Seed-starting shelf units with integrated lights are available from many garden supply stores, but they come in limited sizes and are often very expensive. Design your own shelves to start plants in your home or greenhouse by taking into account your available space, number of trays required for plant starting, and the type and size of light fixtures you will be using.
Consider where you will locate your plant-starting shelves. Choose a location that is warm and free of drafts, and that gets as much direct sunlight as possible. Measure the width, depth, and height of this available floor space with your tape measure, and make note of these measurements on graph paper with a pencil.
Measure the trays you intend to use for starting your plants, using the tape measure. Note the length, width and depth (including the height of any clear plastic domes you may put over the trays) of these trays on your graph paper. Compare this information with the measurement of your floor space. Determine whether the space allows trays to be placed parallel to or perpendicular to the wall or window, and whether one or more trays will fit next to each other on each shelf. Sketch your preferred tray arrangement in pencil on graph paper.
Visit a local hardware store or lighting supply store and examine the fixtures suitable for plant lights. Compare the dimensions of the light fixtures with the measurements of your floor space. Determine which light fixtures will make the best fit, taking into consideration whether they will be hung parallel or perpendicular to the width of your shelves. Sketch your preferred light arrangement in pencil on graph paper. Note the height of the light fixture plus the height of the hardware that attaches the hanging chains to the fixture.
Compare the sketches and dimensions of your seed-tray configuration with your light-fixture arrangement. Determine the most efficient dimensions for the width and depth of your shelves which will maximize the number of lights and the number of trays on each shelf; this may require a compromise on the number of trays or lights you can fit on each shelf.
Calculate how many shelves you can construct vertically by adding the following: the height of the seed trays plus covering domes; an additional 1 to 2 inches for taller plant height; the height of your desired light fixtures and hardware for hanging them; 1 to 3 inches for space between the plants and the lights; the thickness of one layer of your desired shelf-building material (i.e., 1-inch pine, half-inch plywood). Include an extra 2 inches for a base for the shelf unit. Tally these figures, and then divide your available floor space height by the resulting sum to determine how many shelves you can construct.
Multiply the number of shelves times the number of plant trays that fit on each shelf, and purchase that number of plant trays. Multiply the number of shelves times the number of light fixtures on each shelf, and purchase that number of light fixtures and plant-growing light bulbs for each fixture.
Sketch a final building plan on your graph paper that indicates the total height and number of shelves as well as the dimensions of each shelf. Calculate the construction materials necessary, consulting with the personnel at your hardware or lumber store about material options and the best hardware or joinery methods for affixing the shelves to the support framework.