Greenhouse Seed-Starting 101: Tips and Tricks for the Beginner

If you’ve found yourself with a new greenhouse, seed starting just became a lot more fun! However, this unique growing environment also comes with new challenges. Join farmer Briana Yablonski to learn some tips for starting seeds in the greenhouse.

A woman in an apron filling small biodegradable pots with soil on a wooden table surrounded by potted plants and gardening tools.

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Starting plants from seed is one of my favorite winter gardening activities. It allows you to save some money, get your hands dirty, and look forward to the growing season that lies ahead. And having access to a greenhouse makes it even more fun.

Greenhouses allow you to have a dedicated seed-starting space and often provide the ideal environment for seedlings to thrive. However, knowing some tips and tricks related to sowing seeds will help you make the most of your protected growing space.

Join me to learn some helpful hints related to setting up your seed-starting space and keeping your seedlings healthy.

Heat Mat

Epic Seed Starting Heat Mat

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Epic Seed Starting Heat Mat

Rain Wand

Dramm 30

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Dramm 30″ Select Rain Wand

Watering Can

Epic Gardening Watering can

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Promo Watering Can

Order Supplies in Advance

A young man and woman using a laptop to browse for plants, sitting at a table with houseplants in the background.
I recommend ordering supplies and setting up your seed-starting station a few weeks before you begin sowing.

When it’s time to sow seeds, you don’t want to spend your time running looking for seedling trays and your favorite tomato varieties. That’s why I recommend ordering seeds and setting up your seed-starting station a few weeks before you begin sowing seeds.

How to Determine What Seeds to Order

A person holding a green clipboard with a checklist, standing in a room filled with greenery and bright natural light.
Make a list of the new varieties you want to order.

First, go through old packets and take note of which you should keep and which you should toss. Then, you can make a list of the new varieties you want to order.

If you’re not sure where to start, think about what veggies and herbs you like to eat, what growing space you have available, and any new crops you’d like to try growing. Browsing through easy-to-grow vegetables and flowers is another good place to start.

Once you have a list of what you want to grow, order the seeds ASAP! Seed companies sometimes sell out of popular varieties in the spring, so ordering in the beginning of the year is always the best option.

Gathering Seed-Starting Supplies

A stack of black plastic trays with small compartments, arranged on a wooden bench in an outdoor gardening area.
Small cells are great for starting seeds and larger trays allow you to pot up seedlings that have outgrown their pots.

Since no two gardens are the same, there isn’t one ideal seed-starting setup. However, all systems include a light source, a way to water, and trays to hold the soil mix.

If you’re starting your seeds in a greenhouse, you won’t need to worry about light. However, make sure you have a way to easily water your seedlings, whether that’s a hose attached to a spigot or a watering can that you can fill up at a sink.

Finally, figure out what containers you need. You’ll find numerous options ranging from flimsy pots that easily break to sturdy seed-starting trays that last for numerous growing seasons. I like to have numerous sizes of cell trays on hand; small cells are great for starting seeds, and larger trays allow you to pot up seedlings that have outgrown their pots.

Create a Seeding Calendar

A hand marking dates on a green calendar placed on a table with gardening items, including soil and wheat stems.
Create a planting calendar to help ensure that you start each flower, veggie, and herb seed at the proper time.

When you have all of your supplies and the excitement of growing sets in, you may be tempted to plant all your seeds at once. I’m here to tell you this is not what you want to do! If you sow too early, you’ll end up with mature seedlings before you’re ready to plant them outdoors.

Creating a planting calendar can help ensure you start each flower, veggie, and herb seed at the proper time. You can use your predicted last frost date to determine your ideal transplanting date. With this date in mind, you can calculate when you need to sow your seeds.

Many seed packets offer guidance on when to plant. For example, you may see “sow 4-6 weeks before your last frost date” or “seedlings should be ready to transplant in 3-4 weeks.” You can use this information to mark on your calendar when you need to sow your seeds indoors.

Let’s look at ‘Black Beauty’ eggplant as an example. The seed packet says you should sow 10-12 weeks before your average last frost date. Therefore, if your last frost is April 15, you should sow the seeds sometime between January 21 and February 4.

Start With a High-Quality Seed Starting Mix

A flat lay of gardening materials, including labeled bags of peat, perlite, and humus, alongside small pots and wooden tools on a brown surface.
Some bags contain single materials like compost and peat moss, while others contain blends of multiple materials.

With so many types of potting mixes available, it can be challenging to determine which ones will work best for starting seedlings. If they’re all pretty much the same thing, can’t you just choose the least expensive?

Well, it turns out they’re not all the same. Knowing what to look for in a good seed starting mix will help you choose a product that supports excellent germination and plant growth.

First, you need to know the differences between bagged soils. When you walk through a garden center or browse an online retailer, you’ll see all kinds of products in bags. Some bags contain single materials like compost and peat moss, while others contain blends of multiple materials. Some of these blends are designed to support plants that spend their lives in containers, and others work best for quick-growing seedlings.

If you want a mix that grows healthy seedlings, look for a product labeled as a seed-starting mix. These products contain ingredients that allow the soil to remain moist between waterings without becoming saturated. They also contain enough nutrients to support plants during their first four to eight weeks of growth. I love mixes that also contain mycorrhizae, as these beneficial fungi support strong root development and help plants take up nutrients and water.

Be Aware of Ideal Germination Temperatures

Close-up of a small seed starting tray and cell pack with a clear humidity dome placed on a heat mat to aid germination.
You can utilize heating mats to easily provide supplemental heating to crops that require it.

One of the benefits of greenhouses is the warm temperatures they provide. However, it’s important to remember that not all seeds germinate best in hot weather.

Before you stick all of your seeds right next to the heater or in the warmest corner of your greenhouse, note their ideal germination temperature. For example, squash and cucumbers germinate best between 75-80°F (24-27°C), but peas sprout best when kept in soil that’s between 50-65°F (10-18°C).

Since heating the entire greenhouse can be difficult and expensive, it’s okay to keep it at a cooler temperature. You can utilize heating mats to easily provide supplemental heating to crops that require it.

Take Note of Your Available Space

Close-up of vibrant kale leaves with curled edges and a textured surface, displaying a mix of deep green and light frosty hues.
Cool-weather crops like kale can tolerate cooler temperatures, but their seeds require warmer temperatures to germinate.

Before you begin sowing seeds, think about how much space you have to house them. If your greenhouse is divided into a heated and unheated section, note the size of each area. And if you don’t have a heater in your greenhouse, be aware of how many trays you can fit onto heat mats.

While cool-weather crops like broccoli, cabbage, and kale can tolerate cooler temperatures, their seeds require warmer temperatures to germinate. If you have limited heated space, consider starting these seeds in open flats, then moving the seedlings to individual trays when the plants are a couple of weeks old.

Keep Soil Slightly Moist

A tray filled with moist, dark soil placed on a wooden table, ready for planting.
Aim to water your seedlings anywhere from three times a day to once every couple of days.

Water is essential for seed germination and plant growth. However, too much water can cause just as much damage as not enough water.

Constantly moist soil increases the chances that seedlings will suffer from wilting and death, known as damping off. This disease is caused by fungi like Rhizoctonia, Fusarium, and Pythium. Since these fungi thrive in moist conditions, they are more likely to cause problems when soil remains moist.

Moist soil also supports fungus gnats. The adult versions of these tiny flies lay their eggs near moist organic matter, so wet potting soil provides the ideal environment. After the eggs hatch, the larvae feed on plant roots and cause damage.

So, how often should you water? Unfortunately, there isn’t a single right answer. The ideal watering schedule depends on the temperature, light, container size, and humidity.

Your goal is to keep the soil slightly moist. If the soil seems moist to the touch, don’t water. However, if the top inch of soil is dry, go ahead and water. In general, you should aim to water your seedlings anywhere from three times a day to once every couple of days.

Avoid Soil Splash When Watering

A hand-held watering can sprinkling water over a green tray filled with soil, placed on a wooden surface with a small trowel nearby.
A rain wand provides an ideal stream and also comes with a long handle that makes it easy to reach all of the plants in a tray.

Maintaining a clean environment is key if you want to avoid diseased greenhouse seedlings. Sanitizing seedling trays, using a disease-free potting mix, and maintaining good airflow can all help prevent disease. However, avoiding soil splash is another key part of maintaining a tidy greenhouse and healthy plants.

If you water with a hose or watering can, choose an option that provides a gentle flow. A rain wand provides an ideal stream and also comes with a long handle that makes it easy to reach all of the plants in a tray. If you prefer to use a watering can, look for a spout with a rain shower option.

Provide Adequate Airflow

A curved garden cover with an open wooden ventilation flap, set in a green area with bushes and plants.
Open up the doors, vents, or sidewalls of your greenhouse on warm days to allow air to enter.

No matter what you’re growing in a greenhouse, providing excellent airflow is a key part of maintaining healthy plants. Stagnant air encourages moisture to settle and increases the odds that plants will become diseased.

If your greenhouse has doors, vents, or sidewalls, you can open them up on warm days to allow air to enter. Fans also help move air and can be used regardless of the outdoor temperature.

Harden Off Seedlings Before Transplanting

Young, bright green tomato plants sprouting in small pots with vibrant leaves in a sunny indoor garden.
Gradually increasing the amount of time plants spend outdoors allows them to adjust to the direct sun, wind, and cold nights.

When you get your temperature, lighting, and watering dialed in, greenhouses provide the ideal growing environment for plants. They don’t have to worry about heavy rainstorms, harsh winds, or sudden changes in temperature. However, it’s important to remember that the plants will eventually have to survive outside of this perfect bubble.

That’s why hardening off greenhouse-grown seedlings is essential.

Hardening-off involves slowly acclimating plants to their new environment. Gradually increasing the amount of time plants spend outdoors allows them to adjust to the direct sun, wind, and cold nights without experiencing debilitating stress. When it’s time to transplant the plants outdoors, they’ll already be used to much of their new environment.

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