Building a Cheap, Seasonal Greenhouse
Reading Time: 8 minutes
By Missy Ames
A greenhouse can be crucial for extending the growing season. It keeps plants warm while letting in a full day’s worth of sunlight. But a large greenhouse costs thousands of dollars and a small, flimsy one may last a year or less. Small homesteads or urban gardens may have little space to spare, but issues of money and space can be lessened by making the greenhouse seasonal.
If you recycle materials, a 10-by-10 greenhouse can cost as low as $30 per year. You can construct a new one for less than $200, repurposing most of the materials next year.
Choose the perfect location. You can build it over a gardening space and sow seeds directly into the ground. Or park your car on the street for a few months and utilize your driveway. In a corner of your yard, a fence might block the wind or provide part of your framework.
Before you purchase a framework, search the classifieds. Many homeowners tire of their yard gazebos after the fabric rips and the bare frames make great greenhouses. If you can’t find a good deal on a used one, order it online or at a hardware store. A $200 frame can last 10 years with good care, at a cost of $20 per year for your greenhouse.
A pop-up gazebo is a less-expensive and more portable option. At the end of the season, remove the plastic, fold up the poles, and store it in a garden shed until next spring. These don’t last as long because wear and humidity can weaken the joints. But a $50 pop-up gazebo which lasts only five years adds averages to $10 per year.
For a more permanent option that holds up to winds, purchase PVC pipes and joints at the hardware store. A 10-foot length of pipe costs between $2 and $9, depending on circumference. Elbows and tee joints are as low as 30 cents apiece. Free instructions for arched greenhouses can be found online. If you don’t glue the joints together, the PVC can be disassembled and stored alongside a house for the rest of the year.
REINFORCING THE FRAME
Though good arched PVC greenhouses don’t need additional supports, a cheap gazebo does. By bracing the joints of a pop-up frame, you extend the life several more years and give a solid surface for plastic to press against. Look for smooth bare branches, wooden dowels or PVC. Stretch a couple across the roofline, keeping sharp surfaces facing away from the plastic. Install more in T- or X-shaped formations against the frame’s poles. Pallets or upcycled steel racks can fit between supports, forming strong walls to hang lights or baskets. Be sure your reinforcing props allow light to shine through.
If your framework doesn’t have a bottom edge, lay long poles on the ground, stretching from corner to corner on all sides. This gives you a surface on which to fasten the bottom skirting of the plastic.
Fasten these materials tightly to the frame with cable ties or nylon cord. If fasteners have sharp edges, such as cable ties, position them toward the inside of the greenhouse so they don’t puncture the plastic.
To install electricity, lay an outdoor extension cord along the ground and through the framework. Tie the outlet high onto the framework so it won’t rest in standing water. Running the cord before covering the framework with plastic allows you to position it in a place where you won’t step on it.
CREATING A DOOR
Your door can be simple. It must open and close often without damaging the greenhouse, hold back the elements, and allow you to pass through laden with carts or baskets of plants.
Try repurposing an existing door such as a discarded kennel gate. Or build a reinforced rectangle from PVC. Perhaps upcycle old racks or pallets. The door must fit within a larger doorjamb, which can be as simple as upright poles on either side, secured to the frame at the top, with a lintel cross-piece.
Constructing the door on the most sheltered side of the greenhouse keeps the wind from whipping it apart. If the entry faces an outdoor faucet, you can water your plants without wrapping a hose around the exterior. And remember the path of least resistance, especially if you plan to carry many items to and from the greenhouse.
Secure the door to the jamb with flexible material that can take a lot of friction, such as nylon cord, leaving it slack enough that you can open and close the door regularly. Swing the door back and forth to be sure it’s secure and will hold up to use.
Six mil clear plastic is fairly inexpensive, allows light to shine through, and holds up against a lot of abuse. Rolls range from 10-by-25 feet to 20-by-100 feet. The more footage per roll the more you’ll save per square foot. If you intend to build the greenhouse again next year and have a little extra money, buy a 20-by-100 roll for less than $100 and save the leftovers for next year.
Purchase a roll wide enough that you can stretch it all the way over your framework without seams. While this may be difficult on larger frameworks, a wide roll allows you to position seams down low, where heat won’t escape as easily and the wind won’t catch the edge. A 10-by-10 gazebo frame needs a width of 20 feet and will still come up a little short.
Before covering your greenhouse, wrap sharp edges with cloth or duct tape. Then measure the length necessary to cover the greenhouse, adding a few feet on both ends, and cut the plastic from the roll. With help from someone else, center the plastic over the peak of the framework and unfold to hang down both sides. Maneuver the plastic so it covers as much area as possible.
If the plastic doesn’t completely cover the greenhouse, position seams toward the most sheltered area. A seam located at the door saves time because you must cut it there anyway. When you add more plastic to finish the exterior, lay it underneath the original piece so precipitation will drop down the side instead of into the greenhouse. Most tape will not stick to 6mil plastic, but duct tape holds fairly well and only obscures light in a small area. Apply tape when the plastic is completely dry, keeping extra on hand for repairs. Press firmly against the plastic, rubbing with your fingernail so the tape adheres well.
Pull the plastic snugly against the sides, bringing surplus to the greenhouse interior and leaving a smooth exterior to face the elements. Avoid cutting since this leaves flaps to catch the wind; secure surplus to the frame with cable ties, rope, or staples. Carefully trim the plastic around your doorjamb, leaving at least six extra inches to wrap around and attach to the framework.
Pull the excess plastic at the foot of the greenhouse into the interior. Secure it to the running boards or set heavy objects such as buckets of dirt atop the skirting.
Measure enough plastic to cover the door with at least six extra inches on all sides. Keep the surface flat and smooth while you fold slack edges around and secure them to the frame. This can be done by stapling the plastic to PVC or wood, nailing a small board over the sides while trapping the plastic in the middle, or taping the plastic together.
ADDING THE EXTRAS
Within your basic greenhouse, sunlight will shine through during the day, raising the internal temperature. The plastic insulates at night. But you may need extra equipment.
Hang a $10 patio thermometer in the most shaded area, since direct sunlight causes an incorrect reading.
A heat lamp, attached to the framework with the bulb dangling away from combustible objects, can raise the ambient temperature by a few degrees. If you expect a hard frost, purchase a space heater for as low as $25 and turn it on after the sun goes down. Be sure to turn the heater off during the day and to remove it when you water your plants. Heaters or lamps should be positioned low within the greenhouse so heat can rise as it fills the interior.
Just as the greenhouse can get too cold at night, it can overheat during the day. Good ventilation is key. If the internal temperature rises more than 100 degrees, position an inexpensive box fan within the open doorway to circulate fresh air into the greenhouse.
A waterproof table reduces trips back and forth. A hanging shop light keeps you working after the sun goes down and helps you check temperatures during cold nights.
AFTER THE SEASON ENDS
Since UV light is plastic’s worst enemy, and because of wear and tear, you probably can’t salvage the exterior for next year. Cut solid panels from the walls to salvage for smaller projects. Cut off the rest of the plastic and throw it away.
If you need your driveway back, disassemble your framework and store it in a sheltered area. Or put the cloth canopy back on the gazebo and use it for summer entertaining. If the framework is in your garden, consider growing vertical crops such as beans, hanging twine from the supports and tacking it to the ground with landscape pins. Or cover the gazebo with light cloth to protect plants from the blazing summer sun.
A BASIC GREENHOUSE NEEDS:
• Roll of 6mil plastic
• Fasteners, such as cable ties or rope
• Knife or scissors
• Duct tape
ADDITIONAL MATERIALS MAY INCLUDE:
• Outdoor extension cord
• Tables or racks
• Box fan
• Supplementary heat
• Shop light
A vigorous gust can toss a greenhouse across several properties. Temporary structures are especially susceptible.
Consider an arched formation instead of flat walls. Position the curved sides in the direction of the strongest winds. Build the door on the most sheltered side.
If wind can get under the plastic, it can lift the greenhouse. Extend the bottom skirting about 10 feet to either side. Set straw bales on the edges. Tack the edges to the bales then roll until they are encased in plastic and touch the sides of the framework. This holds the plastic against the ground and gives added insulation along the sides.
Avoid seams if possible. Unroll the plastic up one side and down the other, giving yourself plenty of skirting. If an arched greenhouse is less than 20 feet long and you use 20×50 or 20×100 plastic, it will only have seams at the two ends.
Your greenhouse doesn’t have to be large enough to enter. It just needs to retain heat and allow in sunlight. Hoop houses are a classic mini-greenhouse design, but they don’t have to follow that pattern.
Construct a box from PVC, pallets with a few slats removed, or pieces of aluminum. Use either an arched design or install enough cross-bracing for a strong frame. Make it easy to reach and water your plants. Wrap sharp corners with cloth or tape before stretching 6mil plastic over. Hold skirting down with cinderblocks or flower pots.
Mini greenhouses cost less, use less space, and are easier to heat. A heat lamp or strings of weatherproof lights will warm plants. A 10 x 10×8 greenhouse may need an electric space heater during a cold snap while a mini greenhouse can thrive on a low-wattage heat bulb with old quilts thrown atop the structure during the night.
With a smaller greenhouse, you can construct a flap of plastic to pull back on warm days. This eliminates the need to harden plants off if you allow them to experience full sunlight from the time they sprout. Weigh the unattached side of the plastic with materials such as washers or magnets taped onto the edge so it will stay in place once you cover your plants back up.