How to Plan a Greenhouse Garden
If you’re thinking about building a greenhouse, Monica Mansfield outlines what you’ll need to get the most out of your indoor grow space from the different greenhouse structures to automation.
With proper planning and design, a greenhouse can add a whole new dimension to your garden. It can extend your seasons, allowing you to start plants earlier in the spring and harvest crops through the coldest months of the year. It gives you a place to start seeds, propagate cuttings, and store patio plants in winter. By controlling the environment, you can even grow plants that wouldn’t normally do well in your zone.
Types of Greenhouses
There are two main types of greenhouses: lean-to and freestanding. Lean-to greenhouses attach to the side of another building, while freestanding greenhouses are a completely independent structure. There are advantages and disadvantages to each.
While lean-to greenhouses have easy access to electricity, water, and heat, they also offer limited space, light, ventilation, and temperature control.
Freestanding greenhouses provide more light, however, they will need their own separate heating system which can be costly.
Choosing the Right Location
The ideal location for your greenhouse is a spot that gets a lot of sun on the south or southeast side of your home. If you plan on using your greenhouse during the hot summer months, placing it on the north side of a deciduous tree will provide shade during the hottest part of the day. In the winter, the tree’s leaves will fall and allow more light to get through.
Choose a level, well-drained area for your greenhouse. Lay it so the length runs north and south. This way, it will receive as much sun as possible during the winter months.
Make sure you have access to water and electricity. Your life will be much easier if you can automate watering, temperature control, and ventilation.
There are many things to consider when deciding on the size of your greenhouse. You’ll want to include enough space for future expansions, bench space, walkways, and storage space.
It is easier to control temperature in a larger greenhouse. Temperatures in smaller greenhouses can change quickly if someone opens a door or forgets to open a vent. If you build as wide as possible, it will be easier to expand the length in the future.
Consider what you will need to use your walkways for. Will you need just enough space to walk between rows and work at the benches, or will you need enough room for a wheelbarrow? Typically, 24 to 30 inches should give you a walkway wide enough to push your wheelbarrow.
Will your benches be off to the side or down the middle of your greenhouse? Side benches can be as narrow as two to three feet wide, while center benches could be as wide as six feet.
How many plants do you intend to grow? A six-inch pot needs about one square foot of space, so if you grow 100 plants, you’ll need 100 square feet of bench space. A good rule of thumb is that two-thirds of your greenhouse will be bench space, and one-third will be walkways.
How tall are the plants you’ll be growing? If you are just starting plants, then an eave height of five feet will work fine. However, if you plan on growing taller plants or trees, you will want to increase the height.
There are many building materials to consider when constructing your greenhouse, each with advantages and disadvantages. Frames can be made from wood, aluminum, plastic, or iron. Coverings can be made from glass, fiberglass, acrylic, double-wall plastic, and film plastic.
Although glass can be costly, it will last a lifetime if there are no accidents that break it. Glass is strong, transparent, retains heat and humidity well, and provides a weather-tight structure.
Fiberglass is less expensive, strong, lightweight, and takes harsh weather well. However, as it ages and degrades, it will turn yellow and allow much less light to get through.
Acrylic is UV- and weather-resistant, lightweight, and won’t yellow, however, it is expensive, easily scratched, and gets brittle with age.
Plastic has become a popular choice with gardeners because it is inexpensive and yields results similar to expensive glass greenhouses. An added benefit to homeowners is plastic greenhouses are viewed as temporary structures and will most likely not raise the value of your property for tax purposes.
You can choose from double-wall plastic or plastic film. Double-wall plastic is rigid, durable, and usually coated with a UV-inhibitor to make it last longer.
Plastic films can be made of polyethylene (PE), polyvinyl chloride (PVC), and copolymers of these materials. Polyethylene will last one to three years, while PVC and copolymers with UV inhibitors will last longer.
Heating and Cooling
To control the temperature in your greenhouse, you’ll need to install heating and cooling systems. For most plants, night temperatures of 55-65˚F and a maximum daytime temperature of 85˚F will do. If you simply want to protect your plants from frost, then setting your thermostat to 40˚F will be adequate.
Remember to account for the microclimates within your greenhouse. The spaces near walls, the floor, ventilation, and any kind of opening will be cooler than the rest of the area.
To heat your greenhouse, you can use a space heater, hot-water or steam system, electric heater, forced-air heater, or radiant heat lamps above plants. A space heater with a few circulating fans is an inexpensive option for smaller greenhouses.
Arguably the most efficient system for small greenhouses is a forced-air furnace that distributes heat through ducting or a plastic tube system. The tube is placed as high as possible down the length of the greenhouse to distribute heat evenly so there are no cold spots.
To cool your greenhouse, you can use evaporative cooling systems, fogging and misting systems, or shade cloth. Evaporative cooling and fogging systems both increase humidity and work well in drier climates. If your humidity gets higher than 90 per cent, your plants will start having issues and you should bring in a dehumidifier to bring humidity back down between 70 and 85 per cent.
Evaporative cooling systems, also known as swamp coolers, pass hot air though a soaked screen or spray of water to cool it down. Foggers and misters work well, however, can be high maintenance when emitters clog.
Shading your greenhouse will also bring the temperature down. Trees planted on the south side will offer afternoon shade. You can also use a shade cloth, roll up screen, or paint a compound directly on the glass.
Proper ventilation is essential in your greenhouse. It circulates the air and keeps the temperature even. It also exchanges the stale air for fresh air and replenishes CO2.
Vents, intake and exhaust fans, and oscillating fans will create enough air flow for a healthy environment.
Without air flow, warm air would rise to the top of the greenhouse and cool air would settle at the bottom. By placing fans in diagonally opposite corners, you will move air in a circular motion and keep temperatures uniform throughout the greenhouse.
Intake and exhaust fans work together to exchange stale air for fresh air. The fans you choose should be powerful enough to change the air about once per minute.
Vents are helpful when moving air. When the warm air rises and escapes through the roof vent, cool air will be drawn in through the side vents.
Now that you have all the pieces in place, do yourself a favor and automate everything. If you don’t, you will be tied to your greenhouse, opening and closing vents throughout the day and hand watering all of your plants daily. The more money you invest in automation technology, the easier your life will be.
You can hook your heaters, cooling system, fans, and vents up to a thermostat that will turn them off and on depending on the temperature. Humidistats will regulate humidifiers and dehumidifiers to keep your humidity levels in check. Automatic watering systems will ensure your plants never get too thirsty.
A greenhouse is a wonderful addition to any garden. Though it can be costly up front, once it is set up and automated it will extend your growing season, give you space to propagate, and allow you to grow tropical plants you may not have been able to grow otherwise.