Seeds and seedlings need warmth to grow. Without sufficient warmth, they almost always do not grow. In many cold places, greenhouses provide the warmth needed for seeds and seedlings to grow. The greenhouses are usually heated at night to ensure the usual nighttime drop in temperatures does not significantly affect the enclosed environment within them.
However, not all greenhouses are heated. Many of them are unheated. This makes it difficult to start seeds in them because of the lack of heat/warmth needed for growth. Difficult but not impossible.
In this post, I will share with you useful tips on starting seeds in an unheated greenhouse.
Here we go.
1. Understand The Requirements For Germination
For a seed to germinate (start growing as a plant), it needs water, oxygen, soil and warmth. Some seeds also need either light or darkness to germinate.
The germination process has three stages – imbibition, lag phase, and radical emergence. The first stage, the imbibition stage, involves the seed absorbing water resulting in the expansion and softening of the seed coat.
The second stage, the lag phase, involves the activation of metabolism in the seed’s embryo cells. This is enabled by the availability of water and oxygen in the cells. The activation of metabolism results in further expansion of the seed coat and leads to the metabolism of stored food and production of fats and protein.
In the last stage of germination, radical emergence, the radical emerges. The stage involves the seed absorbing water and expanding to the extent whereby the seed coat breaks. It is the breaking of the seed coat that allows the emergence of the radical (a primary root). The root grows downward and starts access water and nutrients and then shortly afterward, the plumule (a rudimentary shoot) emerges and grows backward to emerge above the surface undamaged.
As you see above, germination is a process and for it to work, certain inputs must be available. The inputs are water, oxygen, warmth, and soil. Each is necessary for germination. Warmth is particularly important because without it metabolism cannot happen and germination will fail.
Therefore, when starting seeds in an unheated greenhouse, you should know that heat or warmth is still necessary and you must somehow provide it for germination to occur.
2. Know The Minimum Temperature Needed For Germination
Now that you know that heat/warmth is necessary for germination, you should know the minimum temperature needed for germination. Most seeds will germinate when the average temperature is 60 degrees F or more.
So to start seeds in an unheated greenhouse, all you have to do is to determine the minimum temperature in it. If it is more than 60 degrees F, then you most likely do not need to heat it or do anything else to start the most common types of seeds in it.
If the temperature is less than 60 but more than 40 degrees F, then you should know you can still start many types of seeds in it. This is because numerous seeds including chard, celery cauliflower, cabbage, beet, and parsley have a minimum germination temperature of 40 degrees.
Some seeds such as onion, lettuce and spinach seeds have an even lower minimum germination temperature of 35 degrees F. This means they can start in an even colder unheated greenhouse.
In short, some seeds need less warmth than others to germinate. So even if your unheated greenhouse is cold, all you need to know is the minimum germination temperature of the seed you want to start. This information will allow you to confirm if the seed can start in your cold greenhouse.
3. Increase The Soil Temperature If You Need To
If the minimum germination temperature of the seed you want to start is too warm compared to the temperature inside your unheated greenhouse, you need to bring the temperature inside your greenhouse to the minimum germination temperature. You can do this in three ways.
First, you can increase the temperature in your greenhouse by buying and installing a heat mat. A heat mat is a water-resistant mat. It heats the soil and can increase unheated greenhouse temperature by about 20 degrees F. However, keep in mind that this option requires an electric connection to work.
Second, you can increase the temperature in your greenhouse by buying and installing heating cables. The cables come in various sizes. Some are as short as 12 feet long while others are as long as 48 feet long. So get the size perfect for your greenhouse and install. Heating cables can increase and maintain temperatures at 70 degrees F, which is perfect for starting most common types of seeds. However, you should keep in mind that this option also requires your greenhouse to be connected to electricity.
Lastly, you can cleverly increase the temperature in your greenhouse using a mini-greenhouse inside your greenhouse. You can buy a mini greenhouse. Alternatively, you can build your own. If you build your own, it will free and cheaper than the other two options. A mini greenhouse inside your unheated greenhouse will always be warmer than your bigger greenhouse for the air in your bigger greenhouse will insulate it from the outside world. After installing a mini-greenhouse, you should find out if the temperature inside it is warm enough for the seed you want to start as in some cases mini-greenhouses do not become warm enough for seed germination.
4. Start A Different Type Of Seed
If for some reason you cannot use any of the methods explained in the tip above to increase the temperature of your greenhouse to at least match the minimum germination temperature of your preferred seed, there is only one other thing you can do – start a different type of seed.
As explained in the section above, some needs need less warmth than others to germinate. Therefore, if your unheated greenhouse is too cold for your preferred seed to germinate, you should consider starting a seed or seeds that need less warmth to germinate.
To do this, you should simply find out the average temperature of your unheated greenhouse and then search online for seeds that can start at that temperature or more.
There are two general types of crops in terms of seasons – warm-season crops and cool-season crops. Warm-season crops are those that grow in the warm growing season. They prefer temperatures between 70 and 90 degrees F. While cool-season crops are those that can start and grow in temperatures as low as 40 degrees F.
The crops or seeds you should be looking to start and grow in an unheated greenhouse are, therefore, the cool season crops/seeds.
There are two types of cool-season crops you can germinate and grow – hardy and semi-hardy. Hardy cool-season crops include turnips, spinach, radishes, peas, kohlrabi, kale, rutabaga, onions, cabbage, Brussels sprouts, and broccoli. Hardy crops can be started in relatively cold unheated greenhouses. While the semi-hardy cool-season crops include Swiss chard, salsify, potatoes, lettuce, endive, carrots, Asian greens, arugula, celery, cauliflower and artichoke. They can be started over two weeks before the last frost date.
So once you know the average temperature in your unheated greenhouse, you can use the temperature to find out which seeds or crops can start at the temperature and then grow them. If the temperature in your unheated greenhouse is quite low, then know you will probably need to grow a hardy cool-season crop/seed if you do not want to heat it or warm it.
Seeds need warmth to germinate and grow. For this reason, they are often started indoors or in heated greenhouses. If your greenhouse is unheated, there are several things you need to think about to successfully start seeds in them. You now know these things. Consider them and you should be able to successfully start seeds in your unheated greenhouse.
Happy greenhouse gardening!