Since I miss my home grown vegetables in the winter, I am considering getting a greenhouse. What do I need to know before starting this project? NM

This is the time of year most of us are missing our vegetable garden. Growing fresh produce in our backyards can spoil us. It’s no wonder you are thinking about getting a greenhouse. There are several ways to approach constructing a structure so you can extend your growing season.

First of all, a greenhouse is great, but it may not be where you want to start. High tunnels might be a good way for you to test the waters before diving into a greenhouse. Since cost is almost always a consideration, high tunnels can be constructed for about $.50 per square foot as compared to $20 per square foot for greenhouse construction. So, let’s talk about high tunnels first. 

High tunnels are unheated, plastic-covered, hoop houses that are relatively inexpensive to construct and do a good job of extending your vegetable growing season. They are similar to greenhouses with a few primary differences.

1) High tunnels do not have any installed temperature control system. They are warmed in the winter by the sun and cooled in the summer by the breeze.

2) Your crops are typically grown in the soil or in raised beds inside in the high tunnel, as compared to elevated tables or containers on concrete floors.

3) High tunnel frames are anchored in the ground rather than affixed to a solid foundation.

Aluminum or galvanized steel tubing is the most common material used to frame your high tunnel and these supports are typically sunk into concrete below ground level. A good, solid frame will extend the life of your high tunnel by helping them survive our Oklahoma winds. 

Most high tunnel frames are covered with a single layer of 6 mil polyethylene plastic. The plastic film walls of the high tunnel are usually built in a way that the side walls can be removed in the summer to help keep the crops cool. 

By definition, a high tunnel should be high enough to allow you to walk comfortably. This is in contrast to a low tunnel which may be only 2 to 3 feet tall. 

An ideal site for your high tunnel would be level or nearly level ground. You might consider raising the soil level in your high tunnel to help water drain away from your high tunnel rather than pool inside. 

Your high tunnel should be located where they receive at least 6 hours of sunlight per day and be oriented with the peak of the tunnel running north and south. It’s also not a bad idea to have some wind breaks in place, but this won’t be an option for everyone. 

Since the heating and cooling of your high tunnel are passive in nature, you are essentially the thermostat for your structure. During an average winter day, the sun raises the temperature of the air and soil inside your structure. When the sun sets, this heat helps keep your crops warm as the soil releases some of it’s stored heat. 

While high tunnels will struggle to keep your crops warm enough during extremely cold weather, a space heater can be added to provide additional heat on cold nights. 

Cooling is accomplished through air flow. During warm months, the side walls can be removed to allow for maximum air flow. 

Greenhouse come in a couple of different varieties. There are lean-to structures which utilize one side of your house as part of it’s structure. Free-standing structures are probably the type of greenhouses people are familiar with.

Greenhouse size will likely be determined by how much you are willing to invest, but when building a greenhouse, consider how you will use it. If want to be able to wheel your wheel-barrow into the structure, obviously the doorways will need to accommodate the width of the wheel-barrow. It if is just you, a more narrow doorway will suffice. 

Temperature management in greenhouses will be more critical because greenhouses tend to be used 12 months out of the year. To accomplish this, you will need some sort of heating and cooling system for your greenhouse. While you can use a space heater, a forced air heater, hot-water, or steam systems, you can also use radiant heat lamps over the plants when combined with soil heating cables under the plants. However, if you plan on using a gas, oil, or coal fired heater, you will need a fresh air supply for combustion as well as exhaust venting to avoid carbon monoxide and ethylene accumulation. 

On warm days, your greenhouse can warm up from the sun, so you will need some kind of ventilation system to keep it cool. The cost to operate an air condition as a way to keep your greenhouse cool is likely prohibitive so some type of evaporative cooler would need to be installed. 

If you would like more information on greenhouses or high tunnels, we have links to information in the Hot Topics section of our website www.tulsamastergardeners.org. Happy Gardening!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo courtesy: Lynn Brandenberger