You'll need some sort of large-scale Sun manipulation device. Check eBay for such devices. I think they're under the Deathray category ...or wait until the summer.
if your anywhere near northern california your plants should start flowering in the begining of august. if your really impatient go to www.mylocalalmanac.com find out what time the sun rises where you live, then everyday at that time IN THE EVENING take your plant in a shed or somewhere COMPLETELY dark, then take them back out once its dark, that will give your plant 12/12 forcing into flowering
my indica plants are 14 in tall with 6 heads started from seed in the lastweek of april they are outdoor an get 12 plus hrs of direct sunlight and im a zone 4/5 when can i trigger flowering?
any time you desire my friend
now if you want to keep up with maintaining a 12/12 light schedule with tarps,buckets etc...
or you can wait untill nature takes its course