Top 10 Plant Q&A

Top 10 Questions About Greenhouses

By Mary H. Dyer | January 13, 2019
Image by gemenacom

Top 10 Questions About Greenhouses

by Mary H. Dyer January 13, 2019

Top 10 Questions About Greenhouses

By Mary H. Dyer | January 13, 2019

Here at Gardening Know How we get lots of questions, and this includes greenhouses. Our goal is to provide answers to these inquiries to the best of our knowledge. The following information includes the 10 most commonly asked questions relating to greenhouse gardening.

1. How do you get rid of bugs in a greenhouse?

Surprisingly, even in a greenhouse environment, insect pests are inevitable. These greenhouse pests may range from aphids and mealybugs to mites, whiteflies and more. One of the easiest ways to monitor them is by using sticky pads, which keep many insects under control or at least signal you when their numbers are growing for further control. The use of insecticidal soap and neem oil are additional methods of control and normally safe to use on edibles.

2. What is the ideal humidity for starting seedlings in a greenhouse?

Most plants prefer humidity between 50 and 80 percent. Foliage loses water and wilts rapidly when levels are below 50 percent, but high humidity can cause fungal diseases such as damping off, which causes tiny stems to rot at the base. If high humidity is a problem, you can provide a drier environment by watering plants from the bottom, and by providing adequate heat and ventilation. If humidity is too low, improve conditions by misting plants, with cool mist vaporizers or small humidifiers, or by placing pots on trays of wet pebbles.

3. How to get rid of mushrooms in greenhouse soil?

Mushrooms often pop up in potting mix, which indicates the substance is high in organic material. Although the fungi are harmless, they tend to be unsightly. If you notice tiny mushrooms, pull them up by their base and discard them to prevent development of spores. Water plants from the bottom and be careful not to water excessively. Provide proper ventilation to keep the air moving. If air circulation is limited, a small rotating fan will help.

4. Why is soil turning green on my starter seed trays?

Green stuff on the surface of potting mix is probably algae or moss. While the substance is generally harmless, a thick layer can prevent proper absorption of moisture and nutrients. Often, you can remove the green by breaking up the surface of the soil with a pencil or toothpick. Always start seeds in fresh, good quality seed starting mix. Allow the surface of the potting mix to dry slightly, and don’t water to the point of sogginess. Be sure your greenhouse is properly ventilated to prevent excess humidity.

5. Should cloches have an air gap top prevent over heating?

A greenhouse provides sunlight that warms the soil and protects plants from frost. However, an air gap is essential, as too much heat in an airtight greenhouse can quickly cook tender greenhouse plants. Be sure to close vents at the end of the day when cool temperatures are expected, then open them in the morning while the air is still cool. Automatic vent systems are available. The same applies when using cloches in the garden, either by tilting it to allow for some air to enter or removing altogether when warmer day temps are expected. Just remember to reposition the cloche in the evening.

6. Do you have instructions on attaching shade cloth on the inside of a greenhouse?

Greenhouse experts recommend that exterior shade is more effective than interior screen systems because light is blocked before it enters the greenhouse. Once light is trapped inside, it becomes difficult to remove the excess heat. However, if you want to install interior shade cloth, it’s best to invest in a retractable system that allows sunlight to enter on cloudy days. Mechanized systems, consisting of support cables, motor and shade, can be opened and closed as needed. You can also install a less expensive indoor system consisting of ropes and pulleys.

7. Can I grow herbs in my greenhouse in the winter without a heating system?

If you live in a cool northern climate, it will be difficult to grow herbs in a greenhouse. Most herbs will struggle when temps drop below 40 to 50 F. (4.5-10 C.), while many prefer consistently warm temperatures of 65 to 70 F. (18-21 C.) during the day and 55 to 60 F. (13-16 F.) at night. You can, however, get a jump on spring growing by starting many herb seeds in an unheated greenhouse, but you may need a heat mat for germination. Herbs that tend to be cold hardy include lavender, garlic, rosemary, winter savory, catnip and oregano.

8. How to choose the best greenhouse?

If you want to grow tropical plants, you’ll need a greenhouse capable of maintaining steady temperatures of 65 to 70 F. (18-21 C.) or higher. Most flowers, vegetables and herbs need temps of around 55 F. (13 C.), while a “cool” greenhouse is suitable for germinating seeds and raising starter plants in spring. A greenhouse with clear panels provides warmth and light for sprouting seeds, while a diffused or glazed covering provides balanced levels of light heat for growing plants to maturity. Additionally, look for a greenhouse that is adequately ventilated and insulated to prevent plants from excess heat and cold.

9. How to keep greenhouse cool in hot weather?

A shade cloth, either internal or external, will help block the intense rays of summer sunlight. Make sure shade is in place before the heat of the day, then remove the cloth on cooler days. Ventilation, in the form of roof or side vents, keeps air moving and prevents buildup of heat. If ventilation is limited, you may need to run a fan (or two) to circulate the air. Be sure to keep greenhouse plants well watered during hot weather. A light misting can cool plants on hot days, but mist judiciously, as too much moisture can promote disease.

10. What plants/vegetables grow in a greenhouse?

You can grow nearly any plant in a greenhouse, but it’s all about determining the needs of particular plants. For instance, you can germinate seeds and grow starter plants in an unheated greenhouse, while tropical plants need consistently warm temperatures year round. You can easily start most flower seeds and cool season vegetables such as peas, broccoli and lettuce in an unheated greenhouse, but tomatoes and peppers need plenty of sunlight and warm temperatures between 55 and 85 F. (12-29 C.).  Some flowers, such as impatiens and ferns, thrive in shade, while geraniums and daisies need bright sunlight.

We all have questions now and then, whether long-time gardeners or those just starting out. So if you have a gardening question, get a gardening answer. We’re always here to help.

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