Greenhouses are useful spaces for gardeners to store, propagate, shelter, and grow a variety of plants. By capturing the heat of the sun through transparent glass or plastic panels, they support all-year gardening or a prolonged growing season.
They also shelter plants from the threat of pests, wildlife, and inclement weather. Not all greenhouses are made alike, though, and this article will give you some ideas, solutions, and tips on how to design and build your own.
1. Traditional Greenhouse
The most common type of greenhouse is made of glass or polycarbonate panels that use the sun's heat to warm the space. Often, they resemble a tiny home or shed, but are totally transparent, including the roof.
Hoop houses are the least expensive to build, as they use looped poles and poly sheeting simply built over top of garden space. Traditional greenhouses can be very simple, or high-tech, depending on budget and needs. They're typically situated close to an outdoor gardening space in the backyard.
2. Geodesic Domes
Off-grid greenhouses, called geodesic domes, are becoming very popular for the modern gardener. Their dome shape construction can easily hold up in heavy snowfall and winds, which make them great for harsh climates.
Solar passive heating is their primary heat source, but they often utilize thermal water mass and solar ventilation panels for heating and cooling. All of the fans and ventilation, as well as the heating mechanisms, are run by solar power.
This completely off-grid greenhouse is made of polycarbonate panels that repel the cold, and won’t break like glass. Their materials are also usually recyclable.
These greenhouses aren’t cheap but may be worth the cost if you want to produce food all year round.
3. Shabby Chic
Reusing and retrofitting old materials is the foundation of a lot of shabby chic and DIY projects. Reconstructed vintage doors and windows can make a really cool, cheap greenhouse. This project isn’t easy, though, as multiple windows, especially if they are different sizes, can be a nightmare to design around.
Make sure to plan really well beforehand, and gather up some friends. If you have the time and patience, you can make an interesting, and environmentally friendly greenhouse with a trendy, distressed style. Repaint it with bright exterior paint to help protect your creation, and give it new life.
4. Sunroom Conversion
Sunrooms or covered porches can be converted into proper greenhouses with a little bit of alteration. Many of them already use the idea of passive solar heat to extend the “outdoor living” in cooler climates, but these are generally built for lounging and entertaining, not growing and propagating plants.
It may be as simple as rearranging or changing the furniture—just switch out the wicker furniture for some plant tables, or, you may need to add more windows to the roof to allow for more sunlight. Vents and fans would need to be considered, as well as extra heat sources in the very cold months.
These decisions depend on your climate, but a sunroom or porch has a lot of the infrastructure you already need in place.
5. Garage or Shed Conversion
Garages and sheds can also be excellent choices to turn into greenhouses, especially if they are in need of renovation. Old garages are usually made of strong materials like cedar and other hardwood. Stripping the roof and siding can open up a new canvas where glass or polycarbonate panels can be installed instead.
Use one of the walls to catch the most reflected sun (generally the one facing west from the inside) by installing reflective panels to increase solar heat capture.
Three sides of glass or polycarbonate panels may be enough to heat your garage-turned-greenhouse all year, but again this depends on how cold your winters get.
6. Lanai Greenhouse
In tropical climates like Hawaii, California, and Florida, you’ll often find lanais that offer respite from the heat, or act as a covered area in bad weather. It’s essentially an open-concept porch that has a roof, but few walls.
Much like the sunroom for cooler climates, a lanai can easily be converted into a greenhouse to help store and cultivate delicate plants, in this case by offering more shade. Even if you aren’t interested in propagating or growing, these are great spaces to adorn with plant life, both edible and decorative.
The open-concept makes them more susceptible to animals, high wind, or heavy rain, so add more protection if sheltering young or tender plants.
Choose Your Location
Southern exposure works best for a greenhouse, as it utilizes the most of the sun’s rays. Plans may include large trees to provide afternoon shade, as greenhouses heat up quickly in the summer. In the winter, trees shed their leaves allowing more sunlight to enter when it’s really needed.
Find the most level ground where water doesn’t pool, and whether your area gets bad storms, high winds, or hail. While most glass and panels are durable, older glass and cheap plastic are susceptible to breaking.
Falling tree limbs could easily cause major damage to a greenhouse located directly underneath. Make sure a hose is nearby, set up a rain barrel system, or if you’re feeling fancy, install a sink and faucet for washing up.
Freestanding vs. Attached
Greenhouses can be freestanding, or they can be attached to another structure like your home or garage. Attached greenhouses can benefit from the radiant heat on one side, and it may be easier to connect to the home’s heating and cooling system.
A freestanding structure allows you to choose the best location where you can build exactly what you want. Either way, calculate the square footage you need for the kinds of plants you want to grow and store.
Then, add about 15 percent to that number—more space is the number one thing other greenhouse builders wish they had considered.
Heating and Cooling Options
Natural sunlight is the main way that greenhouses heat up as solar heat is trapped from the sun, and the heat waves expand. Passive solar greenhouses use a reflective wall to produce extra radiant heat.
In moderately low temperatures, small electric heaters can work, but it depends on your electricity prices and the space you want to heat.
Propane and natural gas heating systems are usually better options for heating large commercial spaces, but may not be cost-effective for a small greenhouse. Wood and pellet stoves can keep a greenhouse going in cold temperatures, but they have to be constantly fed, usually every three hours.
Compost heaps give off a decent amount of heat and can supply enough warmth for an entire winter. It’s a win-win since your plants can benefit from a good quality source of compost.
The pile gives off strong ammonia smells, so an outside compost heap with heat pipes running directly into the greenhouse is a better option. Animals also give off heat, so building against the side of a barn can give some extra warmth.
Geothermal heating can be an expensive investment up front but may be worth it in the end if you have a large operation or want to live off-grid. This technology captures soil and water heat deep below ground and transfers it to the greenhouse through pipes and other systems.
Solar geothermal is a bit different: solar panels and climate batteries are used to store energy from the sun and boost the efficiency of geothermal systems. There are many ways to use these systems, either on their own or together with other heating sources.
Thermal water mass technology uses the sun’s heat to warm or cool a water source like a tub or pool inside a greenhouse, often using reflective panels. In the summer when the sun is high, the rays are not reflected against the low-positioned panels, and in the winter, the panels reflect more of the sun’s rays since it is lower in the sky, thus heating up the water. This creates an efficient, all-year heating and cooling system. Smaller containers can help warm up smaller greenhouses, with or without reflective panels.
Cooling options aren’t in demand as much, but, they are just as important since plants can quickly wilt if they get too hot. Roof and wall vents or windows that open and close are the most useful options for releasing hot air quickly while maintaining a moderately warm temperature.
Fans may also be necessary to circulate the air, cool off plants, and remove stagnancy. Natural ways to cool a greenhouse utilize position in relation to the sun in the sky and placement around trees for shade. Attached greenhouses may also utilize the side of the other structure for afternoon shade.
Glass is the most traditional material used for a greenhouse, as it’s sturdy, and has a nice aesthetic. It's also the heaviest and most expensive. Use tempered or laminated glass that's strong and shatterproof, as they can withstand harsh climates, hail, falling tree limbs, and heavy winter snow-loads.
Panels that open and close on an automatic system are expensive, but excellent ways to keep large greenhouses properly heated and cooled.
Polycarbonate panels are a lighter option and useful for various builds and applications as they can be cut easier than glass to fit specific shapes and designs. This material has a shorter lifespan, however, and may not hold up as well over several decades. Polycarbonate panels tend to yellow over time, and low-quality varieties expand and contract in extreme heat and cold.
Polyethylene material is the cheapest, least durable, but most versatile material as it comes in rolls or sheets that can be easily cut and wrapped over a wood, hoop, or steel frame. Make sure to invest in the greenhouse grade poly: it has a four-year lifespan or more depending on its thickness, and is generally the same price as regular poly.
Know Your Zone!
When constructing a greenhouse, the most important thing is to know your growing zone. Calculate your heating needs, and invest in a thermometer. Depending on your location, and the structure’s position against the sun, a greenhouse can heat up two or three times higher than the outside air in warmer months.
In the winter, you may need to mimic natural light with grow lights, and reflective panels. Having a clear design and idea of your needs will ensure you can create a greenhouse that will be long-lasting, and rewarding.
This is a straightforward example of how to build a sturdy, no-frills greenhouse with materials you can easily get at any local hardware store. It’s for a simple, rectangular structure with a lean-to type roof to allow for water run-off.
Create floor support with joists out of 2x4s to whatever dimensions you decide on. Use 4x4s as feet to level the floor, either securing them in cinder blocks, digging post holes, or attaching them to a concrete pad. Saw them off level with the floor joists.
Build the walls to connect the structure also using 2x4s, planning for any windows and doors. 16-inch centers may not be necessary since you aren’t putting up drywall, so construct the wall studs in accordance with the size of your glass or plastic panels. Add a lean-to roof using a similar build as the walls.
Add half-inch plywood sheets for the floor. You can leave it as is, or use composite flooring as an upgrade. Wrap the outside with the same composite paneling at the bottom to act as a skirt, or use whatever weather-proof material you prefer.
Use polycarbonate corrugated sheets as panels for the rest of the walls and roof. Attach the panels with roofing screws to the 2 x 4's. Add a simple door on swing hinges. Cut some holes with a jigsaw or circular saw to add vents to the siding, and the panels on the roof.
Top vents should open and close as needed using hinges, props, and leftover panel scraps. Add wall registers or vents to the siding.
You can also purchase plans to help you start. Simple barn frame plans with an open floor are very popular for first-time greenhouse builders and DIY-ers. The ground is filled with drainage stones to create a level, well-draining, and durable floor.
Galvanized metal panels are used along the bottom, with plastic or glass panels on the sides and roof. You can build your own door, or repurpose an old one. Windows, doors, and extra materials are readily found on buy and sell sites, as well as salvage and reuse stores.
Remember, many plants can withstand cool temperatures and may not need tropical-like conditions. Evergreens, bushes, and certain fruit and vegetables tolerate and even flourish in colder temps. Other plants just need a nice place to overwinter with protection from frost before they can be brought outside again.
A greenhouse could be a great solution to overwintering plants like palms, bougainvillea, or potted trees that you’d rather not find space for inside your home during the winter. Whatever your growing needs are, these greenhouse ideas can help you get the most out of your climate's growing season.