WINTER GREENHOUSE CALENDAR: DECEMBER

This month you reap the rewards of your autumn efforts. Bowls of forced Narcissi and Hyacinths, as well as pots of Cyclamen, Primula and Cineraria will become Christmas decoration. This month you’ll be harvesting fresh vegetables such as Carrots, Mushrooms, Potatoes, and salads of Lettuce and Chicory.

December can be a difficult greenhouse management month. While it’s not the coldest month, it is the darkest one. You may find that plant growth is sluggish. Be sure to monitor the amount of light your plants are receiving. If you don’t have supplemental lighting, keep the soil relatively dry and the air at temperatures of less than 60°F.

GENERAL PLANT TASKS

• Bring in bulbs for Christmas flowering.
• Water Christmas Cacti, Cinneria, Cyclamen and Gloxina sparingly. Lightly mist Cacti to prevent shriveling.
• Pinch back cuttings that may form flower buds.
• Lift potatoes and harvest Lettuce.
• Keep the soil rather dry for plants not in flower — especially Cacti, Succulents and Pelargonium. Damp down the floor and staging only when it’s really necessary, and only in mid-morning.

Bulbs

• Bring bulb bowls inside to hasten growth. The buds of forced bulbs should start to show color this month, just in time for holiday decorations.
• Plant Lilies. Support stems of Freesia and Lachenalia.

Other Ornamentals

• In early December move Christmas potted plants — such as Azalea, Cineraria and Primula — to the warmest part of the greenhouse. Water carefully and add a liquid fertilizer.
• Sow Campanula Isophylia. Propagate Carnations and stake Lorraine Begonias.
• Bring in pot-grown Roses. Cut the stems at 3-4 buds above the compost. (This may also be done in January.)
• Stock up on seed trays and pots for use in the new year. Wash broken pots thoroughly with a garden disinfectant. Buy a multi-purpose compost.
• Pot up shrubs for spring greenhouse decoration. Examples might include: Forsythia, Hebe, Lilac and Weigela. Many herbaceous perennials — such as Aquilegia, Delphinium and Gaillardia — can now be planted in pots.

Bedding Plants

• Inspect your autumn-sown half-hardy annuals for January planting. Make sure they receive maximum light. Don’t water unless compost surface is thoroughly dry.

Garden Perennials and Shrubs

• Complete the cutting back of Chrysanthemums and Dahlias that were lifted and installed in the greenhouse last month. Keep the roots cool and fairly dry; they will begin to grow in January and February.

Vegetables & Fruit

• If your greenhouse is maintained at 50°F - 55°F, tomatoes can be sown in a propagator in late December, but only if your climate will allow you to plant them out in March.
• Sow Lettuce, Carrot, Mustard, Cress and French Bean. Choose varieties recommended for growing under glass. Plant out October-sown Lettuce seedlings into the border or in growing bags.
• Pot up and force Chicory. Box up and force Rhubarb.
• Harvest Lettuce, Carrot, Radish, Mushroom, Potato, Chicory and Rhubarb.
• Plant Peach, Nectarine, Apricot and Grape vine. Prune established Grape vines if you didn’t do it in November. Keep in mind that Peach and Nectarine need an unheated environment, as well as lots of ventilation on dry days.

GREENHOUSE TASKS

• Since bitter cold snaps can occur in December, keep a close watch on night temperatures, and set your heaters accordingly.
• Be sure to finish insulation before the end of the month! Insulation is especially critical to all-glass greenhouses.
• Ventilate when weather permits. Allow as much fresh air into the greenhouse as possible, without compromising your plants.
• If you have a cool greenhouse and are storing some flowering plants, water them to increase condensation. But be sure to ventilate well.
• Monitor the temperature carefully. In a cool greenhouse, aim for a minimum night temperature of 42°F - 45°F and a maximum day temperature of 55°F - 60°F.
• Some ventilation will be necessary to remove condensation and keep air circulating. Ventilate carefully so as not to create drafts or a dangerous drop in the overall greenhouse temperature. On sunny, dry days, open a vent away from the wind. Close vent in mid-afternoon to preserve solar heat before nightfall. Close vents on damp or foggy days.
• Keep inside air dry. Don’t splash floor, staging or leaves. Water potted plants sparingly, except for those in flower.
• Inspect your plants. Spray if grey mould or whitefly is present. Remove dead flowers and diseased or discolored leaves.
• Trim the wicks of paraffin heaters to prevent plant-toxic fumes.
• Start accumulating seed, bulb and plant catalogs so you’ll be ready to order in the new year.