watch the sky from where your planning to grow put a tarp up like a wall, use the height of the tarp to make it so the sun get blocked out by whatever time visit the place your gonna grow like 4 times a day one in the morning when the sun hits the plants, one ate like 3 or whatever and make sure the sun is still there(not blocked off by tree or anything) and stay count from when the sun comes up 12 hours I guess and go there at that time and build the wall so you can still see how it works its hard to describe but you probably get the idea