While it may not seem obvious, keeping your grow lights the appropriate distance from your cannabis plants is one of the most important factors to having a successful grow. 

Having the lights too close will cause the plants to burn and wilt, quickly killing them off. Raise your grow lights too high and the plants will stretch, causing the stems to be long and skinny leading to a very weak branch. Once the weight of bud growth reaches a couple of weeks, the branches will snap due to being so long and skinny.

Now before I show you the appropriate heights, I would like to mention two important items. First, these heights are applicable for both indoor and greenhouse grows. It does not change. Second, when hanging your lights you will want to consider the type of light you are using. For example, and LED light can get much closer to plants without burning them when compared to a HID Light. 

A simple test you can always use to tell if you are too close is hold your hand under the lights right where the tops of your plants are. If it is too hot for you to hold your hand there for 5 seconds, it is too close and the light needs to be raised.

Now lets discuss the appropriate height for your LED lights. The first thing to consider is what stage your plants are in. When in the Seedling stage, you want the light to be around 2.5-3 feet above the tops of your plants. Once you are in Vegetation, you can lower the LEDs to be around 2-2.5 feet above the tops of the plants. Finally, in flower you will want the plants to have a distance of 1-2 feet from the lights.  

As your plants develop and age, they will be able to handle more light at a higher intensity. You just need to slowly work them up to it is all. If you are not using LED grow lights, but a HID or some sort of Sodium bulb, I recommend adding foot to the heights I discussed above. Most importantly, always use the health of your plants as an indicator. Use the hand test as well and you will always be able to have your LED grow lights at the perfect height!