Greenhouse Growing for Beginners: 7 Essential Tips

Almost every gardener I know wishes for a greenhouse. Maybe you dream of moving your seed-starting operation out of your spare bedroom and into an outdoor space. Or you may want to experiment with growing citrus trees that are too big for your cozy yet tiny home.

I get it—whether you’re a beginner or veteran grower, a greenhouse is an exciting addition to your garden! However, it also presents new challenges and a sometimes steep learning curve.

If you want to grow healthy plants in your greenhouse this year, you must know how to manage this space properly. I’ll cover a few essential tips involving maintaining the proper environment, selecting the right plants, and making the most of this unique growing space.

What Is a Greenhouse?

If you’re part of the gardening world, you’ve probably heard people throw around terms like greenhouse, hoop house, high tunnel, and cold frame to refer to protected growing spaces. Since people use these terms so loosely, it can be challenging to determine if they all mean the same thing or if each one has a distinctive definition.

While I’m not here to manage how you use these terms, knowing the exact definitions of each structure can be helpful.

A greenhouse is typically a permanent or semi-permanent structure designed to stay in the same place for years. It’s often constructed of a metal frame and glass or rigid plastic panels, and the structure is often set on a solid foundation. Although it’s not a requirement, many greenhouses contain heating and/or automated ventilation systems.

High tunnels and hoop houses are semi-permanent structures that are easy to move. They consist of large metal hoops covered with a UV-resistant plastic layer. Although some high tunnels have heating and ventilation systems, most do not. To manage temperature and ventilation, you can raise and lower the plastic sides of the tunnel.

Cold frames are small, season-extension structures that can take many shapes and forms. They typically cover a single garden bed and require manual ventilation to control temperature and moisture.

Although some of the following beginner tips apply to all of these structures, I’ve written them with greenhouses in mind.

Establish Your Desired Use

Before buying and growing in a greenhouse, consider how you will use it. Do you envision it as a seed-starting space, a place to overwinter your cold-sensitive lemon trees, or a spot where you can grow tomatoes early in the season? As long as you set up the space appropriately, it’s okay to use your greenhouse for multiple purposes.

Determining how you will use your greenhouse will help you select the proper structure and build it appropriately.

For example, if you plan to use it solely for seed starting, an eight-foot by eight-foot structure will work just fine. Since you won’t be growing any larger plants, you’ll have space to build a potting bench, tables for seedlings, and shelves for storing pots and soil mix. Since you’ll regularly water seedling trays, consider covering dirt floors with gravel or landscape fabric to avoid soil splash and weed growth.

However, if you mainly want to use the space to overwinter cold-sensitive plants like citrus, ginger, and figs, you’ll probably want a different layout. You’ll need open areas where tall plants can expand and an opening that allows you to move them in and out of the greenhouse easily.

And if you want to grow plants in the ground, you’ll want an entirely different setup! Rather than building a concrete or gravel floor, you’ll want to develop rich soil or build raised beds. You’ll also want to consider irrigation systems and ensure pathways are large enough to accommodate harvest baskets or bins.

The moral of the story? Think about how you plan to use your greenhouse before filling it with plants so you can avoid common beginner mistakes. Designing a proper layout and systems ahead of time can save you countless headaches down the road.

Pay Attention to Moisture

Since greenhouses are closed structures, humidity is a major concern. While it’s possible for the humidity to get too low in a greenhouse, growers usually face the opposite problem.

Irrigation and plant transpiration can lead to moisture buildup in closed greenhouses. This high humidity means plants are more likely to develop fungal diseases like powdery mildew, downy mildew, and gray mold. Therefore, your goal is to keep the humidity at moderate to low levels.

The easiest way to control humidity is to use vents and fans. They push out humid air and draw in dry air. Some greenhouses utilize sensors and automated systems to open vents and turn on fans when the greenhouse reaches a specific humidity level. However, you can manually open vents and turn on fans to decrease air moisture.

Another option is adding a dehumidifier to your greenhouse. However, this won’t add a beneficial increase in airflow like fans and vents.

If your greenhouse consistently has consistently high humidity, consider your irrigation practices. Watering your plants more than necessary will increase the air moisture levels. Plus, constantly moist soil will increase the likelihood of pests like fungus gnats.

Regularly Scout for Pests

Since greenhouses are closed structures removed from the outside world, greenhouse-grown plants often face different pest pressures than their outdoor-grown counterparts. There are a few main reasons for these differences. 

One way to prevent pest damage is to regularly scout your greenhouse for unwelcome critters. Set aside 15 minutes each week to thoroughly inspect your plants. Turn over leaves and check the base of the plants near the soil. Note any insects and their numbers.

If you’re not sure what the insects are, spend time figuring it out! Remember, not all insects are bad for your plants. Start by browsing through this list of common pests and common beneficial insects.

Once you determine you’re dealing with a pest, you can take appropriate control measures. Removing the pests may be as simple as wiping the pests of your plants. Use a soapy rag and continuing to monitor their numbers.

Avoid Sudden Swings in Temperature

One major benefit of growing indoors is the ability to control and regulate temperature. With the help of a heater or a sunny day, the air inside your greenhouse can be 70°F (21°C), even if it’s below freezing outside. And if you’re not careful, the same greenhouse can reach over 100°F (38°C)!

While you may be tempted to allow hot days to offset cold nights, you should avoid sudden temperature changes. It’s okay if the temperature is warmer during the day and cooler at night, but you shouldn’t see a swing of 50 degrees (10°C).

A better option is maintaining a relatively steady temperature throughout the day and night. A combination of heaters, vents, and fans can help you accomplish this. Once the temperature reaches above 70 or 80°F (21-27°C) during the day, open vents and turn on fans to decrease the temperature. You can also use heaters during the night and program them to turn on when the temperature hits 50°F (10°C) or 40°F (4°C).

When setting the temperature, remember that each plant has an ideal temperature range. Brassica and lettuce seedlings don’t mind temperatures near freezing, but many tropical plants experience stress when the temperature dips below 50°F (10°C).

Select Proper Varieties

If you’re growing in the ground in your greenhouse, pay attention to the varieties you select. This applies whether you’re growing tomatoes in the spring, lettuce in the winter, or citrus year-round.

Since growers often plant vegetables like tomatoes and cucumbers in protected culture, plant breeders have created varieties that thrive in these growing conditions. For example, there are cucumber varieties well-suited for greenhouses because they produce fruit without pollination. Since greenhouse tomato plants remain dry but are often grown in high humidity, resistance to leaf mold and gray mold is more important than resistance to early blight.

If you plan to grow greens like lettuce, kale, and spinach throughout the winter, you should also look for well-suited varieties. While you can try growing any variety in your protected structure, cultivars resistant to common greenhouse diseases, such as downy mildew, are more likely to perform well.

Avoid Wet Leaves

Since wet leaves can lead to fungal diseases, it’s best to keep plant foliage dry. This is especially important in a greenhouse since airflow is often limited, and disease can spread quickly in the confined space.

Water potted plants near the base of the plant to avoid wet foliage and soil splash.
A watering can, hose, or drip irrigation system can all provide proper irrigation. You can also choose to bottom water potted plants and trays filled with seedlings.

If you’re growing plants in the grow, opt for drip irrigation instead of overhead watering. Drip emitters apply water directly to the soil and keep foliage dry.

If you choose to water seedlings with a hose, ensure you provide excellent airflow to dry the foliage and prevent disease. I recommend running fans near your seedlings, especially if you utilize overhead irrigation.

Keep a Journal

Although beginners think they’ll never forget that intense aphid outbreak or that impressive tomato harvest, it’s easy to forget greenhouse milestones, let alone minuscule details. That’s why I recommend using a journal to keep track of planting dates, temperatures, pest problems, and more.

If you notice some of your plants show signs of disease or cold-damaged, you can look back on your notes to try to determine a cause. For example, you may realize you brought disease into your greenhouse when you moved outdoor plants inside.

You can also look back on your journal year after year to remember planting dates, first harvest dates, and when seedlings were large enough to transplant outdoors. Then, you can use this information to fine-tune your greenhouse practices and advance from a beginner to an expert indoor grower.