How to Start Seeds in a Cold Frame

Cool weather plants, such as pansies and lettuce, can be started in a cold frame instead of indoors or waiting for the weather to warm outside. Cold frames are boxes with clear tops, usually made of glass or plastic. There are permanent structures, but most cold frames are temporarily set onto the garden soil and then removed once the weather warms. While cold frames are most often used to protect plants going into winter, they can add valuable seed-starting space in spring for cool weather plants.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Cold frame
• Seed flats
• Potting soil
• Blankets

1. Place the cold frame on a level section of garden bed. Push the frame into the soil a few inches so no cool air drafts come in through the bottom edge of the cold frame.

2. Remove any dead plant material from the garden soil inside the frame. Lay a fresh two-inch layer of sterilized compost on top the soil and till it in to a six-inch depth.

3. Fill seeds starting trays or pots with a sterilized potting soil. Sow seeds inside the flats to the recommended depth and spacing as detailed on the seed packet. Water the soil just enough to moisten.

4. Place the seed flats into the cold frame and shut the lid. Cover the glass with a blanket at night and during cloudy days to preserve the warmth generated inside the cold frame during sunny days.

5. Ventilate the cold frame after seeds have germinated and sprouts appear. Open the lid sash on sunny days when the outside temperature is above freezing. Open the sash on the side opposite any wind or breeze, and close again in late afternoon.

Tips & Warnings

• Keep the glass clean to allow maximum light transmittance.
• Cover the glass with blankets or straw during prolonged periods below freezing, especially if freezing winds are blowing.
• Harden-off the seedlings before transplanting to their permanent beds in the cold frame. Open the lid a little bit more each day until you finally remove it after seven to 10 days.
• Insects are drawn to the first foliage in spring, which is inside your cold frame. Check the plants daily and treat with a chemical or organic control if insects pose a problem.
• Keep an eye on the cold frame on warm days and vent as necessary to keep from cooking the plants inside.

References

• University of Missouri Extension