When can you start seedlings in a greenhouse?

Categories: Gardening Tips and Information

Plant seedlings in potting soil in seed trays (photograph by Marcus Spiske on unsplash.com)

Garden greenhouses contribute to an ideal environment for sowing seedlings. Being able to administer a controlled climate as well as receiving sunny conditions gives you the flexibility to start seed starting your plants prematurely, which in turn gives you a head start on the sprouting and growing season. But when is the best time to start planting your seed trays in a garden greenhouse?

Firstly, what is a seedling (in case you didn’t know)?

A seedling is a baby plant that has just germinated and is starting to develop. They can have a head start in life from being placed in a garden greenhouse generated from seeds or cuttings from your favorite plants or vegetables.

When can I put seedlings in an unheated greenhouse?

It is best to germinate seeds in colder months so late winter through to early spring is ideal for germination.

You can put seedlings in an unheated garden greenhouse when they are still tiny, such as when they are in their seed starting trays or pots. Generally speaking seedlings can be transplanted when they have just two sets of leaves upwards. The time frame for this is usually when they are 2-4 weeks old.

What is the ideal temperature for an unheated greenhouse to start seedlings?

Arthur Jack water butts next to a cold frame and greenhouse

Preferably, the temperature in a garden greenhouse should be around 18 degrees celcius or 65 fahrenheit when you are starting seedlings with a nighttime temperature of approximately 10 degrees celcius or 50 fahrenheit. Realistically if the temperature drops too much it will be tricky for the seedlings to develop like it would be on the other end of the spectrum, with too warm conditions. Anything over 24 degrees celcius or 75 fahrenheit would definitely be considered too warm for them.

How cold is too cold for seedlings in a greenhouse?

Strictly speaking there is no definitive answer to this question purely because it depends on where you are in the world and what climate you’re in. However, most seedlings can be safely grown in an unheated garden greenhouse when the temperature is between 10 degrees celcius or 50 fahrenheit and 21 degrees celcius or 70 fahrenheit.

As mentioned above if the temperature drops below 10 degrees celcius or 50 fahrenheit or rises above 21 degrees celcius or 70 fahrenheit, steps to protect them will be in need of. If the weather is too cold with a low soil temperature you may have to bring them inside or if it’s too warm being able to provide them with shade, an example of this would be covering them with blankets. Seedlings in an unheated garden greenhouse can tolerate colder temperatures than if they were outside; however, monitoring the temperature is key to make sure they are not struggling with the cold.

Most seedlings at a push can tolerate temperatures as low as roughly 4 degrees celcius or 40 fahrenheit, but you should check the specific variety to be sure. There are measures you can put in place to help provide a bit more warmth like horticultural fleece, just make sure that during the day you remove it so your seedlings have ventilation.

Please remember in winter months your plants won’t grow as swiftly as they would in warmer months but you really don’t need to worry about this. In some cases they may give themselves a break for several weeks particularly in the colder months and I don’t blame them as we all need a break now and again!

How long should seedlings stay in an unheated greenhouse?

The length of time seedlings should stay in an unheated garden greenhouse varies depending on the type of seedling and the climate. In general, it’s best to start seedlings earlier so they are large enough to survive the cold. If you live in a climate with harsh winters you need to plan to start your seedlings 4-6 weeks before the last frost date, the last frost date in the United Kingdom is usually mid to late April so you should begin seed starting during a week towards the end of February.

If you’re not confident as to when to start seed starting, check with a knowledgeable gardener or your local gardening center. They can help you figure out the best time to start seedlings based on your specific area and climate.

Remember, it’s better to plan to start seedlings earlier rather than later. If seedlings get too big before they go into the ground, they can be transplanted to larger pots. But if seedlings are too small when they’re transplanted, they may not survive when you transplant them. So go on the side of starting seedlings early so you don’t have to worry about them getting too cold in the greenhouse. Personally (if you have the space) try and grow a few extra just in case something goes wrong.

What is the hardening off period?

Plants can be really damaged if they go from a stable environment to an unpredictable one. An unheated greenhouse is a great tool for hardening off plants. The process takes up to two weeks usually. Start them off gradually for an hour or two, then moving onto a few hours before leaving them to fend for themselves overnight. Advantages are the doors and windows can be adjusted to provide conditions similar to being out in the open.

So there we have a rough guideline for planting your seedlings. I hope that you enjoy this process and I wish you all the success for happy seedlings.

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1 Comment

  1. John Diver

    An excellent outline for the beginner. Thank you

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