Spinach is a cool weather crop that requires six to eight hours of sunlight per day in order to grow. Insufficient light is the main cause of an indoor-grown plant's failure to thrive, according to Utah State University. This can be easily remedied by supplying the young spinach plants with supplemental light and, according to Utah State University, fluorescent lights are the best choice for indoor gardening projects. You can purchase suitable lights from most large gardening centers and online.
Mix together equal parts of potting soil, vermiculite, perlite and peat moss. Pour the soil mixture into each cell in the seeding tray and water it until the water runs out of the bottom of the tray. Allow the tray to drain completely.
Sprinkle the seeds lightly across the surface of the soil, several seeds per cell. Using your fingers, lightly rake them into the soil. Do not plant them too deep; the spinach seeds should be barely covered with soil.
Place the tray in a bright room where temperatures, in the daytime, will remain between 65 and 70 degrees F. Nighttime temperatures should not go below 40 degrees F. Check the soil daily and if it begins to dry, spray it with the misting bottle until it is consistently moist, but not soggy.
Place the flat six inches under the light source when the spinach seedlings have their first set of leaves. They will need to remain under the lights for 14 to 16 hours per day.
Fertilize the spinach seedlings with fish emulsion, the same day you place them under the lights. Water prior to fertilizing and apply the fish emulsion at the rate suggested on the packaging.
Raise the light source as the spinach plants grow, always maintaining six inches between the plant and the light source.
Water the seedlings when the top 1/2 inch of soil is dry by placing the tray in a slightly larger pan or tub filled with two inches of warm water. Allow the tray to sit in the water until the top of the soil is wet. Allow the tray to drain for five minutes before placing it back under the lights.
Transplant the spinach seedlings into four-inch pots as soon as they can be easily pulled from the seeding tray. Use standard potting mix and fill the pot to within 1/2 inch of the rim. Dig a hole the same depth and width as the rootbal and place the roots in the hole. Back fill with soil and tamp lightly around the base of the spinach plant. Place the pots back under the lights.