try a shade box (though this normally only used to help a plant thats trying to revegitate stay in flower). or you could make sure they only get 12 hours of light each day by completely covering them and depriving them of light at a certain time each day.
transplanting a plant thats in the ground already sux ass. you dont know exactly where the roots have gone and as said before its a pain in the ass lol. can be done easily enough and the more you do it the better they will do.. make sure the dirt you transplant them too is similar though if you can. less stress involved
peace out
j88