December is not the coldest month but it is the darkest one. So the days are much shorter. It impacts the growth of the plants which becomes sluggish. In this case the light is controlling factor. Make sure that you are aware of the lighting requirements.
Can you use a greenhouse in the winter?
You can use a greenhouse in the winter because it offers a controlled and consistent environment. Depending on what you want to grow you can have a cold greenhouse, cool greenhouse, warm greenhouse and stove house.
Cold greenhouse – not heated expect by the sun. Minimum temperature – 28°F (-2°C) when outside temperature falls to 20°F (-7°C).
Cool greenhouse – heated during the cooler months. Minimum temperature – 45°F (7°C).
Warm greenhouse – heated during most of the months. Minimum temperature – 55°F (13°C).
Stove house – heated all year round. Minimum temperature – 65°F (18°C).
What to grow in December?
Bulbs: you can plant Lilies in December. Support stems of Freesia and Lachenalia. Bring in the greenhouse more bowls of bulbs from outside to hasten growth.
Ornamental Plants: sow Campanula isophylla, propagate Perpetual-flowering Carnations and stake Lorraine Begonias in December. Pot up shrubs of Forsythia, Hebe, Lilac and Weigela for greenhouse decoration in spring.
Plant in pots a number of herbaceous perennials like Lupin, Aquilegia, Delphinium and Gaillardia. Take the cuttings of Chrysanthemums and Dahilas. Keep these roots cool and dry for the planting in January or February.
Vegetables: you can sow these seeds in December:
- Garden Cress
- French Bean
Harvest Lettuce, Carrot, Radish, Mushroom, Potato, Chicory and Rhubarb.
Fruit: plant Peach, Nectarine, Apricot and Grape vine. These fruit trees need unheated conditions and ventilation on dry days. These requirements are not compatible with tender plants. You can prune established Grape vines if you didn’t do that in November.
Greenhouse December Jobs Checklist
- It is a perfect time to get seed trays and pots ready. Get rid of the broken ones and wash the rest thoroughly with a garden disinfectant. Stack them neatly on the shelves. It is also a good idea to buy multipurpose compost at this time.
- Monitor the temperature carefully. If you grow in a cool greenhouse, the minimum night temperature should be 45°F (7°C) and during the day maximum temperature of 55° – 60°F (13° – 15°C). You can use a wireless monitor system which sends low/high temperature e-mail and text alerts to your device. So you can avoid high temperatures which lead to soft and lanky growth.
- Ensure there is sufficient ventilation which removes condensation and keeps the air moving. Avoid creating droughts and sudden drops in temperature as it leads to plant death. If the day is damp or foggy, don’t open the vents. Open the vents on the side away from the wind. Close the vents in the mid-afternoon to conserve the heat of the sun before nightfall.
- Keep the soil dry in December for plants like Cacti, Succulents and Pelargonium. Damping down of the floor and staging in only necessary if the plants are in flower. Damp down in mid-morning.
- Inspect the flowers regularly. This includes removing dead flowers or yellow and diseased leaves. Use mildew killer spray if grey mold or whiteflies are present.
- Make sure to insulate the greenhouse before the end of the month. When insulating don’t block the vents. You can use insulating plugs if there are slots in the glazing bars. Also, you can try using double-sided sticky pads.
- Check that your heater is in a perfect working condition. If you use a paraffin heater, trim the wick to prevent production of plant-toxic fumes.