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 is another option.

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…