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How to Make a Seed Starter Sun Box

By Karen Ellis ; Updated September 21, 2017

A seed starter sun box is the same thing as a cold frame. The only difference is that it’s used to start seeds early, protecting them from late frost. A cold frame is used to protect vegetable plants from early frost so they can be harvested later. The project starts with a wood-framed window salvaged from old buildings when they are replaced with modern, air-tight windows. They are available from building salvage dealers, building contractors or flea markets.

Measure the length of the window frame with a tape measure. Measure, mark and cut two pieces from the piece of lumber that are the same measurement, using a measuring tape, pencil and saw.

Attach a board on each side of the window frame with your drill, screwdriver attachment and 1 1/2-inch screws. Place the window flat on the work surface. Place a cut board on either side, on its long edge. Screw through the board and into the window frame on each side. Place a screw approximately every 4 inches.

Measure the width of the window frame, which now includes the 1 inch of each attached board. Measure, mark and cut two boards from the remaining piece of lumber.

Lay one of these cut pieces, on the long edge, on either side of the width of the frame. Use the drill and attachment to screw them onto the frame and ends of the first boards.

Place your pre-seeded containers in a sunny area of the garden. Place the seed starter sun box over the top. During the warmest part of the day, prop up one side of the box, with a couple of stacked scrap boards from your project, after seedlings have appeared. This will allow needed air flow for a few hours a day.


Things You Will Need

  • Wood-framed window
  • Tape measure
  • Pencil
  • Lumber, 1 inch thick and 10 inches wide
  • Saw
  • Drill and attachments
  • 1½-inch screws


  • To create a cold frame for taller vegetable plants, use 12- or 14-inch-wide lumber. This will give the mature plant more space. The taller cold frame can still be used as a seed starter the next spring.
  • If outdoor temperatures are expected to dip below 20 degrees Fahrenheit, cover the seed starter sun box with burlap or a blanket. Remove it when the temperature rises again.

About the Author


Karen Ellis has been a full-time writer since 2006. She is an expert crafter, with more than 30 years of experience in knitting, chrocheting, quilting, sewing, scrapbooking and other arts. She is an expert gardener, with lifelong experience. Ellis has taken many classes in these subjects and taught classes, as well.