When To Start Planting Seeds In Greenhouse? All The Answers

What if the weather outside is frightful? If you have a greenhouse or planning to build one, the outside climate conditions won’t be much of an issue. In the outdoors, seeds and plants are subject to weather conditions and extreme temperatures. You are limited to grow seeds from late spring to early fall. Of course, this depends on the region you leave and the last frost date.

For instance, if you live in northern England or Scotland, propagation and germination are complex and almost impossible for plants that need warm environments.

Interior of a greenhouse

This is why a greenhouse is a perfect solution for you. In a controlled environment, as the one greenhouses offer, seeds can grow at almost any time of the year. You can have, that way, plants and fruits that would be impossible to grow without a greenhouse. Keep up with this guide to find everything you need to know on when to plant your seeds and many other tips that’ll help you become a pro gardener!

The benefits of greenhouse gardening

Whether you have a greenhouse or planning to make one, you should know all the benefits that it provides. There are a few, and they’re substantial and, in many cases, life-changing.

1. Weather protection

As mentioned above, constantly changing weather conditions could be a nightmare for a gardener. One frost day can destroy an entire seeding season. You lose money, and all the effort you put into planting and seeding goes in vain. A greenhouse provides the ultimate weather protection all year round, not only when it’s cold but also in extremely high temperatures. And if heavy snow and UV rays are not enough reasons for you, you’ll have peace of mind from those nasty animals that sneak in gardens eating everything and digging holes, making a total mess. 

2. All-in-one place

Having a greenhouse also saves you space and organizes your garden better. It’s better to have all your plants and vegetables tucked in one protected place than overflowing your entire lawn. Also, it’s ideal for keeping all your gardening equipment, tools and seed packets in there so you won’t lose them again!

A man is working inside his greenhouse

3. Pest and weed prevention

Having a secure place for your plants will prevent the ten plagues of Egypt from happening. An enclosed structure will keep algae, mould, weeds and all sorts of pesky pests outside. In a completely controlled environment such as a greenhouse, there is no room for unwanted visitors.

4. Plant whatever you want, whenever

A whole new world of planting possibilities opens in front of you. Now that you control the environment and the soil temperature, you can grow a wide variety of different plants. From melons and pumpkins to radishes, sweet peas and cucumbers

Getting started: the requirements of germination

You can’t simply wake up one day and decide to sow seeds out of the blue. There are many things to take into consideration. In this section, we will make a list of them to know exactly what needs to be done before starting seeds

1. Timing 

Timing is vital, as with most things in life. It is crucial for you to know your region’s average last frost date. A quick search on the internet will help you find the date and plan the germination process accordingly. Usually, 6-8 weeks prior to the last frost date is the best time to begin the growing season. This can vary from late winter to early spring, depending on where you live. You might think that this is too early since it’s freezing outside. Well, as said, greenhouses protect your seeds from frost, offering a significant head start on cultivation. The rule of thumb is not to begin seeding until after Valentine’s Day, as the natural light is low in the winter months. Keep up with this guide to find exactly what types of seeds you can plant at any given time. 

2. Heating

Even if a greenhouse protects your plants from cold, frost can still penetrate. That’s why you should make the necessary adjustments and install a heating system. Of course, an unheated greenhouse could still be practical in warm months, but without proper heating, you will not survive the winter. 

green house in the home garden

  • A cold frame is necessary, to begin with. It’s a shallow wooden pit covered with a glass lid and the sun penetrates easily, raising the temperature of the inside up to 5 degrees Celsius. It’s not that much, but cold-tolerant vegetables like broccoli will germinate with ease.
  •  Most gardeners nowadays use a propane heating system for maximum efficiency. Propane is a clean-burning fuel, eco-friendly and cost-effective. It raises the temperature as much as you want, making seed germination possible in late winter. The environmental impact is minimized, and productivity is increased. Propane is ideal for heating larger pots and plug trays
  • If you want a more centralized heating system, consider going with a heat mat. It’s ideal for seedling beds and seed trays and provides all the necessary warmth to initiate germination. If you want as much control over your seeds as possible, you can also use a heated propagator, creating ideal conditions for seeds and young plants to grow. 

Helpful tip: The ideal temperature for most seeds to grow is 15 degrees Celsius or more.

3. Lighting

If you want to eliminate the chances of your seed being controlled by the outside environment, you should consider adding artificial light. That way, the question of when to plant your seeds won’t be much of an issue. A life source above the seeds will keep them adequate and give all the necessary nutrients to grow. Although they act as a replacement for the sun, natural light is always something to look for. If you start seeding in the late winter, though, a grow light will be somewhat helpful. Bear in mind that plants need 6-8 hours of sunlight per day. In the UK, sunlight is a luxury, so artificial light is an effective solution. The costs of having lamps glowing over your plants might be high but have in mind that you’ll produce vegetables all year round

Seed Starting Indoors Growing Under Lights

4. Humidity

Although it’s not completely necessary, many professional gardeners use humidifiers to control the humidity levels of their greenhouses. Surely it’s not overkill, especially in the UK, where it rains almost every day. Humidity levels are essential to remaining stable for consistent and productive cultivation. A humidity dome is a good addition to your system. It’s inexpensive and helps to maintain stable atmospheric conditions

Tip: The more humidity, the merrier, but we don’t want radical changes! 

Plants to grow on different seasons

Now that you know when to start the seed germination process, we should list different kinds of vegetables and when you should begin planting them. Generally, there are 2 categories of plants: those that are winter-tolerant and the warm-season plants. 

1. Winter-hardy plants

Are you wondering what to grow in your greenhouse in winter? In this category, you can find plants and green vegetables that can be planted 8 weeks prior to the last date of frost. They do not require extensive heating. A solid cold frame is sufficient for them to survive the frost and cultivate. Such plants are:

  • Cabbages
  • Brussels sprouts
  • Leeks
  • Kale
  • Broccoli
  • Parsnips
  • Beets

2. Warm-season plants

Those plants are growing in hotter climates and require more than just a cold frame. You plant them usually in spring, 2 weeks prior to the last date of frost. Bear in mind that apart from extra heating, they need tons of light to cultivate correctly. Such plants are:

Wicker basket filled with freshly picked runner beans

Indoors and freestanding greenhouses

Now that we analyzed the benefits of a heated greenhouse and what they offer let’s provide some information about 2 types of greenhouses that can elevate your vegetable garden and make you feel like a pro. 

1. Indoor mini greenhouse

An indoor greenhouse is helpful when the outside temperatures are lower than the optimum. It’s useful for hardening of the houseplants and an ideal place to keep them safe from frost until late spring when you can transplant them in your garden. 

2. Freestanding greenhouse

A greenhouse of that type allows you to start a variety of seeds on shelves or benches. You can add some fans to regulate the temperature. Before planting your seeds, consider adding some seed starting mix for the best possible results. Some artificial lighting, as mentioned above, could also be helpful. 

What to expect

No matter the weather, even in low temperatures, your plants can survive if you care for them properly. During the winter months, plants usually do not grow as quickly as during spring and summer. For instance, tomatoes are summer-time plants. They need warmth and sunlight. You can, of course, plant them in a heated greenhouse in winter, but don’t expect them to reach their full potential. Some plants might not grow at all during the winter. But don’t panic. As long as you keep them alive, they will start to grow when the conditions are ideal

A few tips for successful winter seeding

Here are some extra tips to have in mind when planning to start your seeding process in the cool season:

    • Space: Make sure you have enough room in your greenhouse. If it’s overcrowded, plants will not grow fully, and some of them might even die.
    • Diseases: If you see any signs of illness as rotten leaves or roots, remove the plants immediately. This is to prevent diseases from spreading to other healthy vegetables.
    • Pests: Plant marigolds. They’re naturals pest repellants, and they’ll keep the bugs away.
    • Ladybugs: Let them live in peace. They are your allies as they keep pests under control.

young woman working in her cold frame greenhouse
You’re now ready to grow your own plants in a frost-free environment, having peace of mind. Greenhouses are perfect for cultivating plants at all seasons, not only for pros but also for home gardeners. In this guide, you learned everything there is to know about the time of planting, the benefits of having a greenhouse and some additional tips to make you look like a pro. Ready to plant some veggies? Coming next: Learn how to water your greenhouse plants when away.

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