, You can sow tomato seeds inside your greenhouse after the last expected frost dates. This is the earliest you can go with an unheated greenhouse.

You Know! tomatoes seeds take 5-6 weeks to become strong seedlings that are ready for transplanting.

I would recommend sowing your tomato seeds indoors 5-6 weeks before the expected frost date according to your geographical zone. When the frost date is over you can then transplant your tomato seedlings inside your unheated greenhouse. In my region, the last expected frost ends in the first week of April and I sow my tomato seeds indoors in late February.

How to Grow Tomato Seedlings Indoor?

  • Buy a seedling tray (plug tray) for your tomatoes. I recommend using LOVEDAY Seed Starter Trays (See Price) because the dome keeps the humidity high and helps in fast germination.
  • Fill the seedling trays with compost and press it slightly to flatten the compost. You can fill any multipurpose compost or Seed starting mix. (I have good experience with Espoma Seed Starter but you can use any seed starter mix).
  • Place your tomato seeds on the surface of the compost and cover the seeds with 0.5 – 1 cm of compost.
  • Water the compost to make it damp (Don’t saturate it) and put the lid on.
  • The ideal temperature for germinating tomatoes is 70-80°F (21-27°C) If your indoor temperature below 64°F I will strongly recommend using a heat mat to heat the seedling trays from the bottom, It will greatly improve the germination speed of your tomato seeds. I myself use iPower Seedling Heat Mat (see price) and it works fine.
  • Install a grow light above your tomato seed tray. Use the well known BESTVA LED Grow Light (See Price). You cannot only use it for growing seedlings but you can also use it for growing tomatoes indoors. Install this Grow Light 12-24 inches above your tomato seedlings.
  • During the first few days of germination turn off the Grow Light. After your tomato seeds have successfully germinated give them about 18 hours of artificial light per day.
  • After 5 weeks when the expected last frost date is past transplant your tomato seedlings inside your unheated greenhouse.

For more details on growing greenhouse tomatoes, you can check this extensive Greenhouse Tomato Guide that covers everything.

Check out this video from Youtube channel Project Diaries and see how he is growing tomatoes from seeds.

What is the Perfect Time to sow Tomatoes Seeds?

The perfect time to sow your tomato seeds indoor is around 5 to 6 weeks before the last expected frost date in your geographical zone. The main reason why seeds are sown indoors is to make sure that they grow as healthy seedlings.

The seedlings should then be transplanted into your unheated greenhouse after the last expected frost dates, according to your geographical zone.

Can I directly Sow Tomato Seeds Inside my Greenhouse?

You can definitely directly sow tomatoes seeds in your unheated greenhouse. Then you will be surely 5 weeks behind schedule.

Furthermore, the temperature of the greenhouse might be slightly above 10°C (50°F) when the last frost is over. But it is still low for proper germination of tomatoes seeds because tomatoes are warmth-loving plants and the ideal germination temperature is between 70-80°F (21-27°C).

Your tomatoes will germinate in your greenhouse but it will take long for them to grow into strong and healthy seedlings which will again delay your expected harvesting dates.

What you can do is, heat your greenhouse so that the temperature is high enough for your tomato seeds to grow into healthy plants.

grow tomatoes inside greenhouse greenhouse tomatoes guide

What can I do to extend my Growing Season Even More?

In order to extend your growing season even further, you will have to heat your greenhouse in the late winter and make sure your greenhouse plants are getting at least 8 hours of sunlight.

If not then you will have to supplement it with 2-4 hours of artificial lighting for your greenhouse tomatoes. Use the same light I mentioned above (BESTVA LED Grow Light (See Price)). Install this light 24 inches above your greenhouse tomatoes.

What Happens If You Plant Tomatoes Early or Too Late? 

The appropriate time to plant tomatoes in an unheated greenhouse is a major factor that determines whether you will fail or succeed. So many environmental factors will work against your tomatoes if you take a wrong turn.

However, the questions asked above causes controversy among tomato farmers. At the same time causes wrong decision making.

Planting Tomatoes Too Early In Your Unheated Greenhouse:

Planting seedlings too early (probably in the late month of January and early or late February when the intensity of light is low and is not enough to grow tomatoes) will result in leggy and poor plants.

This will eventually produce fewer tomatoes later in the season! The seedlings will become stunted as they grow to maturity. The pale color of their leaves will also indicate that they are not getting enough light.

To combat this, you have to heat your greenhouse to keep your greenhouse tomatoes happy and You will have to supplement your plants with artificial lighting as discussed above.

Planting Tomatoes Too Late In Your Unheated Greenhouse:

It is perfectly fine to plant tomatoes late in your unheated greenhouse if you are OK with the short growing season.

You see, If you plant tomatoes late inside your unheated greenhouse then the length of your growing season will decrease accordingly. This will have a serious impact on your profits if you are growing for the market. Obviously, you will get more profit while sending your tomatoes to the market as early as possible, when there is high demand.

Can I heat my Greenhouse for Free ?

Of course, you can heat your greenhouse for free, There are many ways by which you can warm your greenhouse without the use of a heating system which consumes gas or electricity.

For example, you can add thermal mass inside your greenhouse it will help to regulate your greenhouse temperature. The thermal mass will absorb heat during the day when the sun is high up in the sky and during the night it radiates the heat back helping to keep the temperature high.

Besides adding thermal mass you can also add decaying compost in your greenhouse. The compost will release heat while decaying and will help in maintaining your greenhouse temperature. You can also use solar heaters to heat your greenhouse.

I mean, there are a number of different ways by which you can heat your greenhouse without paying for electric or gas bills. For more information on this topic, check out the following links:

Do I need Greenhouse Ventilation During Winter?

During Summer, Greenhouse ventilation is a must. If you don’t provide your greenhouse tomatoes with proper ventilation then no doubt, your greenhouse tomatoes will burn to death.

But what about winter?

Even in late winter when its a full sunny day your greenhouse temperature may increase dramatically especially during noon when the sun is high up in the sky. So, I will definitely recommend installing a good automatic ventilation system for your greenhouse tomatoes. You can check this detailed article on the DIY greenhouse ventilation system.

Do You Want to learn more about Growing Greenhouse Tomatoes?

If you want to learn everything about growing greenhouse tomatoes follow this detailed article:


This detailed guide include the following topics.

  • Greenhouse Site Selection for tomatoes.
  • Selecting the Tomatoes Varieties
  • Growing Medium
  • Greenhouse Tomatoes Plant Density
  • Planting Schedule for Greenhouse Tomatoes
  • Humidity, pH, and Temperature requirement for Greenhouse Tomatoes.
  • Growing and Hardening Seedlings
  • Transplanting Seedlings in Greenhouse
  • Greenhouse Soil Requirement for growing Tomatoes
  • Pruning, Training and Pollinating Greenhouse Tomatoes

This article covers everything and I will strongly recommend you to read it. It’s a very lengthy article with lots of information, but it is worth reading.

It would be nice if you spare some time and subscribe to our newsletter. We will keep you updated with more interesting information on growing in greenhouses.

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 Favorable weather conditions before transplanting

This particular period of the year (5-6 weeks before spring frost) is the most favorable for transplanting tomato seedling. Tomatoes need 3 to 4 months of warm conditions that will be available after the last frost.

Tomatoes need consistent night temperatures between 55°F and 75°F to set fruit.  Planting your tomatoes too late will expose them to temperatures above this range, thus stopping the production of a plant growth hormone (gibberellins).

This causes delayed ripening of fruits when night temperatures rise above 85°F, and at the same time plants will quit growing when temperatures go above 95°F. Therefore planting late or early does not favor tomatoes production in an unheated greenhouse.

Furthermore, your greenhouse ventilation system should be quite adequate especially during summer. You can check this article for more details.

Presence or absence grow Lights

The introduction of grow light to grow tomatoes in unheated greenhouses when the seedlings are planted early has proven to be successful. Seedlings may be planted in late February if a grow light is used to provide more light for the seedlings to grow. The presence of the extra grow light takes care of issues of the seedlings becoming leggy, (too tall with thin stems).

The benefit of using a grow light is that tomatoes can be planted early. Ripe tomatoes can be picked early. Therefore the presence or absence of grow lights determines exactly when to plant tomato seedling. A situation where additional grow light is produced, a farmer may decide to begin his or her planting as early as possible in turn maximizing his or her opportunities.

On the other hand, it’s absence means that planting will be delayed until the light intensity is sufficient enough to support the proper growth of tomato seedlings in an unheated greenhouse. So if you don’t have a grow light, never be in a hurry!!

grow tomatoes inside greenhouse greenhouse tomatoes guide


The right time to carry out an operation on a farm is a very important aspect of agriculture as a whole and not only in tomato production. Knowing exactly when and how to carry out an operation is a major factor that determines the success of a farmer.

The right time to plant tomatoes seedling is an important operation and should not be taken with levity if a farmer wants to be successful. In order words, timing and high productivity are related directly.

Accurate planting time gives you high productivity, Unlike Heated greenhouses that are approximately not affected by any weather conditions. A farmer using an unheated greenhouse will likely be faced by so many environmental conditions that can damage his or her tomatoes.

This makes accurate timing very important. Start too soon and your tomatoes seedlings will be subjected to environmental conditions that can cause severe damages due to the low temperatures at night. Hence the low light intensity during the daytime is not adequate enough for proper growth will cause damages. Effective growth can only be possible with the introduction of growing Lights!

Tomatoes planted at the most appropriate time in a very good condition will most probably overcome the tomato plants that were planted early in the season when the environmental factors were not favourable. In due time the later will be more productive than the former.

This particular case is very common among tomato farmers with low planting time knowledge, a scenario called PLANTING PEER PRESSURE. This occurs when a particular tomato farmer decides to run to his/her greenhouse and start planting just because other farmers are doing the same. But it should also be noted that if you sow seeds very late and the season will end before the tomatoes have had time to reach maturity, especially in areas with the extremely short season.


Plant tomatoes at the right time

The major secret, at the same time a tip to success, using an unheated greenhouse is to preferably sow seed indoors around the middle of March and then towards the end of April. The seedlings should be transplanted to your unheated greenhouse. If you intend to take freshly harvested tomatoes to the market for sale earlier, use a grow Lights for proper growth of your tomato plants.

 Avoid Frost

Frost can cause serious damage to your tomato seedlings. This gives an opportunity for other diseases to attack the young seedlings which will then increase the cost of maintaining the effects of disease caused by the frost in the long run.

The last frost dates are different for different regions and can only be determined based on the local weather conditions of your locality. Based on this fact, farmers should know that weather conditions differ completely in another region, and assumptions of planting time should not be based on other locality but based on the locality where your greenhouse is located.

When to grow tomatoes in a greenhouse, Well this is a question which is asked allot of time, The perfect time to grow tomatoes in an unheated greenhouse depends on the region in which you are growing the tomatoes

What if you are Late?

If you have missed out in the “sow by date” operation. It is a bad idea to start tomato plants all over again from the seeds because it will delay your production by a couple of weeks.

You can just buy tomato seedlings in plug trays from the garden center with a sounding reputation of growing healthy seedlings. This will save you time, but the choice of varieties will be less.


It is always very easy to be tempted to start too early due to the financial reward pictured when the estimation of the rate of return is carried out. I get the same urge myself to start sowing as early as the first week of February rather than waiting for the best time which in my case is late February.

But based on my experience, results will always be better when you wait patiently and sow in late February which is the most appropriate time to sow (INDOORS) in my region.

Though the window of opportunity is small, making maximum use of this opportunity will end with good outcomes.  And also getting the timing right, that is knowing exactly when to run down to your greenhouse to plant tomatoes seeds is crucial for success.

Likewise getting it wrong on the other hand can be an utter disappointment especially in short-season areas, where little or no amends can be made to turn back the hands of time.

On this note, other crucial points for success in unheated greenhouse tomato farming cannot be ignored. And must be included, put into consideration, and practice in an unheated greenhouse.

  •  A proper check on the Light intensity.
  • Geographical temperature and relative humidity control.
  • Water check.
  • Fertilizer application at the appropriate time.
  • The temperature of the growing media.

It would be great if you save this awesome pin to your Pinterest “Greenhouse Growing” board! See you SOON 🙂

How to grow tomatoes in an unheated greenhouse. Time is the most important factor when it comes to growing tomatoes in an unheated greenhouse. Most of the time the fall of the frost during the mid February is considered to be the perfect time for growing tomatoes.


  1. I am an urban farmer in Boston and the farm I manage includes an unheated 30X68 high tunnel. It’s too early to fully review well founded suggestions and recommendations, but base on my experience, the content of your article is so far so AWESOME!!! I was not going to grow tomatoes in the high tunnel, but even without completing the article I believe it make commercial sense to grow them in our high tunnel. Thank you for your awesome articles, which you can be certain I will share with my collogues.

    • It really makes us happy when We see our Guide helping our readers. Thankyou so much for your kind comments. Definitely growing tomatoes in High Tunnels gives the grower a head start in the season, so his/her tomatoes reaches the market earlier than others when the demand is high. As a result he earns more.

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