Extending Your Season

Extending Your Season With Cold Frames

For an early start to your spring planting season, sow seed in a cold frame and transplant it into the garden later (see Figure 1). Seed may be started as much as six weeks earlier than outdoors.

Locate the cold frame on the south side of a garage or dwelling. If built with a tight-fitting lid, the cold frame will hold sufficient heat from the sun to keep seed and seed­lings warm at night. On warm, sunny days (50F or warmer), prop the lid open to prevent buildup of excessive heat. Close the lid in the late afternoon to trap enough heat for cold evenings.

Cold frame.
Figure 1: A simple cold frame made with 2-inch x 2-inch lumber. Cover hinged lid and sides with translucent (clear) polyethylene plastic. For better insulation against cold, cover both inside and outside to leave an airspace between layers of plastic. An 8-foot frame requires 10 pieces 2 inches x 2 inches, each 8 feet long.

As the season progresses, gradually expose the plants to longer periods of outside temperatures, as long as the air temperature does not go below 50F. Treated in this way, they develop into sturdier plants that are better able to adapt to fully-exposed garden conditions at transplant time. This is particularly true of the hardy annuals and biennials that prefer to develop in cooler temperatures: cabbage, broccoli and lettuce. Use Table 1 to determine when to start seed in the cold frame.

Table 1: Starting times for seeds grown indoors and
in cold frames.
Number of weeks to start seed
before average frost-free date.
Plant name
In cold frame
Indoors
Ageratum
6
8
*Amaranthus (summer poinsettia)
4
6
*Bachelor’s button
4
4
Broccoli
6
4a
Cabbage
6
4a
*Calendula
4
4
*California poppy
4
4
Calliopsis
4
6
Cauliflower
6
4a
China aster
4
6
*Cosmos
4
4
Dahlia
6
8
Dimorphotheca (African daisy)
4
4
*Gaillardia
4
4
*Gomphrena
4
4
Larkspur
4
6
Lettuce (head and semihead)
6
4b
Lobelia
6
8
*Marigold
4b
4
*Morning glory
4
4
*Nasturtium
4b
4
Pepper
4b
6
Petunia
6
8
Phlox (annual)
4
6
*Poppy (Shirley)
4
4
Salvia
4
6
*Scabiosa
4
4
Snapdragon
6
8
Statice
4
6
*Straw flower
4
4
*Sweet alyssum (lobularia)
4
6
Tomato
4b
6
Verbena
6
8
Vinca (annual)
4
6
*Zinnia
4
4
*Plants best suited for direct seeding in garden two weeks prior to the average frost-free date.

aShould be kept in coolest room. Best at 55F to 60F.bIf outside temperatures are below 20F at night, delay planting or use artificial heat to keep temperatures above 50F.

 

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3 Comments Add yours

  1. I love the cold frame for hardening off the plants before they go into the ground. I recently visited a gardens from mid 1800 in the UK where they grow pineapples in long cold frames. The frames are heated by piling up fresh horse manure on both sides of the frame!

    Like

    1. CropLuck says:

      Pineapples in the UK? That’s pretty awesome! We don’t have much of a use for cold frames here in Southern California as it’s pretty much 70 degrees all the time but they are such a necessity in the NorthEast where I was first introduced to the idea. They are so simple to make and can easily be constructed from reclaimed wood and old windows.

      Liked by 1 person

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