The flower and vegetable plants that I start indoors get tall and spindly. Why?

FAQ
Question: 

The flower and vegetable plants that I start indoors get tall and spindly. Why?

Answer: 

Tall, spindly growth is a common problem when growing transplants indoors. Poor (insufficient) light, excessive watering, high temperatures, excessive fertilization, and crowded conditions are factors that contribute to spindly growth.

Proper growing conditions should produce short, stocky transplants. Immediately after germination, move the seedlings to an area with a temperature of 60 to 70 degrees Fahrenheit and place them under fluorescent lights (a sunny window usually doesn’t provide sufficient light). A standard fluorescent shop fixture containing two 40-watt tubes (one cool white and one warm white) works fine. Position the fluorescent lights no more than 4 to 6 inches above the seedlings. Leave the lights on for 12 to 14 hours a day. Thoroughly water the seedlings when the soil surface becomes dry to the touch. Fertilize every two weeks with a dilute fertilizer solution. Seedlings growing in flats should be transplanted into individual pots or cell packs when the first “true” leaves appear.