Plants grown for their fruits, vegetables, or flowers often require a dark period in order to force them into their flowering stage. Detailed information on how to do this can be found on my flower forcing page. If you are trying to grow peppers indoors, for example, many varieties require a 12 hour dark period. The plants need to receive this dark period day after day, with the lights going on at the same time and off at the same time each day. Also, it is important that NO LIGHT should interrupt the dark period.
http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/how-long-do-you-leave-grow-lights-on.html
Set the petunias in a sunny window where they receive at least six hours of direct light a day. If you don't have a suitable window, use fluorescent grow lights to provide enough light for the petunias to produce healthy, compact growth.
http://www.ehow.com/info_7948148_much-sunlight-do-petunias-require.html
We use grow lights, but are not sure if they should be left on 24/7 or turned off in the evening. Please let us know. Answer: Rita-the answer to your question depends partly on what you are growing, and partly on what you want from your garden. Let me give you a few examples and hope it will clarify things. Any time you intend to grow plants in the vegetative stage, they should receive at least 18 hours of light per day.
http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/how-long-do-you-leave-grow-lights-on.html
Crops requiring at least 6 hours but can grow with less than 8 hours of sunlight per day (Partial Sun): 1  Beets. 2  Broccoli. 3  Cabbage. 4  Carrots. 5  Cauliflower. 6  Collard Greens. 7  Kale. 8  Leeks. 9  Onions. 10  Peas. 11  Potatoes. 12  Radishes. 13  Rhubarb (perennial-can also grow in full sun). 14  Swiss Chard.
http://blog.gardenharvestsupply.com/2014/03/12/how-much-sunlight-do-growing-vegetables-need/
So, for the purpose of describing shade or sunlight—it is not an exact science; it can depend on where you live. For instance, full sun in the northern part of the country can be 8 to 10 hours of direct sunlight per day.
http://blog.gardenharvestsupply.com/2014/03/12/how-much-sunlight-do-growing-vegetables-need/
Petunias may not flower as well when planted in partial sunlight, and the plants may become leggy and weak looking. Prune the plants by up to half their height at midseason if they are weak and spindly. Pinching helps encourage a new flush of growth and makes the plants appear fuller.
http://www.ehow.com/info_7948148_much-sunlight-do-petunias-require.html
Full Sun Beds. Plant petunias in beds that receive full sunlight whenever possible. Petunias flower best and produce the most compact growth when supplied with six to eight hours daily of sunlight. Before transplanting petunias to a full-sun bed, harden them off so they don't suffer leaf scalding from the bright light. Place potted petunias in an outdoor area protected from direct sunlight.
http://www.ehow.com/info_7948148_much-sunlight-do-petunias-require.html
1 Partial Sun—at least 6 hours of direct sunlight per day. 2  Partial Shade—at least 4 hours of direct sunlight per day. 3  Full Shade—less than 2 hours of direct sunlight per day. 4  If your prospective garden area experiences this type of shade, we do not recommend planting vegetables.
http://blog.gardenharvestsupply.com/2014/03/12/how-much-sunlight-do-growing-vegetables-need/
Indoors. Most indoor growers provide 18-24 hours of light a day (known as 18-6 or 24-0 light schedules). Giving your cannabis plants more hours of light each day in the flowering stage will encourage faster growth. Lingo: When a grower provides 18 hours of light a day and 6 hours of darkness, this is commonly known as the 18/6 light schedule. For 24 hours a day, this is referred to as the 24-0 light schedule. Outdoors.
http://www.growweedeasy.com/light-schedules
As long as your plant is getting plenty of light a day, your plant will automatically stay in the vegetative stage from late spring until late summer. Every strain is a bit different. Flowering Stage. Cannabis starts budding when plants get at least 12 hours of uninterrupted darkness each night.
http://www.growweedeasy.com/light-schedules
how many hours a day do my petunias need a grow light
Petunias need six hours of light a day to grow. Your petunias need a grow light for six hours a day.