Learn How to Start Seeds in a Greenhouse

Sowing seeds in the springtime is often a hit-and-miss due to unexpected frosts but in this article, we will examine how to start seeds in a greenhouse to give you a leg up on your garden.

How to Start Seeds in a Greenhouse


9 Tips on How to Start Seedlings in a Greenhouse


1. When to Plant Seeds in a Greenhouse

If you want to know when to start vegetable seeds in the greenhouse, remember all seeds have a different growth rate. Look at the back of the seed package to get a good idea of what you can expect as far as germination and growth rate.

Many people are surprised to learn that you can start seeds any time of year if you have a greenhouse. I have started cold-weather seeds such as turnips, beets, carrots, radishes, onions, garlic, rutabagas, and parsnips in the fall and successfully grown them through the winter months for a spring harvest.

When to plant seeds in a greenhouse will vary depending on what seeds you plan to plant and where you live. The controlled environment of the greenhouse lets you safely start seeds in the greenhouse in the early spring and then translate them to the garden when they are six to eight weeks old and all danger of frost has passed.


2. Germination Temperature Considerations

Seeds germinate best when the temperature ranges from 70 to 80 degrees Fahrenheit during the day with night temperatures dropping to 50 to 55 degrees F. You’ll want to monitor the temperature in your greenhouse, especially if a freeze is forecast. I place a thermometer in my greenhouse so I could regularly check any temperature fluctuations.

You can use seedling heat mats to help keep the soil warm if the temperature in the greenhouse dips too low. Remember, seeds will not germinate if the soil is cold.

When the temperature gets too hot in the greenhouse during the day, you can use a fan to cool the interior down or open a window, door, or vent.

If you are starting seeds in an unheated greenhouse and the temperature is cold then you might want to use a heating mat

A heating mat encourages faster germination by increasing the soil’s temperature. Invest in a mat with a thermostat so you can control the temperature and ensure that it does not increase too much. Most seeds prefer a soil temperature of 70 to 85 degrees Fahrenheit to encourage germination.


3. Starting Seeds in a Greenhouse

When starting seeds in a greenhouse, you’ll want to use either individual plug trays or seed trays. You’ll want to research the needs of the seeds you will be growing. Some seeds require overnight soaking in warm water, others might need stratified or scarified prior to planting.

You’ll place the seeds in rows. Try to properly space the seeds, but you’ll probably have to thin out the seedlings to ensure healthy growth. The seedlings should always have air circulation and you’ll need space between each plant to water, fertilize, and trim when needed.

If you plant the seeds in rows in a flat tray but you don’t want to thin the seedlings, then you can always transplant them into individual cells or pots.

Place only one or two seeds per cell when using single-cell trays. Personally, I like planting in single-cell trays over a flat tray.I feel that each cell holds water nicely and creates a cocoon-like environment that retains heat well.

I keep my seedlings in plug trays longer than a flat tray because I don’t have to worry about the roots becoming entwined with the other seedlings. Also, transplanting the seedlings into my garden is a breeze. All I do is pop out the seedling from the plug and transplant the small plant right into the garden. I don’t even have to worry about thinning the seedlings when I plant the seeds in plug trays. Overall, they save time and work wonderfully.

If you are watching your pennies, then you can always use egg cartons or plastic containers to start seedlings.


4. Soil and Water Considerations

You’ll want to invest in a quality growing medium formulated for starting seeds. Pour the soil into a large bucket and mix with water using a trowel or your hands. The seed starting mix will rapidly absorb the water. Add more as needed. If you are using a peat-based seed-starting mixture then it takes several minutes to effectively absorb the water. Ideally, you’ll want the seed starter to feel moist but not overly water-logged.

Follow the instructions on the seed packet to learn how deep to place the seeds. Most require a depth of ⅛ inch to ½ inch. Cover the seeds lightly with the soil. After planting, mist the surface of the soil with water.


5. When to Put Seedlings in an Unheated Greenhouse?

When gardening in Central Oregon, the weather is always unpredictable. The spring months might be wonderfully warm or bitterly cold. After I built my first greenhouse, my first question was, “When can I put plants in an unheated greenhouse?”

If the night-time temperature drops below freezing then you’ll want to take steps to protect your seedlings and encourage germination. Remember, seeds will not germinate if the soil is not warm.

Provide protection in the greenhouse such as one or two layers of horticultural fleece over the seedling trays at night when the mercury drops.

You can also place bubble wrap to layer the inside of the greenhouse to provide an insulative layer of protection. During the day, the interior of the greenhouse will heat up from the sun’s rays and at night the insulation will help trap the heat to keep your plants warm even when it’s frigid outside.

In extreme cold, place barrels of water inside the greenhouse. During the day the water will heat up and then at night release the heat to keep your seedlings warm.

Place the seedling flats in a sunny location in the greenhouse. You’ll want to keep the growing medium moist but not overly wet to encourage germination.


6. Label Your Seedlings

If you are growing a variety of seedlings then it’s easy to become confused over which seeds are in which flat. You should label each pot so you know exactly what you are growing and where.

Invest in affordable plastic markers. You can label the markers and place them in each flat. Many gardeners invest in metal plant markers which always look lovely.


7. Germination of the Seedlings

To successfully germinate seeds, always take the time to read the seed packet. Sow the seeds to a depth of twice the seed in most situations. Press small seeds into the soil’s surface using your fingertip. Large seeds will require a greater depth.

Another option for germination is to place the seeds onto the top of a moist paper towel and place the seeds/towels into a sunny location in the greenhouse. This germination method usually takes about a week to effectively produce sprouts. You can then transplant the sprouts into the soil.


8. Thinning Your Seedlings

When your seedlings emerge from the soil and start to grow then you’ll need to thin them out if they are crowded. Overcrowding can lead to weak growth and diseases as the seedlings compete for light.

Thin your seedlings after they have a few sets of leaves. Pinpoint the weakest seedlings and remove them from the flat to make room for the stronger ones.


9. Fertilizing Seedlings

For optimum growth, your seedlings need a fertilizer application before you transplant them from your greenhouse to the garden. I always use liquid fertilizer on my seedlings for the best results. It is easily absorbed and promotes sturdy growth. Mix the fertilizer at ¼ of the strength.

Fertilize your seedlings after they have developed at least two leaves. Normally, I only apply one application of liquid fertilizer to my seedlings and then I wait at least a week or more before I transition them out of the greenhouse to the garden for transplanting into the soil or pots. When learning how to start seeds in the greenhouse, remember the ultimate goal is to give your young plants a protected headstart before transplanting them into the garden.



Some seeds will survive if you sow them directly in the garden each spring. However, they need to be a hardy variety in case the area is hit by a late frost. Clearly, when trying to start seedlings in the garden, timing is everything. You don’t want to plant too early or too late. If you are tired of the guessing game and feeling like you are that the mercy of Mother Nature, why not start your seeds in a greenhouse for optimum success?

Living in Central Oregon, I lost a lot of seedlings from late frosts each spring. Finally, I got tired of the struggle and decided to build a greenhouse on my property. I wanted to successfully grow year-round – even in the snowy winter. Also, I wanted to have a safe place for my seeds and protect my tender seedlings.

A greenhouse provides a stable environment with a consistent soil temperature to encourage seed germination.

How to Start Seeds in a Greenhouse

About Kimberly Sharpe 8 Articles
Based in Florida, Kimberly Sharpe has worked as a full-time freelance writer since 2006. She writes for numerous online sites and publications in a wide array of industries such as gardening, home design, DIY, real estate, home remodeling, lighting, cultivation methods, and more. Gardening, hydroponics, and outdoor design are hobbies she is passionate about in her spare time.