time release ferts. sprinkle some around the base of your plants(no more than the recommended amount,slightly less than that might even be a safer bet)or mix in with the soil. As your plants get watered the ferts slowly leech out. Lots of MJ growers arent too fond of time release fertz for lack of being able to control your feedings, but if your doing a guerrilla style outdoor, and cant see your plants very often, its your best route. As long as you dnt use too much for the amount of soil your using, and your plants dnt get really overwatered, you should be good. a long period of rain could cause problems, more water=more ferts dissolved which could lead to burns and salt build ups, ultimately compounding more problems. As i said use less than the advised amount so if this does happen it wont be nearly as much ferts being drained into the plant at once. Its easy to add a little more fert to your soil after showing deficiency signs, not so easy to take away fertilizer after your plants are burning up