[JayG]

Can't do any better than to quote this Wiki entry.

(For my aquarium I use an Interpet "Tropical Daylight" tube which has a blue/purple tinge which is good for plant growth but needs an additional "daylight" tube both to provide greater overall light intensity and to improve the colour appearance of the fish.)

[GilbertGoodman]

Only really know about lighting for aquarium plants, although the principles are presumably similar.

It's not just the amount of light but the colour spectrum which has to be correct to allow plant growth.

As far as I can make out led light with sufficient power and the correct spectral balance are available but are indeed very expensive to buy and still quite expensive to run (forget about 3W consumption - LED arrays with enough power to grow plants will use a lot of electricity and are considerably less efficient than you might expect, although that aspect is improving all the time.)

In these days of ever-increasing energy prices I suspect many people are reluctant to invest even in heating to extend the growing season given that for many "growing your own" is (at least in part) a way of saving money if at all possible. 

Thanks for this useful post but is there any particular wavelength or color which helps in plant growth.. Please reply thanks in advance:)

Still waiting for reply

[JayG]

Have you not read my reply (#16 in this thread - i.e. the post before yours?)