Greenhouse Gardening: What You Need to Know Before You Grow

We sell tens of thousands of seedlings per season out of our retail greenhouse to pay the bills and keep the lights on. And let me tell you, greenhouses can be amazing, or the worst thing ever… if you don’t manage three important factors!

If you’re growing in a greenhouse or want to grow in a greenhouse, this blog post is for you. These tips for improving plant health and maximizing success in greenhouse gardening apply to greenhouses of all sizes. 

A Greenhouse is a Luxury

Many home gardeners think that greenhouses are the ultimate gardening tool. The thought is, “If only I could just get a greenhouse; it would be so easy to grow a ton of plants!”

While greenhouses are great gardening tools that allow you to grow large numbers of plants earlier in the season, they are also a lot of work, and they’re not foolproof.

If you don’t have the luxury of a greenhouse, don’t worry. There are other great methods for extending the growing season and gardening in the winter, like seed starting indoors, building a DIY cold frame, and using frost blankets for plants in the garden.

Buying A Greenhouse

If you have been wanting to buy a greenhouse, the MIgardener store now carries a selection of the best greenhouse kits! From the USA-made All-Metal Round Hoop House kit to the DIY Gothic Greenhouse kit to the build-your-own Hobby Greenhouse kit, we only sell high-quality products we can stand behind!

Greenhouse Gardening

A greenhouse is simply a tool that allows you to grow plants earlier by utilizing the sun’s power. No matter the size, the mechanics of how a greenhouse works stay the same, so the tips and tricks that I am sharing will apply to all greenhouses. 

The three most important factors for successful greenhouse gardening are:

• Controlling Humidity

• Regulating Temperature

• Combating Fungal Problems

Controlling Greenhouse Humidity

Humidity is the amount of moisture in the air. Humidity is beneficial because it helps seeds germinate, prevents plants from dehydrating, and promotes plant growth.

But humidity is only good for plants to a certain point. Greenhouses act like a giant humidity dome, trapping moisture in the air. Too much humidity will lead to mold, mildew, and other issues. To control the humidity in a greenhouse, both ventilation and airflow are needed. 

Venting lets excess humidity escape the greenhouse, helping things dry out a bit (keep in mind that it also cools down the temperature). We use vents (louvers) and roll-up sidewalls in our large greenhouse. In a small greenhouse, you may only need to install a vent in one end or roll up the sides. 

To increase success, you really should have some airflow in your greenhouse! We use fans to circulate air, dry off wet plant leaves, reduce mold and mildew, and strengthen seedling stems.

Regulating Temperature in a Greenhouse

The next thing you need to successfully grow in a greenhouse is heat! It’s so important that I dedicated an entire blog post to how to heat a greenhouse.

Regardless of what you’re growing, you absolutely should have a heat source (besides the sun) to keep it warm at night. It’s the low nighttime temperatures that pose a problem for many greenhouse growers.

There are various methods for heating a greenhouse; our method of choice is a 125,000 BTU forced-air heater. Whatever method you choose, the goal is to reduce temperature fluctuations that can occur between daytime and nighttime. 

Temperature swings will stress seedlings out and cause infrequent germination and spotty growth. If at all possible, try to keep the temperature in your greenhouse constant. 

I also recommend getting a thermometer to keep in your greenhouse. It doesn’t have to be anything special or expensive, just something that reliably tells you what the temperature is, so you know when to vent and can track your temperature data. 

Pro Tip: If you are setting up a retail greenhouse or need to monitor temperature when you are not on-site, I highly recommend getting a Temp Stick. With it, you can get minute-by-minute updates on temperature and humidity, no matter where you are.

What’s the best temperature for a greenhouse, you might ask? That depends on what you are growing. Brassicas like 55° to 65°F, while tomatoes and peppers like warmer temperatures. I keep our greenhouse at around 70°F because I can grow a wide variety of crops.

Combating Fungal Issues When Greenhouse Gardening

One of the main issues you will likely face when greenhouse gardening is fungal problems. Even in our large greenhouse with lots of ventilation and heaters, it's an issue, especially in the late winter and early spring, because the outside temperature is too cold to vent the greenhouse.

It’s important to use fungicide to control fungal issues, or your plants will be harmed! You don’t need to get anything crazy; I recommend a simple, organic solution like copper fungicide. 

So many plants are susceptible to fungal issues, including herbs, flowers, and ornamentals, as well as tomatoes, peppers, and other fruits and vegetables. It is highly recommended to spray greenhouse plants with a fungicide every 2 to 3 weeks as a preventative measure.

To spray, add 2 teaspoons of copper fungicide to a gallon of water in a pump sprayer. Shake it up and spray plants until they are dripping. This makes leaf surfaces inhospitable for things like gray mold to colonize, even in high-humidity environments. 

Greenhouse & Seed Starting Supplies

I hope you learned something new and this spurs you on to Grow Bigger! Check out the MIgardener store for all of your greenhouse and seed starting supplies like grow lights, grow bags, seedling heat mats, seed starting trays, seed starting pots, and more.