How to Grow Seedlings in a Greenhouse

If you have the space for a greenhouse, learning how to grow seedlings in a greenhouse is a great way to get a jump start on your gardening season.

Greenhouses can extend your garden growing season in spring and fall. Plus a greenhouse allows you to save money by growing your own seedlings instead of having to buy them!

2014 was the first year I had a decent sized greenhouse so I was able to grow quite a few hearty seedlings to plant out in our garden.

At our old house, we didn’t have a sunny spot that was big enough to put a decent sized greenhouse.

Instead I had a tiny 2’x6′ greenhouse on the deck of our old house. Space was limited and I often ended up starting my seeds indoors instead.

Back then I used the tiny greenhouse mainly just to harden off my seedlings. But in 2014 I finally got to grow all my seedlings outside in my new greenhouse!

Choosing a Greenhouse to Grow Garden Seedlings

2014 was our first full year at our old homestead and we had plenty of sunny space for a greenhouse. We installed a 6’x8′ greenhouse kit (this is the one we bought ).

We bought this on sale and calculated that it was cheaper for us to buy it on sale than buy the supplies and build one from scratch.

We chose this kit for several reasons. First, it was a nice size upgrade from the tiny greenhouse I used at our old house. I still dream of having a large hoop house greenhouse to grow seedlings someday but that was not in the budget for us when we bought this greenhouse.

The other main reason we bought this greenhouse is because it was in our price range. We had so many other projects on our to-do list and the financial challenges of homesteading made our budget pretty tight.

We knew this green house kit we bought wasn’t a super sturdy, top quality greenhouse. It was better quality than the flimsy little thing I had at our old house though!

Because we get a lot of high winds in our area throughout the year, my handy husband added a few things to our greenhouse to boost the sturdiness and hopefully make it more durable in our wild Montana weather!

Why Grow Seedlings in a Greenhouse

Here in Montana, the growing season is quite short. Our average last frost is the end of May. Our first frost usually comes late August or early September.

To get a jump on the growing season, I’ve learned it is necessary to start seedling indoors or in a greenhouse.

When I grew seedlings in the house, I was always limited by space and could only start a few pots. It also was a bit tricky to water the seeds without making a big mess in the house if the water spilled over (which it often does!)

Plus the seedlings grown in our house always get a little leggy because the sunlight isn’t as good as it would be if we grew the seedlings in a greenhouse.

With a greenhouse, I’ve been able to start my gardening season in late winter and garden all the way through late fall and even early winter depending on the type of season we are having.

If I wanted to pay to run our little heater in the greenhouse all winter, I probably could extend our gardening season even longer!

When to Start Growing Seedlings in a Greenhouse

We set up our new greenhouse in early April. I was finally able to get some seeds started on April 10. This was six to eight weeks before our average last frost so a good time to get seeds started inside.

I have a friend who is an organic farmer an hour from here. She starts her seeds even earlier as early as January in large hoop houses since she plants the seedlings out in her fields before the last frost and uses row covers to protect them.

Once our greenhouse was set up, I could start growing seedlings in our greenhouse in late winter each season.

If you live in a warmer climate, you could start growing seedlings in a greenhouse even earlier than us.

I love to use these handy resources and charts to figure out what seeds to plant when specific to our area!

Setting up Shelves in a Greenhouse

Since I have a small greenhouse, I needed to maximize the vertical space with multi tiered shelves. This also is beneficial because some plants like warmer temperatures and some are cool weather plants.

The tomatoes and peppers love being on the top shelves of my greenhouse where it is toasty and humid.

The broccoli and cabbage seedlings started to wilt being up that high. They thrived on the lowest shelves of the greenhouse where it was much cooler.

We have a random collection of shelves in our greenhouse. You don’t have to buy new shelves, repurpose what you have!

Here are the shelves in our greenhouse:

• Two multi tiered metal shelves reused from our old tiny greenhouse
• One rectangular multi tiered metal kitchen shelf given to us
• One tall, skinny, multi tiered metal shelf given to us
• One waist high wood shelf my husband built from scrap wood (this also doubles as my planting table)

Starting Seeds in a Greenhouse

To figure out which plants to start in the greenhouse, I started sorting my pile of seeds. I organized the seeds according to when they could be planted outside.

I found this information by reading the back of the seed packet and using this free handy planting chart.

Here are the four piles of seeds I made.

• Cool season plant seeds that grew best when direct seeded into the soil before the last frost
• Cool season plant seeds that could be started as seedlings indoors and planted out as transplants
• Warm season plants that grew best when direct seeded after the last frost
• Warm season plants that could be started indoors and transplanted out after the last frost.

I chose to plant several cool season and warm season plant seeds to get a jump start on our growing season.

My first year with my new greenhouse I started tomatoes, peppers, tomatillos, broccoli, cabbage, onions, cucumbers, lettuce, kale and several varieties of herbs and flowers.

Except for the peppers and herbs, the seedlings were all be planted out in our gardens.

The peppers and herbs I grew in containers in the greenhouse throughout the season.

Peppers grow best in hot conditions. Montana summers can warm up a bit, but usually not as much as peppers like.

I’ve tried growing peppers in my garden several times over the last decade and always end up with a very tiny return for all the effort put into growing them.

My bell peppers would be the size of a lime instead of a baseball size. With a greenhouse, I always get a more productive crop by keeping the pots in the warmer greenhouse all summer.

Watering Seedlings in a Greenhouse

The first year I had my greenhouse, Little A and I spent the day on April 10th planting a wide variety of seeds in our stash of DIY upcycled seed pots filled with my homemade potting soil.

Gardening with a toddler is always an adventure (and sometimes a challenge!) but overall it was a fun day getting our seeds planted.

To water the seeds, I used this handy watering wand . I’ve wanted one of these for a long time and was thrilled to finally get one!

It made watering so much easier and I could adjust the water pressure so the seeds and seedlings weren’t disrupted.

When the seeds were first planted, I made sure the soil stayed moist but not too soaked everyday.

Since it was still fairly cool in April here in Montana, I kept the greenhouse door and windows closed. This increased the temperature and moisture  in the greenhouse and the seeds soon started germinating.

As the seeds began to germinate and the seedlings grew, they needed more water. I make a habit of checking on my seedlings every evening.

I water the seedlings that are dry an inch or more down from the top layer of soil. If you over-water your seeds and seedlings, you can cause them to rot.

Seeds that are too wet can rot in the soil before germinating. Seedlings that are watered too much can develop root rot and will die off.

If you let the soil dry out too much, the plants will wilt and be stressed. Or worse they could die if they dry out too much.

I’ve noticed that the warmer it is outside and the bigger the seedlings are, the more water they need. Makes sense doesn’t it?

By the end of May, the greenhouse was bursting with plants. I would keep the door and windows open 24 hours per day at that point to harden the plants off and ready them for planting in the garden.

They all need to be watered every day since it gets so warm in the greenhouse.

Heating a Greenhouse Full of Seedlings

Since I started seeds in our greenhouse when it was still getting down into the 20’s and 30’s at night, I knew I needed to figure out a heat source if I ever wanted the seeds to germinate.

I did some research and read that it is best to keep the greenhouse temperature at a minimum 50 degrees and maximum 90 degrees.

To track the greenhouse temperature, we used this snazzy little solar powered thermometer.

We sat it on the front outer ledge of the greenhouse and ran the cord into the greenhouse. I dangled the metal heat gage on the end about the height of the top shelf of the greenhouse.

This placement would allow me to see the highest temperature of my top row of seedlings to ensure they didn’t cook if it got too hot.

One day I would love to get one of these things so I don’t have to remember to run out and check the temperature and adjust the windows throughout the day!

For now, we always just have to remember to run outside in the morning to open the windows up before it gets too hot and remember to go out and close them when it cools off.

My organic farmer friend has a large hoop house, actually it is my dream greenhouse. It is about 15’x20′ so needs a powerful heat source.

She uses a pellet stove to heat her greenhouse in the early season. My greenhouse doesn’t need that big of a heat source!

We used two types of heat sources in our greenhouse:

Passive Heating

Years ago I bought a couple rain barrels with spigots on the bottom at a yard sale. They were only $5 each and I was thrilled with my bargain.

We put one in each corner of the greenhouse. I filled it with water from our well.

During the day, the water naturally heated up from the warmth inside the greenhouse. Then after the sun set and the temperatures started dropping, the warmth of the water helped keep the greenhouse a few degrees warmer.

Electric Heater

We bought a small electric heater with a built in thermostat. We ran a power cord from the shop to the greenhouse to power the heater.

I set up the heater to turn on when the temperature in the greenhouse dropped lower than 55 degrees. We have low electric rates in our area so this only cost us a few extra dollars a month on  the power bill.

Growing seedlings in a greenhouse is a great way to extend your growing season. When you live in a cool weather climate like Montana, having a greenhouse can make a big difference in how much food you can grow in a season so it is worth the investment!

Do you have a greenhouse? Do you have any tips on how to grow seedlings in a greenhouse?

Updated February 2023