9 tips for planning and building a greenhouse

June 23, 2015

A greenhouse is a substantial investment, but to serious gardeners its benefits are incalculable — including a much longer growing season and the opportunity to raise exotic flowers. If you have your heart set on a greenhouse, here are nine tips for turning your dream into a reality.

9 tips for planning and building a greenhouse

A garden greenhouse: a haven for planting

Gardeners have used glass houses to propagate and grow tender plants since the 19th century.

  • These days, a sunroom or attached greenhouse can be a relaxing haven when it's too cold outside to garden.
  • Or, if you just want to grow more plants in your garden, a small greenhouse — or even a cold frame — will provide ideal conditions for starting plants and extending their lives.

1. Consider the cost

When designing a greenhouse, take into account not only the price of the structure but also the long-term cost of operation, especially providing heat.

  • A "warm" greenhouse should be kept above 16°C, while a "cool" greenhouse must always be kept above 7°C.

2. Location is key

Site the greenhouse to receive as much sunlight as possible during the winter months.

  • Attached greenhouses should have southern exposure, while a freestanding house's long axis should be oriented north to south.
  • Protect the greenhouse from prevailing winds with a hedge or berm.
  • Plan for easy access to water.

3. Add winter warmth

For winter warmth, instead of heating the entire greenhouse, create an alcove in one corner for your most tender plants.

  • Block off the space by hanging sheets of plastic and keep the area warm with a small, thermostatically controlled electric heater.

4. Go with the flow

A greenhouse needs ventilation to let stale air escape and fresh air circulate.

  • To provide proper airflow, install at least one vent in each side of the roof and each side wall.
  • Open the vents in the morning when the temperature is rising and then close them in the afternoon to conserve heat.
  • Crack the vents even on windy or cold days.

5. Eliminate guesswork

Regulate airflow by installing automatic vent openers.

  • Activated by a thermostat, they constantly adjust the ventilation when cloud cover or other factors cause temperature fluctuations.

6. Provide summer shade

To keep a greenhouse from becoming unbearable in hot months, block the sunlight with blinds or fabric shades, which can be rolled into place with pulleys.

  • You can also shade a greenhouse by planting deciduous trees nearby. Locate them on the south side, far enough away that falling leaves and branches won't land on the greenhouse.
  • Or train vines, such as clematis or morning glory, to climb tall trellises that will cool the greenhouse with their shade.

7. Create a potting area

  • For maximum convenience use tables, or "benches," about 75 centimetres high and one metre wide, so you can work without bending over and comfortably reach to the far side of the tabletop.
  • Store pots, soil, tools, hoses and similar items underneath the benches to keep walkways uncluttered.

8. Organize your plants

Organize plants according to their needs.

  • Keep plants with fuzzy foliage, which should be kept dry, away from those that like to be misted.
  • Place sun lovers on the south side of the greenhouse and shade lovers on the north. Make sure no leaves are touching the sides of the greenhouse in winter.

9. Prepare plants for overwintering

Prepare plants for overwintering in a greenhouse about six weeks before you move them in.

  • Clean up damaged stems and foliage, pot up transplants and treat the plants for pests and diseases, which can spread rapidly in the confines of a greenhouse.
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