The science of greenhouse growing is at an all-time high. Running a greenhouse in the summer and the winter are two completely different things, however—what sets them apart?
Your Winter Greenhouse
Most people recognize the advantages of a greenhouse when it’s cold outside. A greenhouse in the winter can play a crucial role in providing gardeners with a means of extending their growing season. During the day the sun warms the air, plants and soil, while at night the greenhouse provides protection from cold temperatures. In order for this process to work the greenhouse must be sealed, with only ventilation fans and intakes as sources for fresh air. With supplemental lighting and some heat, a ‘controlled’ environment is now achievable.
A controlled environment is one in which outside factors like temperature extremes, dry conditions or long periods of darkness are not allowed to adversely affect crops growing within it—all these factors are set and controlled by the grower. But the grower must create and maintain an environment that nurtures plants completely—just because the air temperature feels right doesn't mean the soil or water temperature is actually ideal for the plants.
Some growers have found that adding lights and above-air heat has actually resulted in up to 40 per cent decreases in production—when supplemental lighting and root zone heating are added to the mix, though, these problems disappear. Ambient air temperature should not be your main focus. Whether soil or soilless gardening is your thing, root zone temperature will play a huge factor in determining your end result. Find a way to warm up the water or get a radiant flooring system if you want to increase crop production in your greenhouse in the winter months.
Your Summer Greenhouse
The sealed winter greenhouse should give way to a more open and naturally ventilated greenhouse during the hot summer months. Some simple ways of naturally ventilating your greenhouse might be taking the end wall coverings down or reattaching the sidewall covers a few feet off the ground to improve airflow. Let’s face it—we all know what its like to be in a car with the windows rolled up on a hot summer day. Lots of greenhouse growers are also beginning to install diffused greenhouse coverings.
Diffused light has a positive influence on production, especially during the summer. This positive effect can be explained by a change in light penetration into the crop and by an increased capacity for photosynthesis.
A crop like cucumber can utilize diffused light better than direct light. In addition, diffused roof material results in a lower crop temperature—especially higher in the crop canopy—which likely leads to more optimal conditions for photosynthesis. Polyethylene/polyweave covers are stronger and last longer than normal film. Greenhouse film is usually only four to six millimeters thick and very easy to puncture—we see it in landfills far more often than the polyweave material. The film is quite a bit cheaper, but its lack of longevity and the problems you’ll face using it in the summer are just not worth the savings.
Most people think glass is the ultimate greenhouse covering. I would say if you are in the Netherlands or somewhere that never experiences hot summers, glass is great. Glass does not insulate, however, so it can turn out to be an expensive way to cover your greenhouse while unfortunately making it less efficient at the same time. It's also twice the cost of polyweaves and polycarbs. Glass in the higher elevations of your greenhouse also tends to magnify light, which in turn will create hot spots on the leaf surfaces of your plants.
As the greenhouse warms up, your plants begin to transpire by bringing water from the roots to the leaves, the natural way plants cool or protect themselves. Once the water transfers to the leaves, we begin to see the humidity increase and the plants begin to sweat. At first this process can be protective to the plant, but as the process continues we see more problems begin to develop—the humidity creates water droplets or condensation on the inside of the greenhouse roof. Rain on the inside! These droplets also act as little magnifying glasses all over the roof, which creates hot spots and the potential for mold.
Add a power outage in the middle of the day—rendering exhaust fans and circulation fans useless—and you’ve got a recipe for disaster. A simple diffused polyweave or diffused polycarb would have helped this situation and is half the cost of glass. A twin-wall polycarb is also a better insulator than glass, so it is much more efficient. A one reason person like glass is the idea they are getting more lumens.
In some ways that is true—however, the sun is so intense that in the heat of the summer you will actually gain more light by diffusing it rather than by allowing direct sunlight. By diffusing the light you are taking a light particle and breaking it into more light particles, which can then actually penetrate deeper into the canopy and provide more light to the lower branches.
The ideal greenhouse for summer can be anything with a diffused cover and good ventilation—preferably, something with roll-up side walls and doors at both ends. That way you can avoid solar gain, cool your plants and the soil (even if the air temperatures are in the 100°F+ range) and keep the air fresh. Stagnant air that is continuing to heat up is what you are trying to avoid.
Having a house with a roof vent is also a very good option—roof vents allow you to release the hot air that can gather in the peak of your greenhouse, allowing you to keep the cooler air that stays lower in the structure. On a hot summer day a greenhouse with a roof vent, roll-up sidewalls and doors on both ends will allow you to open up, avoid solar gain and in turn cool both the plants and the soil.
This will result in decreased plant stress and allow photosynthesis to continue even when outdoor plants are suffering—and you won't even need to run the big exhaust fans or evaporative cooling systems. In fact, some small horizontal airflow (HAF) fans might be all you need throughout the heat of the day.
Evaporative greenhouse cooling systems are like swamp coolers—water runs over a thick pad at one end of the greenhouse while exhaust fans pull air from the other end through the wall of cool water. These tend to work well in moderate temperatures and dry areas but they can cause humidity levels to rise, which isn't always desirable—and the electric bill for cooling a greenhouse can be another issue.
Most companies outfit greenhouses for cold winter use only—it never seems to occur to them you might want to use it in the summertime. Sometimes just switching out your roof covers to match the season will do the trick—a polyweave cover in the summer will protect you from rain and the sun's intensity, while switching this cover out to a double inflated poly film could add to the efficiency of your greenhouse come wintertime.
Circulation fans are another very important part of a greenhouse. Some growers find themselves near the coast or in a foggy mountain area that gets higher humidity in the morning or evening. Take Florida for instance—they have pretty good humidity even on a warm day and you can't fire up a heater to dry out the air.
HAF fans are the key to drying out the air in these situations since they actually get the air moving in a specific direction, rather than just scattering it around like oscillating fans do. The air will dry out faster and more efficiently if moved in a specific direction. Some greenhouse companies recommend fewer fans, but there is a benefit to adding more circulation in this department for dryer—fresher air.
Adding a light deprivation or blackout capability to a greenhouse can be another important factor in increasing productivity. Normally when growing outdoors or in a greenhouse you have to wait until fall for the harvest season. With a blackout cover, though, you can force flowering as early as July.
A light deprivation cover can be manually pulled over the garden or greenhouse and as long as you create the correct light cycle every day, you should be able to start your harvest long before Mother Nature intended. Internal and external systems for light control are both options.
For internal systems I recommend a breathable blackout material—this helps avoid any condensation drips that can occur after the curtain is pulled. Remember, the fans and intakes will be restricted because of light leaks. With that being said, it’s obvious you are going to see a bit of a heat and humidity spike since the sun is still out and you are covering your crop.
For external systems, I recommend a non-breathable blackout material. If you have a greenhouse cover and you try to pull the breathable cover over the outside of it, it won't breathe and you’ll still have the same issues with ventilation. The next step would be to use underground ventilation since you can’t go through the walls of the blackout and you are cut off from the exhaust fans you might use throughout the day. Going underground would mean a bit more work when building your greenhouse, though—and if you already have a greenhouse, underground ventilation might be a real pain to install.
There is another solution. Forever Flowering Greenhouses has unveiled a new product called The Breathable Wall. The Breathable Wall was created to allow full airflow through your exhaust and intake without any light getting through—the designers basically created a baffle that breaks up the light and not the air. Even indoor growers can find a use for The Breathable Wall for covering holes in walls for vent and air conditioning systems.
Shopping for Greenhouse Options
When shopping for a greenhouse you have many options—price should not be the only factor you consider. Always look for product reviews, checking things like the thickness of the frame and the type of framing—rolled steel, tubular or square. Make sure the greenhouse can handle the wind and snow conditions in your area as well.
Greenhouses can start out really basic and be upgraded over time. If you can't afford roll-up walls now, add them next season. You might need to keep your screw gun handy because you can always reattach the cover to the ground when it gets chilly—it might be a bit more work, but in the long run making do will save you a few bucks.
The problem with buying your greenhouse from a big box dealer is that the catalogs may show a nice picture of an ideal greenhouse but you might not actually be getting everything that’s in the picture—you might get the frame and cover but not the doors or the roll-up walls. Or if you do get the roll-up walls you’ll have to get the polycarb roof separately.
Your greenhouse should not be the cheapest thing on your list. Keep in mind that most greenhouse companies sell parts for you to do it yourself. Covers, locking hardware and other miscellaneous parts might be all you need to create a growing area that suits your needs.
A greenhouse is your number one weapon in your own personal war against everything that might cause harm to your precious plants—make sure you buy it from somebody who cares about your passion for gardening!