Drip feeding is one of the most popular hydroponic methods among commercial applications. It is often used to grow longer term crops like tomatoes, cucumbers and peppers. In this system, the nutrient solution is delivered to the plants through drip emitters usually on a timed system. The emitters are usually scheduled to run for approximately 10 minutes of every hour depending on the stage of development of the plant and the amount of available light. The drip cycle flushes the growing medium, providing the plants with fresh nutrients, water and oxygen. In a commercial drip system, the plant roots are most commonly grown in a medium of expanded clay grow rocks, perlite or rockwool. The biggest variables in a drip system are in the growing medium and the container that holds that medium. Perlite is often used in thin, plastic sleeves. Holes are cut in the bag and plants, usually 3 - 4, are set in with the roots growing down into the perlite. Rockwool, coco peat and expanded clay grow rocks can also be used.