How to Over Winter Plants in an Unheated Greenhouse

Written by kimberly johnson
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Overwintering is the act of protecting outdoor plants from the cold during winter months. Usually this involves bringing fragile plants indoors or into a greenhouse until warm weather returns. Since most greenhouses are designed to heat themselves with the sun's rays, no additional steps are usually needed to care for plants. However, if an unheated greenhouse is in a location that receives no sunlight or if the greenhouse is covered with snow, additional steps are needed to protect plants.

Skill level:

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Things you need

  • Thermometer
  • Bubble wrap
  • Tarp
  • Pesticide
  • Plastic garbage bag or tarp

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  1. 1

    Move cold sensitive plants inside the greenhouse before the first frost. In most areas of the country the best time to place plants inside is in mid to late October.

  2. 2

    Trim any overhanging tree limbs and vines. The most critical item plants need during winter is sunlight. Since winter light is less abundant, cutting back foliage and limbs allows more sunlight to get to the plants in the greenhouse.

  3. 3

    Verify that the door of the greenhouse has an airtight door sweep. This prevents the outside cold air from penetrating the greenhouse as well as controls humidity. If the door sweep is cracked or has visible gaps, replace it.

  4. 4

    Place a thermometer in the greenhouse and monitor it regularly. The temperature should be maintained at no lower than 4.44 degrees C. If temperatures drop lower than 4.44 degrees C, the plants may be damaged or die. To increase heat retention, wrap the outside of the greenhouse in bubble wrap. Bubble wrap provides insulation while allowing light to filter through.

  5. 5

    Provide adequate circulation inside the greenhouse, especially during times of snow. Snow, like rain, causes humidity levels to rise. Opening the greenhouse vents or door allows air to circulate and removes some of the moisture.

  6. 6

    Water plants only when the soil is dry. Most plants do not grow much during the winter, even when inside a greenhouse. As a result, plants do not require as much water in winter. Depending on the humidity inside the greenhouse, water plants only once or twice a week.

  7. 7

    Examine plants carefully for insects and pests. Inspect for pests before bringing plants into the greenhouse and on a weekly basis thereafter. Treat any pests with pesticide.

  8. 8

    Protect the plants from frost. Most people do not realise that greenhouses do not protect plants from frost. When frost threatens, cover plants with blankets, paper bags, plastic trash bags or tarps. Be sure to remove the coverings later in the day when the temperature warms up.

Tips and warnings

  • Tropical plants should not be placed in unheated outdoor greenhouses. Because they need temperatures of around 21.1 degrees C to survive, these plants should be placed inside the home or some other heated area.

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