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45 Best Starting Seeds in an Unheated Greenhouse

What are the best starting seeds in an unheated Greenhouse? Well, if you want to grow plants all year round in an unheated greenhouse, this is how you do it.

Germination is the start of life for every plant. It is the process by which a seed or spore transforms into a seedling.

For a seed to germinate, it requires several conditions, including water, air, and warmth. These factors will, however, vary depending on the type of seed you want to grow.

For some seeds to start germination, they must be exposed to light.

However, some seeds will also germinate in total darkness. And, with the right conditions, the germination process includes three distinct stages, as we will see in a bit.

However, today we delve more into germination in a greenhouse, especially an unheated one.

Which are the best starting seeds in an unheated greenhouse? What do you need to do to get the best results?

Best Starting Seeds in an Unheated Greenhouse

unheated greenhouse

Before we get into the ideal starting seeds for an unheated greenhouse, we need first to understand how these seeds germinate.

What takes place when you provide those seeds with the ideal conditions?

Seeds Germination Stages

  • Imbibition

This is the first seed germination stage. Here, the dried seeds absorb water, one of the factors necessary for germination, softening the seed coat as it also swells.

  • Lag phase

When the seed coating softens, water and oxygen easily reach the embryo cells. This initiates a metabolic activity, which makes the cells start enlarging.

The process leads to protein production, as well as the metabolism of stored fats and starches. That’s completes the second germination stage.

  • Radicle emergence

This is the third and final stage of germination. Here, the seeds will take in more water, increasing their weight. And as the coat soaks with more water, it eventually breaks, and a radicle (primary root) emerges, growing downwards towards the source of water.

From here, a plumule (shoot) also emerges on the opposite side of the radicle. And instead of following the water, the plumule follows the source of light, hence grows upwards.

With that knowledge, we can now try to figure out the best starting seeds in an unheated greenhouse. As mentioned earlier, seeds need warmth to germinate effectively. So, how will you be able to grow seeds in an unheated greenhouse?

Well, first things first. Since heat is an essential germination factor, let’s see how much of it, different seeds will require to germinate.

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Optimal Seeds Germination Temperatures

Seed types

Minimum Temperature (F)Optimum Temperature (F)Maximum Temperature (F)

While seeds will germinate in minimum warmth, the process will be quite slow. Only at optimal temperatures will the seeds germinate rapidly. But, if the temperatures are also too high, this will also negatively affect the process.

Ideally, the optimal temperature range for most seeds to germinate is between 65°F and 86 °F.

However, greenhouse owners encounter temperature variations, which might affect germination for their seeds. For instance, at night, temperatures are mostly low, while they soar up during the day.

What is an Unheated Greenhouse

greenhouse planting

This is a type of greenhouse that’s heated by the sun. This means that when the sun is down, its source of heat also goes down. However, this greenhouse, also known as a cold greenhouse, is designed in a way that traps warmth during the day, keeping your plants warm during cold nights.

The only challenge comes when the temperatures go too low, especially during winter nights – although the greenhouse can maintain an environment much warmer than the outside environment.

Generally, temperatures inside an unheated greenhouse differ from that on the outside, with around 7°F.

For example, when the outside temperatures are around 20°F (-7°C), it’s around 28°F (-2°C) inside the greenhouse. This means that the greenhouse is approximately 8°F warmer than the outside.

For this reason, understanding the optimal germination temperatures is crucial when trying to figure out the best seeds to start with, in a greenhouse.

This way, you also know whether it’s necessary to heat the greenhouse if the temperatures go too low.

Tips on Using an Unheated Greenhouse

When growing plants in an unheated greenhouse, you might need to consider a few things, unlike in a normal heat-regulated greenhouse. For instance, the types of starting seeds ideal for your greenhouse should never be overlooked.

If the plan is to start growing your plants in cold weather, the experts always advise going with cool-season plants. These are plants whose seeds will germinate even in cold temperatures. Remember, different plant seeds will germinate at different temperatures.

This means identifying the ideal starting seeds for that cold weather, considering the greenhouse is unheated and warmth from the sun is at its minimum, is crucial.

Depending on the type of unheated greenhouse structure you own, you can plant your seeds through either of these two options.

  • Trays
  • Ground

Note: When sowing the seeds directly into the ground, prepping the soil to the right temperatures is necessary. You can do this by positioning your cold greenhouse facing the sunrise about a week before sowing the seeds. This allows the greenhouse to absorb warmth from the morning sun, bringing the soil’s temperatures to optimal germination levels.

On the other hand, for warm-season plants, sow them approximately 1-2 weeks before your area’s average last frost date.

As for the trays, make sure you have the right ones for the job.

You can buy high-quality ones from Amazon, such as the 10-Pack Starter Trays, which are humidity adjustable, or the 5-set Seed Tray Kits that offer 40 seed planting cells per tray.

Top Seeds for an Unheated Greenhouse

Deciding what to grow in an unheated greenhouse is not always easy, especially if it’s your first time. The fact is, it’s not impossible to do greenhouse gardening even in the cold weather and enjoy fresh produce.

If you yearn for fresh vegetables and herbs even during the cold of winter, there are numerous starting seeds you can sow in your unheated greenhouse.

Generally, you have two types of plant seeds you can grow in your unheated greenhouse. These are categorized as cool and warm-season seeds.

For the cool season seeds, they will germinate and thrive at temperatures around 35°F – 40°F. On the other hand, the warm season plants do well at temperatures ranging from 70°F to 95°F.

If you are planting the cool season seeds, ensure you do so during the cool weather before it starts to get hot. However, even the cool season plants are categorized into two subsets – the hardy and the semi-hardy ones.

The hardy ones will germinate in low-temperature soils, and their seedling can also survive through heavy frost.

As for the semi-hardy ones, they will germinate in not-so-cool soils and withstand moderate to light frost. For this reason, these need to be planted around two weeks prior to your area’s last spring frost.

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Hardy Starting Seeds in an Unheated Greenhouse

vegetables to grow in an unheated greenhouse

Here are some examples of hardy seeds that you can grow during the cold season.

Leafy Greens

  1. Collard
  2. Endive
  3. Cabbage
  4. Mustard greens
  5. Radicchio
  6. Swiss chard
  7. Kale
  8. Spinach
  9. Arugula


  1. Onions
  2. Coriander
  3. Asparagus
  4. Cabbage
  5. Brocolli
  6. Beetroot
  7. Silverbeet
  8. Snow peas
  9. Peas
  10. Celery
  11. Cauliflower
  12. Carrots
  13. Radishes
  14. Garlic

These plants are strong enough for the cold weather, and you can easily find some in Amazon, like the Green Arrow Peas.

Hardy Flowers

  1. Chrysanthemum
  2. Snowdrop
  3. Calendula
  4. Pansies
  5. Winter heather
  6. Cyclamen
  7. Mahonia
  8. Winter clematis
  9. Daphne
  10. Winter aconites

Hardy Perennial Herbs

  1. Parsley
  2. Thyme
  3. Lavender
  4. Chives
  5. Fennel
  6. Mint
  7. Coriander
  8. Tarragon
  9. Garlic bulbs
  10. Chicory
  11. Chervil
  12. Rosemary

All these represent some of the best seeds for an unheated greenhouse, which will thrive in cold weather. This means you need not worry about the cold season nearing. You can still grow various crops in your unheated greenhouse without minding the frost.

Benefits of Greenhouses

  • Protection against Harsh Weather

Without greenhouses, it would be impossible to grow most plants during weather extremes. With a greenhouse, your plants are always protected against scorching heat or freezing winter temperatures. This way, you can continue enjoying fresh produce from your indoor or backyard garden, even in the worst weather.

It also means that there is not a single time you won’t have fresh crops from your garden – be it in January or December.

  • Do all your Gardening in a Single Place

Depending on the size of your greenhouse, you can grow all your crops in a single location. This allows your crops to grow uniformly, and you can manage them better. And, no matter the type of plants, a greenhouse ensures that they receive ideal conditions for optimal growth.

As for the larger greenhouses, they can even act as your tools storage room.

  • Protection against Pests

Pests can really be a nuisance and can ruin even the most stunning indoor or outdoor garden. However, with the right greenhouse, you won’t have to worry about them destroying your plants anymore.

Since it’s an enclosed structure, it’s easier to control pests and provide a thriving sanctuary for your plants. The structure also comes in handy to protect the plants against various insects and birds that might harm the plants.

  • Grow any Plant Type

While some small indoor greenhouses might limit you to the types of crops you grow, larger outdoor ones do the opposite. They give you a chance to grow a wide variety of plants, depending on what you want.

Whether you wish to grow some vegetables, flowers, herbs, perennial crops, or fruits, these structures make it possible. You won’t have to be restricted to vegetables and flowers only, but rather a whole spectrum of amazing crops.

All you need to do is understand the right conditions for the different crops to ensure optimal production. You can grow your crops simultaneously or grow different ones at different times of the year.

How to Start with Seed in an Unheated Greenhouse

If you want to germinate seeds in an unheated greenhouse, you should first ensure that the soil temperature is warm (at least the minimum warmth necessary for germination). This will ensure that your plants don’t experience difficulties in germinating.

Here are a few ways to warm the soil:

  • Heat Mat

With this water-resistant mat, you can easily heat the soil in your greenhouse and maintain it at 10°F to 20°F, above the outside temperatures. One limitation of this mat is that it’s an 18 Watts mat and is only ideal if your greenhouse has an electricity source.

  • Heating Cables

These are 12ft to 48ft long waterproof PVC cables that you can use in heating your soil. They will heat and maintain your greenhouse soil’s temperature between 70°F and 76°F, the ideal warmth needed for most seeds to germinate. It also requires electricity to operate.

For more greenhouse heaters, check out our piece on the best solar greenhouse heaters.

  • Mini Greenhouse

If you have a large unheated greenhouse, you can buy a smaller greenhouse (heat controlled), where you can first germinate the seeds. There are numerous options of mini-greenhouses online that you can buy to facilitate your farming.

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