If your outdoor plants are not receiving enough light, then you can supplement natural sunlight with HID lights. All you have to do is make sure that the lights are in a waterproof container. You can cover the area with a tarp to keep the moisture off the light, but then you will have to water the garden manually, because you have blocked the rain from the plants.
How Long Should I Run My HID Lights?
To get the best growth out of your plants, you should run your HID lights for 10 to 12 hours every day. They should start around sunrise and, even if sunset is earlier, should run for 10 to 12 hours. This may seem excessive, but plants grow best in tropical climates, where sunlight and darkness are almost equal throughout the year. You can supplement this by using LED lights before and after your regular cycle, to simulate sunrise and sunset.