Hi everyone,
I'm a newbie and so don't know much. How do I know when I should switch to 12/12 and HPS (currently running 400W MH)? I'm growing a sativa bred to be short (arjan's haze #3 from greenhouseseeds.nl). Greenhouse's web site says it should be ready for harvest in 11 weeks, but doesn't say when to start flowering. (Does counting start from when the germinated seed is planted?) I'm at 6 weeks after germination and planting, plant is about a foot tall (~30 cm) and is getting fairly bushy. Any advice would be much appreciated. Thanks!

Flower now. That plant will double or triple in size during flowering. Most people flower based on height. If you have three feet of available grow height then you should flower when it is one foot to allow room for growth.