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DIY: How to build a backyard greenhouse

By: Sierra Laverty, Adult Education Specialist | 02/12/2019

Building a small, backyard greenhouse can be a rewarding, easy, and even less expensive solution to indoor grow lights. A greenhouse can allow a home gardener to effectively start their own seeds, keep year-long stock plants (see below), or overwinter plants that don’t survive Idaho winters.

Follow these steps when considering background greenhouse construction.

Step 1) Consider what you will use a greenhouse for.

A greenhouse opens up the entire realm of plant propagation!

• Spring and fall, cold season vegetables (lettuce, kale, kohlrabi, cabbage, etc.)
• Summer vegetables (tomatoes, squash, okra, etc.)
• Native wildflowers (Rocky Mountain bee plant – Cleome serrulata, Penstemon, buckwheat – Eriogonum)
• Ornamental, cold-tolerant perennials (roses, Veronica, Clematis, Viola, )
• Ornamental, tender perennials (Coleus, sweet potato vine – Ipomoea batatas, dusty miller – Centaurea cineraria)
• Ornamental annual flowers (Nasturtium, Zinnia, Nicotiana, Gomphrena)
• Tender succulents
• Culinary herbs (basil, rosemary, lemon verbena, etc.)
• Carnivorous plants
• House or patio plants

A greenhouse can aid you in growing marginally hardy plants like rosemary.

Step 2) Decide how large you want the greenhouse to be.

Growing capacity: 12 feet by 14 feet (168 square feet) greenhouse is large enough to grow hundreds of transplants, but small enough to fit on most properties. Remember, maintaining a greenhouse that is mostly empty is much easier than expanding an existing structure. Choose a size that you can “grow into” and increase your plant production over time.

More than 600 plants are growing in this 12′ x ’14 portion of IBG’s production greenhouse.

Step 3) Choose an optimal site.

Drainage: Select a site that is level, and allows for ample water drainage. Avoid constructing a greenhouse on top of heavy clay soils, as the clay will hold the moisture and create a swampy environment. A cement or chat (crushed, decomposed granite) pad would be best, but coarse sand or fine gravel will also help increase drainage.

Light: Access to full sunlight is critical to the health of plants grown in a greenhouse. Choose a spot with as much sun as you can provide (ideally 10-12 hours in summer). We will discuss how to cool your greenhouse below (see Shade).

Water: Greenhouse plants will need regular water (often daily in the heat of summer). Select a site that is easily accessible with a garden hose, or one that you can install irrigation at a later date.

Electricity: A heated greenhouse will allow you to grow vegetables, annual flowers, tender perennials, house plants, and carnivorous plants. To grow these plants, position your greenhouse with access to electricity to power possible heat mats, heat lamps, or a small electric heater. Many of our native flora can be started from seed or cuttings in an unheated greenhouse, as can non-native, cold-tolerant perennials. Access to electricity will also allow you to place fans for ventilation (see Consider these amenities below). Use heavy-duty, outdoor extension cords.

Wind: Gardeners living in extremely windy conditions may encounter difficulty maintaining the plastic sheeting. Two thinner plastic sheets can be layered (1-2 millimeters), or additional PVC ribs can be added to increase areas of attachment for the plastic.

 Step 4) Create a materials list and construct your dream greenhouse!

A backyard greenhouse can be constructed out of essentially PVC pipe, treated lumber, and 4-6 millimeter plastic sheeting for about $150.

A hobby greenhouse might seem like a gardener’s dream, but it can easily be a reality using simple construction techniques!

Follow this guide — North Carolina State University Extension guide, “A Small Backyard Greenhouse for the Home Gardener” for construction plans and other considerations about materials.

Essential tools and equipment:

• Fluid Level
• Power drill
• Hammer
• Screwdriver
• EMT clamp
• Hand saw or pipe cutter
• Disposable gloves (to apply PVC cement)
• Staple gun
• Steel tamper (to compact chat or sand floors)

 Step 5) Consider these amenities

Shade: Idaho summers can be brutal in some regions of our state. As temperatures approach 70 degrees outside, an unventilated greenhouse will act much like a parked car: inside temperatures can reach 10-30 degrees above outdoor temperatures! A 30-50% polypropylene shade cloth will protect plants from the blazing summer heat, and allow light for growth.

Ventilation: We open our greenhouse doors when the outdoor temperature reaches above 55 degrees. A screen door or magnetic screen curtains can keep out unwanted insects or mammalian pests. An open door may not provide enough ventilation alone in the middle of summer. Consider installing an electric fan to increase air circulation, which can also reduce fungal and bacteria disease pressures.

 Step 6) Use your resources, and continue learning.

There are a number of local resources at your disposal to learn more about greenhouse growing! Follow your curiosity, and reach out to professionals:

• Join our IBG Continuing Education class Starting From Seed: The Basics (Wednesday, March 13, 2019, at 6:30 p.m.) taught by our Greenhouse & Nursery Coordinator, Nell Lindquist
University of Idaho Extension Offices
Idaho Nurseries, Garden Centers and Greenhouse Growers

University publications
University of Georgia bulletins, Hobby Greenhouses
Washington State University: “Hoophouses and Greenhouses”
Utah State University: “Greenhouses”
University of Minnesota: “Deep Winter Greenhouses”
Colorado State University presentation: “Backyard Greenhouses, Sunspaces, and Cold Frames”