What do you mean by "harden"??? You need to provide more info. Are you still vegging or in flowering? How hot is it where you are? Humidity? The best approach is to slowly supplement your artificial lighting with natural and slowly increase the amount of natural sunlight until you are 100% natural. This way you reduce the risk of stress and a hermie. One other point, make sure you pot is insulated (you can put it into a bigger pot or wrap in cardboard, etc.). If you pot gets too hot outdoors, especially if it's plastic, it can "cook" the roots and damage your plant. Good luck.