Greenhouse Planning: What Growers Need to Know
So, you’ve decided to start growing in a greenhouse. That’s great! It might be tempting to start with selecting a frame, but wait. There are some other important factors to consider before building and growing in a greenhouse.
When planning for a greenhouse, most people start with the frame. This may seem like a good start, but there are some other important factors for commercial operators to think about before they can ever pick a style of greenhouse frame.
The first and most important part of greenhouse planning is the property. Is it owned, leased or borrowed? All of these things can help determine what structure you’ll choose. If it’s owned, the sky’s the limit. If leased or borrowed, you might need something less permanent in case you need to tear down and rebuild without causing a financial crisis.
Next on the list is zoning and the location of the property. Residential properties can be a bit of a challenge due to the permitting laws in these zones. The counties will look at the size of the structure, coverings you choose, concrete and offsets. When considering permitting, pay attention to your wording. A high tunnel is considered temporary, whereas a greenhouse is permanent. It’s a good idea to call everything a high tunnel when applying or requesting information.
Then, the greenhouse manufacturer requires a permit. You may think it is not an issue, but you never know if your neighbors will be pleased with your new structure. If they call the building department, it’s better to know all the information on building specs and adhere to the code. Commercial and industrial zoning can be somewhat easier since it’s usually out of residential areas. Some areas have dual zoning and can cause even more red tape in the long run. Some structures are exempt from permitting. So, before rushing into purchasing your frame, be clear on your property’s zoning.
Once you’ve talked to the building department and have gotten some feedback and somewhat of a verbal OK, it is time to start picking out the greenhouse. The greenhouse will need to meet the building codes for wind, rain and snow ratings for agricultural structures in your zone and property. That will narrow down the choices if you’re in a high wind or snow zone.
After consulting with your greenhouse expert, you should be able to go forward with the engineering process. Most commercial operations in commercial and industrial zones need engineered plans for permit approval. This is an official set of drawings that carry an engineer’s stamp on the bottom of the front page, which puts all the responsibility on the engineer that the building will meet the required code for that project. If the county needs a revision, it goes to the engineer. If there is a failure or discrepancy in the structural portion of the plans, the engineer is the go-to person. This does not always grant you a permit.
It’s only after the permits are approved that you should put money down on a structure. In the meantime, the county will tell you what’s needed for site preparation, erosion control, water catchment or diversion, etc. Some areas require more infrastructure than others. It’s always best to work with a project manager from the area. They typically know the proper terminology and are prepared for curveballs in the permitting process.
Once the permits are conquered, it's time to order!
A qualified greenhouse company with a background in agriculture can consult on what’s the right style for different types of regions, properties and weather. Most hoop houses, high tunnels and gothic frames can be ready to ship in one to three weeks, depending on size and accessories. Larger commercial structures can take six to eight weeks before the first truck can roll out. This is usually what we call gutter-connect structures.
Most greenhouses we commonly see around farms and nursery supply centers are single structures. This means there is just one greenhouse with nothing connecting it to another structure. This freestanding structure might have guttered connections, or it might not. Some greenhouse models automatically come with gutters, while others just let the water fall to the ground. The greenhouses that do have a gutter can usually be gutter-connected. This means we put another greenhouse directly next to another and join them at the gutter.
A greenhouse is typically made up of four sides (two end walls where you normally see doors, and two side walls) and a roof. The side wall height can also be called the gutter height and eave height. Back to the gutter-connected, this type of greenhouse allows for acreage-size structures. It’s also called a range of greenhouses. For example, you might have a range of 11 42-ft. by 120-ft., gutter-connected structures and in that range, six are flowering, three are veg and the other two are processing. The processing can be clad in sheet metal like a warehouse while still gutter-connected to the flower and veg range of houses.
Grading, piping, gravel, concrete and electrical preparation all help you get ready for the greenhouse. Concrete slabs are not a structural component of the greenhouse but are a nice addition. A slab can be floated in after the columns/stubs are set around the perimeter. Most people will use crushed gravel. Road base material can be watered and smoothed almost to as nice a surface as concrete for less than half the cost. Another thing to keep in mind is floor heating.
You’ll save yourself in overhead with root-zone heating. Remember that 30 to 35% of the heat from air-fired heaters reaches the plants. Floor or bench heating will make 95% of that heat concentrated to the plant. It makes a huge difference in quality and yield stability.
Now we move onto the installation process
So, the trucks have arrived, the pad is ready and the process can begin. A reputable greenhouse company should offer parts that are punched, pressed, pre-drilled, pre-bent and ready to assemble. Be aware that when buying a lower-priced greenhouse frame, the frame itself might be cheaper upon initial purchase, but you will have to hire contractors to do the grinding, welding and drilling, which can increase overall costs.
As the assembly comes together, electrical work can begin—wiring, fans, vents, lights and blackout systems. Once all these things come together, the installation process comes to a close and the growing can begin.
Growing in the greenhouse is a whole new ball game. Centuries have brought us the art of growing when it is cold, but what about when it is hot outside? Moving out of the greenhouse in the summer months is what some folks do, but in my opinion, that’s not an option. The secret is in the selection of the cover or glazing. Clear glass, polycarbonate and films sound like the right choice for winter, but if you’re in a warm, sunny area, the clear materials are going to mean heat challenges.
Typically, we would exchange the air with exhaust fans, but they cost money to operate and might not be able to keep up. Someone might suggest you get an evaporative cooling system or a wet wall. That works for some specialty crops, but not all. We’ve seen the adverse effects of too much moisture and no cooling. It has benefits for certain regions, but not all. For example, the average relative humidity in Oregon or Washington is much different than in Arizona or Colorado. These factors, in particular, can cause a person to re-think the available equipment.
Read More: Humidity 101 - Basics for Your Indoor Garden
Dealing with covers
Since clear covers can encourage too much light during the summer, I recommend a diffused material. Diffused polycarbonates and polyweaves tend to help encourage optimal sun values while blocking out un-wanted rays like UVB. Typically, shade cloths can help reduce UVB exposure, but it does cut the growing area off from par. The black shade cloths can also encourage more heat than relief.
Greenhouse covers are not the only source of relief in a greenhouse. While the cover is designed to cool a plant’s leaf surface, air temperature and humidity can play a factor. While air temperature can remain high, it’s the solar gain that needs to be addressed. Solar gain, or the greenhouse effect, isn’t what we’re looking for on a hot day. The simple answer is to open up the greenhouse. The use of sidewalls and roof vents are the best uses for natural airflow. The transpiration rates in plants are what we keep a close eye on.
When a plant overheats, it begins to transpire by taking water up from the roots into the leaves and expires from the underside of the leaf. It’s a natural process, but it can also inhibit the plant’s growth. Hot leaf surface and soil temperatures add to this cycle. Another contributing factor to heat is the colors in a greenhouse. Black ground cover and black shade cloths tend to encourage way more heat than what a plant or soil can handle for effective growth.
Add a black grow bag or container and you’ve got a solar oven. I believe in offsetting these colors with white groundcover, white shade cloth and tan grow bags. These small steps equate to several degrees difference—a difference that saves water usage and encourages better growth.
Circulation fans are the key to helping provide fresh air with less dead zones. Circulation fans are best at moving air in a direction rather than scattering the air. When the air moves in a direction, we can see a dying effect in more humid zones and also a cooling effect for the plants. When air passes the underside of a leaf, it has a cooling effect much like a radiator in a car.
And finally, don't forget about moisture
Plants and people are similar in the sense that blood runs through our veins as water runs through the plant’s system. That moisture may not be visible, but it’s there, and as air blows across it, the cooling process is achieved. Plants are easier to cool than the air in most cases.
Indoor growers typically pay close attention to the reflector. The reflector or hood is what spreads the light. Some are more uniform than others and can have trouble spreading light, thereby creating hot spots that increase leaf surface temperatures and encourage higher transpiration rates. The result is typically yellowing leaves that start to appear. Stress begins to set in and bugs and pathogens easily take over. This happens because of inconsistent leaf surface temperatures.
Outdoor plants droop in the afternoon because of water weight, but indoor plants tend to yellow. The air conditioners and air-cooled lights won’t let the water weight gather enough. It’s almost like the plants can’t decide what to do, so getting sick becomes the end result. The sun is the ultimate indoor light and needs to properly reflect and disperse for certain crops in warmer regions. Good airflow and a proper cover is the first line of defense.
There are many considerations to make when planning a greenhouse. These are only a few of the many suggestions that go along with building and growing in a greenhouse.