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HPS is a High Power Sodium lamp.
They contain a lot of the red spectrum, which simulates autumn sun. They also give off a high amount of Lumens, e.g. lots of light. They also give off a lot of heat, so you'll need to think of temps and ventilation. You've got 8 plants? I think the very mimimum you could get away with is a 600w HPS, if you want to be successful. Lots of people will probably say thats too little. They can be combined with warm white fluorescents, but again, a lot of people will say that those lights can't really compete with an HPS.

You should change when you want them to begin to flower. Preferably, the plants should be showing signs of sexual maturity first, like pre-flowers, or "alternate nodes", which means the new branches are no longer growing opposite each other.

Some people suggest it shold be phased in over a couple of days to simulate a gradual autumnal approach, but most people will tell you that that is unneccessary.

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