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How to Build a Raised Bed Heat Sink

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a raised bed with a heat sink installed with tomatoes growing inside
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A passive-solar heat sink, made by adding multiwall polycarbonate (PC) panels to a raised bed.

Photo Credit
Rob Schuster

Passive-Solar Alternative to Greenhouses to Extend the Growing Season

Janet McNaughton
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Many gardeners long for a greenhouse, but greenhouse growing brings challenges, what with regulating the ventilation, watering, pollination problems, the risk of fungal diseases and pests (aphids and whiteflies, to name a few). Not to mention the expense and the space requirements.

There is a low-cost alternative that avoids the pitfalls while providing many of the benefits of a greenhouse: a passive-solar heat sink made by adding multiwall polycarbonate (PC) panels to a raised bed. It is open to sunshine, rain, pollinating breezes, and beneficial insects, allowing the soil to trap and hold more of the Sun’s heat. The polycarbonate panels are fitted as closely as possible to the top boards of the raised bed. The hinged panels are flush-mounted on support posts so that the sides of the bed make continuous walls when closed. This helps to trap heat.

The directions here are for a 4x8-foot raised bed (any depth). Adjust accordingly for larger or smaller beds. Learn how to build a raised bed in our step-by-step guide.

This project typically costs under a few hundred dollars. The multiwall polycarbonate is available as 4x8-foot panels, sold by some suppliers of plate glass and most greenhouse supply stores. These come in various thicknesses (I used 3.5-mm); the thickest cost more but retain more heat. While these panels are the main expense, they are also lightweight, almost indestructible, visible to birds, and trap more heat than glass. Vendors might cut them to size for you; to do it yourself, use a fine-toothed circular saw, a Dremel tool, or a straight-edge and strong box cutter. Use eye protection.

The other materials are found at hardware stores. Weatherproof hardware is best.

You will need:

  • duct or greenhouse tape
  • 3 sheets of 4x8-foot multiwall polycarbonate, cut to make 6 4x4-foot pieces
  • 6 5-foot lengths of 1x3-inch wood strapping (aka furring strip board), for support posts
  • 18 1½-inch deck screws
  • 16 pan-head wood screws (length depends on the thickness of panels)
  • 8 flush-mount cabinet hinges with screws
  • 4 4-foot lengths of wood trim (narrow but thick enough to hold the hinge screws)
  • 4 bungee cords, each about 3 feet long
  • 12 brass cup hooks
  • cordless drill with screwdriver bits to match screws, plus a bit to predrill 5-foot wood strapping

How To Make the Heat Sink:

1. Cover the sharp edges of the polycarbonate with duct or greenhouse tape, pressing the tape evenly over the edges.

2. Secure the 5-foot-long support posts to the inside of the raised bed with three deck screws in each. Screws go through the wider, 3-inch side of the post. Leave a 4-foot-long portion of each post above the bed frame. (Predrill the screw holes to prevent splitting.)

Secure four 5-foot-long posts at the corners inside the 4-foot ends of the raised bed.

Attach the remaining two posts at the middle of the 8-foot sides of the raised bed, one on each side.

building a raised bed heat sink for your garden
Cover the sharp edges of the polycarbonate with duct or greenhouse tape, pressing the tape
evenly over the edges. Secure the 5-foot-long support posts inside the raised bed with three deck screws in each. Leave slightly more than 4 feet of each post above the bed frame.
Illustration by Rob Schuster 

3. At a 4-foot side of the raised bed, rest a PC panel upright against two posts, with the bottom resting on the top board of the raised bed beneath. Attach the panel to the outside of each post with pan-head screws at 16-inch intervals, using a light hand on the drill.

4. Repeat on the other 4-foot end of the bed.

5. Attach the hinges to the remaining four panels: Place a panel on a flat surface. Lay a 4-foot length of wood trim under it, along one edge; match the edge of the panel with the edge of the wood trim. The hinges have a flat door-leaf side and a cylindrical pin side. Place the door-leaf side so that its edge by the pin sits flush with the edge of a panel, 6 inches from the top. Attach the screws that match the hinges to fasten the door-leaf side to the panel and wood trim. These screws are so small that you may wish to do this by hand. They may go all the way through the wood.

Attach another hinge 6 inches from the bottom of the panel.

Repeat until all four panels have hinges at the edge of one side, 6 inches from the top and bottom.

Attach a panel to the 4-foot end of the bed using screws at 16-inch intervals. Repeat on the other 4-foot end of the bed. Attach the hinges to the remaining four panels. The remaining panels go on either side of both center support posts.
Illustration by Rob Schuster 

6. The remaining panels go on either side of both center support posts (you will need help at this stage). Rest the hinged side of a panel snug against the left side of a center post. The pin sides of the hinges will rest flat on the post, facing you outside the bed. The bottom of the panel should rest exactly on the top edge of the raised bed, with no overlap or gap. (Note that the unhinged side of the panel will extend slightly beyond the end of the raised bed’s 8-foot side.) Have a helper hold the panel in place while you screw the pin part of the hinges to the center post. A second panel goes on the right side of this center support post. Repeat on the other side of the bed until all four panels are in place.

7. Screw 3 cup hooks into the outer surface of each of the raised bed’s 8-foot sides at both ends and the middle. Put a cup hook near the top of each post, inside, where the wood is not covered by the panels. To keep the panels closed, fasten a bungee cord to the end hooks, top and bottom. To access your plants, fold a panel flat against the side of the bed and move the bungee cord to the middle hooks.

Screw 3 cup hooks into each of the raised bed’s 8-foot sides at both ends and the middle. Put a cup hook near the top of each post, inside, where the panels do not cover the wood. To keep the panels closed, fasten a bungee cord to the end hooks, top and bottom. To access your plants, fold a panel flat against the side of the bed and move the bungee cord to the middle hooks.
Illustration by Rob Schuster 

The design proposed here helps in just about any environment. In cooler climates (or growing seasons), the heat sink enables tender melons, winter squashes, cucumbers, peppers, and eggplants to produce more, faster, and with less stress. In generally frost-free zones, the heat sink could be used all winter to shelter frost-hardy crops like cabbage, kale, arugula, brussels sprouts, and carrots. Check out our raised bed garden layout ideas.

Plus, anywhere (or time) the wind blows to excess, the heat sink’s barrier configuration is an effective windbreak.

All this, and access to your plants is as easy as ever: The panels are hinged to let you weed, water, and harvest. The entire structure is easily removed to be stored in winter to extend its life and moved to a new bed each year for crop rotation.

Your heat sink will last 10 years or more. Heat-loving plants will flourish in their shelter!

About The Author

Jennifer Keating

Jennifer is the Associate Digital Editor at The Old Farmer’s Almanac. She is an active equestrian and spends much of her free time at the barn. When she’s not riding, she loves caring for her collection of house plants, baking, and playing in her gardens. Read More from Jennifer Keating

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