Germination is a process of growing an organism from a seed or spore. To germinate seeds need water, warmth, oxygen and soil. However, these factors depend on the particular seeds. Some need exposure to light while other germinate in darkness. In order to have a better understanding of germination process let’s look at the structure of the seed.
A seed consists of embryo, supply of food and seed coat. When exposed to optimal environment, germination process starts:
Imbibition – first stage of seed germination. Dry seed absorbs water so the seed coat starts to soften and swell.
Lag phase – second stage of seed germination. Due to access to water and oxygen embryo cells activate metabolic activity and start to enlarge. It produces proteins and metabolizes stored starches and fats.
Radicle emergence – third stage of germination. During this phase seed takes more water and the weight of the seeds increases. Eventually, seed coat breaks and a primary root also called radicle emerges. The radicle grows down into the soil to access water there. Then shoot or plumule emerges which grows backward through soil so it doesn’t get damaged as it contains leaves and stem.
So, now that you have an idea on how seeds germinate, what about starting seeds in an unheated greenhouse?
Starting seeds in an unheated greenhouse is possible if you maintain the lowest temperature at which seeds can germinate effectively. Although minimum temperature will let seeds germinate, they will only germinate rapidly and uniform at optimal temperatures.
What temperature do seeds germinate at?
Most seeds germinate at temperatures from 65°F to 86 °F, however, it depends whether you are growing cool or warm season crops. This table specifies soil temperatures for a variety of seeds:
| Minimum (F) | Optimum Range (F) | Optimum (F) | Maximum (F) | |
|---|---|---|---|---|
| Beet | 40 | 50-85 | 85 | 85 |
| Cabbage | 40 | 45-95 | 85 | 100 |
| Cauliflower | 40 | 45-85 | 80 | 100 |
| Celery | 40 | 60-70 | 70 | 85 |
| Chard | 40 | 50-85 | 85 | 95 |
| Cucumber | 60 | 60-95 | 95 | 105 |
| Eggplant | 60 | 75-90 | 85 | 95 |
| Lettuce | 35 | 40-80 | 75 | 85 |
| Melons | 60 | 75-95 | 90 | 100 |
| Onion | 35 | 50-95 | 75 | 95 |
| Parsley | 40 | 50-85 | 75 | 90 |
| Pepper | 60 | 65-95 | 85 | 95 |
| Pumpkin | 60 | 70-90 | 90 | 100 |
| Spinach | 35 | 45-75 | 70 | 85 |
| Squash | 60 | 70-95 | 95 | 100 |
| Tomato | 50 | 70-95 | 85 | 95 |
Source: Soil Temperature Conditions for Vegetable Seed Germination by Kemble and Musgrove (2006).
The problem that many growers can encounter is that during the day greenhouse is warm but the temperature falls down at night. Generally, the temperature inside unheated greenhouse is 28°F (-2 °C) when outside temperature is 20°F (-7 °C). There are ways to heat greenhouse without electricity or you can get a cheapest greenhouse heater to warm the structure.
Will seeds germinate in a cold greenhouse?
In a cold greenhouse the air temperature inside greenhouse is just a few degrees above the air temperature outside. However, the soil temperature is not the same as air temperature, so you need to test it first. As long as you can keep the minimum soil temperature for germination, seeds will germinate in a cold greenhouse.
How to germinate seeds in unheated greenhouse?
To germinate seeds in unheated greenhouse you need to warm the soil temperature to at least the lowest temperature at which seeds can germinate effectively. There are a few ways to do so:
Heat mat – it is a water resistant mat which runs on 18 Watts. This option can heat the soil and maintain the temperature of around 10℉ – 20℉ above ambient air temperature. However, you can only use this option if you have electricity in your greenhouse available.
Heating cables – made of waterproof PVC these cables vary from 12 to 48 feet long. This option promises to heat and maintain soil temperature from 70 – 76℉ which is an optimal germination temperature for most plants. Again, as with the previous option you need to have access to electricity.
Mini greenhouse – that is a very cheap way to germinate seeds in a greenhouse. You can buy or build a small greenhouse inside the existing greenhouse. That way you will have a smaller growing area where you can increase the temperature. Check out what you can grow in a mini greenhouse.
Whether you choose any of these options, you need to ensure that soil temperature is consistent for at least a week before germinating seeds. Also, I personally like to separate greenhouse in sections and insulate one of the benches for seed germination.
What can I grow in unheated greenhouse?
There are two different types of vegetables: cool season and warm season crops. As the name suggests you can plant cool season crops when the soil temperature is at least 40ºF . Warm season crops prefer temperatures from 70ºF to 95ºF (21-35ºC).
To thrive you need to plant cool season crops when the weather is still cool so they can grow before the summer heat. Moreover, depending on how cool season crops can withstand frosts, they are further divided into hardy and semi-hardy crops.
The seeds of hardy cool season crops can germinate in cool soil and their seedlings will withstand heavy frost. These hardy vegetables are:
- Broccoli
- Brussels Sprouts
- Cabbage
- Collards
- Onions
- Rutabaga
- Kale
- Kohlrabi
- Peas
- Radishes
- Spinach
- Turnips
On the other hand, semi-hardy vegetables can tolerate light frosts and you can plant them two weeks before the last spring frost. These semi-hardy vegetables are:
- Artichoke
- Cauliflower
- Celery
- Arugula
- Asian Greens
- Beets
- Carrots
- Endive
- Lettuce
- Potatoes
- Salsify
- Swiss Chard
While some of these benefit from sowing indoors, other vegetable seeds you can plant directly in soil. For example, you can germinate Kale, Peas, Spinach and Turnip seeds directly in the ground in unheated greenhouse. Root plants, like carrots, actually benefit from direct sowing.
I have put together step-by-step guides on how to germinate seeds in a greenhouse directly in the soil and in seed trays.
