Extend your gardening season with cold frames

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener

Rows of milk jug for seed starting.
Seed starting medium with sprouts.
WSU Chelan-Douglas Master Gardener Emilie Fogle grew germinated native plant seeds in milk jugs. Using milk jugs to stratify seeds is an alternative to using a cold frame to achieve the same result. – Provided photos/Emilie Fogle
Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

It is not too early to think about how to harvest greens most of the winter. Starting seeds in a cold frame while the soil is still warm will give the plants a healthy start to survive the winter.

Who wouldn’t want to harvest arugula, spinach, kale, lettuce, radicchio, lettuce, dill, parsley and chives? The delight of winter salads! And, best of all there are no insect pests nor weeds. Most of the plants will be cold-tolerant such as you plant in early spring. For example, tomatoes are too big and not cold tolerant.

This is the time to build a cold frame, which is a small greenhouse. A cold frame captures sunlight and creates a micro-climate. The internet has dozens of suggestions for building plans. One example from Iowa State University Extension can be found at bit.ly/isucoldframes. You can use repurposed materials of old window glass, Plexiglas or heavy-gauge clear plastic to make the lids. The lid frames can be PVC or wood. Cold frames can also be purchased.

Here are some considerations:

  • The location should be on a south or west wall. The wall of either a fence or your house provides an important warming feature to the north side of the cold frame.
  • South or west sunlight is the strongest in the winter.
  • The cold frame is a bottomless box and should extend 16-24 inches deep into the soil. The soil retains heat. Also in the Wenatchee Valley, the ground seldom freezes deeper than 8 inches.
  • Whether you make the frame from brick, wood or straw bales, mulch or soil piled up on the outside of the frame provides more insulation for the cold frame.
  • If the outside temperature is 40 degrees or higher, be sure to lift the cold frame’s lid a few inches so the sunlight does not cook the plants. Then lower the lid when it cools to keep in the heat overnight.

Another use for the cold frame is to cold stratify seeds that need a month or two of chilling before they germinate. It is also possible in March to start seedlings of your garden vegetables if you do not have a space in your house to grow seedlings.

Rather than building a structure, many gardeners have found that they can stratify seeds, especially seeds of native plants, and germinate other slow growing plants in plastic containers such milk jugs with holes punched in the bottom and set slightly into the soil. These types of containers are usually seeded in December. There is not a chance of them overheating because the lids are not placed on the milk jugs.

Raised beds can be made into a type of cold frame for at least part of the winter. A hoop or lid for the raised bed can be made from PVC and heavy gauge clear plastic, or one that fits the raised bed can be purchased.

Enjoy an enhanced garden season.

A WSU Chelan and Douglas County Master Gardener column appears weekly in The Wenatchee World. To learn more, visit bit.ly/MGchelandouglas or call (509) 667-6540.