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Growing plants in a greenhouse requires balance and finesse.

By Nicole Faires Posted Jun 2, 2017

interior of greenhouse with plants
A greenhouse is a necessity for most serious gardeners, and has been for over 700 years when they were first built in the Vatican to preserve tropical plants brought back by explorers. As the poet William Cowper observed, “Who loves a garden loves a greenhouse too.”

The concept is a simple one: capture heat from the sun by housing plants under a layer of glass or other light-trapping material. However, because plants are such complex living things, the actual practice of growing plants in a greenhouse requires a little bit of balance and finesse. It also involves some care to avoid common mistakes that can challenge your greenhouse growing conditions.

Here are ten things to watch for when gardening in a greenhouse.

1. Neglecting to control the temperature

One of the biggest mistakes gardeners make is forgetting to monitor their greenhouse temperature on a daily basis. Generally, the ideal summer temperature for a greenhouse is 75-85° F during the day and 60-76° F at night. In the winter, this changes to 65-70° F in the day and 45° at night. The best way to control the temperature is through ventilation, shade cloth, and heating. Use a basic hanging thermometer, or for only slightly more money, purchase a digital thermometer that also includes the relative humidity, which is very critical to know for preventing heat damage.

2. Not considering nearby trees

One of the number one regrets of many greenhouse owners is placing their greenhouse in the wrong location. This almost always has to do with a nearby tree: not only can a tree shade your greenhouse over a large part of the growing season, it can also drop debris throughout the year. Leaves must be removed from the greenhouse or you risk further shade, and a falling limb can cause extensive damage. Roots from nearby trees can also invade your greenhouse from underground, eating up nutrients and moisture meant for your plants. To protect your structure and whatever is growing inside, consider the placement of nearby trees and locate accordingly. If your greenhouse already has issues with trees, serious pruning or even removal are other options.

3. Forgetting to provide shade where needed

By the same token, there are times when you do want your greenhouse to be shaded—in a controlled manner. Even though you may be religiously monitoring temperature, it can still be very easy to accidentally allow plants to get heat stressed. As the greenhouse heats up due to different wavelengths of solar radiation, the air temperature will rise, as well as the leaf temperature. Because these are two separate things, shade cloth used in conjunction with ventilation will prevent heat damage and reduce how much water your plants need. The shade cloth works by blocking some solar radiation from the plants themselves.

close-up of red and green tomatoes growing in a greenhouse

4. Not controlling the humidity

Humidity is a natural part of the greenhouse water cycle. As plants grow they take in water through their roots, and then transpire that water into the air around them. However, the air can only hold so much water, and that ability is decreased as the temperature drops. Rapid temperature changes in a greenhouse can seriously damage plants, especially plants like tomatoes and cucumbers, which are common greenhouse fare. You could purchase a high-tech monitoring system, but for a hobby gardener, the real secret is maintaining stable temperatures. Ventilate the greenhouse during the hotter part of the day so it does not have a temperature spike, and make sure cooling fans are turned off and ventilation windows are closed well before night or cloud cover to prevent excessive cooling. In the winter, consider using a low-tech heat sink like barrels of water to stabilize temperature. If you purchased a humidity monitor, you will be able to see what the humidity is when dew is forming on your plant’s leaves, which is what you want to avoid.

5. Failing to ventilate

Ventilation is the key to balancing your greenhouse’s temperature, and often greenhouses just don’t have enough. To calculate how much ventilation you need, divide your total floor space by five. That number is 20% of your total floor space, which is the minimum area that should be able to open in the form of windows, vents, or rolled-up walls. However, just because you’ve opened the window doesn’t mean the greenhouse is venting – the air needs to flow through. Since hot air rises, this means pushing out that warm air at the top. You can suck cool air in through the bottom and use roof vents to allow the warm air to escape. Sometimes a small fan is necessary to keep the air moving.

6. Encouraging fungus

Unfortunately, warm, moist environments are the ideal places for mold and fungus to grow. If the relative humidity in your greenhouse is high (over 85%), there is little air circulation, and water or dew is sitting on the leaves, mildew will grow. Air circulation is key to preventing this, which is another good reason to use a fan. Adequate spacing between your plants is also very important. Using drip irrigation is another critical prevention step to keeping leaves dry. Common fungi include grey mold, powdery mildew, and black sooty mold, with symptoms of spots, blight, rot, and other reactions from plants. Sometimes you may see these signs without actually seeing mold, because the fungus is in the soil. It’s a good idea to clean your greenhouse and tools every year with vinegar or oxygen bleach to start with a clean slate.

wet strawberry leaves against rich soil

7. Depleting the soil

Soil management in a greenhouse is similar to the rest of your garden, but with a few extra challenges. Because you’re often growing the same things in the same spot over and over, the soil can easily become compacted, lose fertility, and attract pests. Besides the basics of adding compost and fertilizer routinely, consider using a blended soil mix when preparing your beds. Never use old potting soil, which will introduce pests and disease, and in fact, many professional organic growers recommend using a soilless mix in order to prevent these problems. These mixes contain peat, coconut fiber, perlite, vermiculite, worm castings and sometimes compost. However, because you would have to continuously purchase these materials, for the hobbyist, just using really excellent compost can have the same benefits: retaining the right balance of moisture and preventing fungus.

8. Watering too much or not enough

Yes, drip irrigation is usually best for a greenhouse. However, quantity is just as important as method. It’s very easy to under or over-water greenhouse plants because of the humidity levels. To gauge your plant’s needs, water in the morning, and then see how dry plants are by the end of the day. This gives plants time to transpire before the temperature gets higher. At the same time, you don’t want to give them too much water. If the weather gets cloudy, plants will take up a lot of water but may not be able to use it. It’s better to check them later in the day to water again. In the spring and summer, you will have to water more frequently when plants are young and growing rapidly and when temperatures are high.

9. Limiting light

We’ve already talked about limiting light intentionally with shade cloth, but you may not realize that you are accidentally limiting the amount of light your plants are getting without even noticing. Greenhouse coverings come with a light transmission rating. The standard plastic cover is 6mm, which is durable and usually rated with 91% light transmission per layer. However, this transmission rate will degrade as your plastic ages and turns yellow. You can also purchase a solar power meter to keep track of this: direct sunlight is 360 BTU per square foot per hour and under 6mm plastic, should be around 327 BTU per square foot per hour. If you have an 8mm polycarbonate greenhouse, it is usually rated at 80% light transmissivity, and will block UV rays that burn plants. Polycarbonate greenhouses don’t yellow with age, but they do get dirty and this grime can reduce the light transmission by 10% or more. Condensation can also reduce light transmission by another 10, so keep your greenhouse cover clean and up to date.

10. Growing the wrong plants inside

One of my biggest mistakes in greenhouse growing is getting greedy and trying to start everything early and thinking warmer is better for everything. As fun as it is to experiment, proper garden planning can take you from being a mad scientist to being a pro. Check ideal plant temperatures, and maximize your garden space by using it to extend seasons. This means growing vegetables in the winter, and then as temperatures warm up, use your greenhouse to start seedlings. When things are really warm, you can transplant the tomatoes and other heat-loving crops. Stay focused to get real results.

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Top 10 Greenhouse Gardening Mistakes

Author photo
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Nicole Faires is an urban farmer and best-selling author of books on sustainable agriculture and food policy. Originally from Montana, she now lives with her family on the West Coast. Find out more at http://deliberatelife.ca or connect with her on LinkedIn.
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  • Shannon Cowan

    I’ve learned a lot from my digital greenhouse thermometer, which includes a history of the highest and lowest temps in a 24 hour period. Knowing this history can help you unravel the mystery of why tomatoes may not fruit, among other things. A great list! Thank you Nicole.

  • Torstein Rosenvold

    I think I make 10 out of 10 of these mistakes. Thanks for the good info ..

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