Bottom Heated Flat for Seeds

By Charlie Nardozzi

It's well known that seeds germinate faster and seedlings grow better when they have heat around their roots. You can buy heating cables or mats to place under seedling flats, or if you're somewhat handy, you can build a simple, heated seed-starting box from scrap materials found around the house as member Robert Carney of Delmont, Pennsylvania did.

Build an 8-inch-deep 24- by 48-inch box from 1/2-inch plywood. Insulate the interior sides of the box with 1-inch-thick blueboard insulation or plastic foam. On the bottom of the box, mount four outdoor light sockets, evenly spaced and connected in serial together. Drill a hole in one wall for the electrical cord. Screw 40-watt bulbs into the sockets.

For the top of the box, cut a piece of metal sheeting to fit across the top to the outside edges of the box. Paint the underside of the metal sheet black to retain the heat from the light bulbs. Fasten the metal sheeting securely to the box with metal L-brackets.

With the lights enclosed in the insulated box and turned on 24 hours a day, pots of seedlings placed on top stay between 70 and 75° F, even in an unheated basement. Place an outdoor thermometer on the metal sheet to measure the temperature. If the temperature goes above 75° F, turn the lights off for a few hours.

Be careful to place the pots in a plastic tray and to remove the seedlings before watering. Never water them on the metal sheet because water may leak into the box and short-circuit the lighting fixtures.

Charlie Nardozzi is a senior horticulturist at National Gardening.

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