Morning, Nico… I have a flowering plant that’s getting burned by my 400-watt MH light bulb. If I cut my plant down to an appropriate height what damage could I cause to her? Is this the best solution? Thanks for any advice you can lend. – Rob S. via NicosNuggets@hightimes.com
Hello, Rob – Thanks for reading and sending in this week’s cultivation question!
Lighting is one of the hardest aspects of indoor horticulture to master. However, lighting your garden does not have to be hazardous to your plants if you follow a few basic rules. These rules will help keep your plants happy, healthy and producing primo buds to smoke.
To start, let me address the focus of your question, which asks if cutting your plant to an appropriate height is an acceptable solution to plant burn. Answer: Do Not Cut Your Plant To Adjust Height. Unlike pruning, chopping inches off your plant top will indeed be detrimental to the growth and development of your plant.
Rather, you must adjust the height of your lamp, not the height of your lovely lady. Or, if you can’t adjust the lamp, lower the plant by putting her on a lower level or on the floor if need be. If she is already on the floor and the light cannot be adjusted, your grow space is too likely too small for cultivating flowering plants. Remember also that cannabis plants will still grow quite a bit taller during flower, so ample space is needed. If you know your space is too small from the onset, savvy growers may attempt to train their plants by weaving branches through a lateral trellis or latticework, but this is a more difficult and advanced grow technique and still requires a good amount of (horizontal) grow space.
This brings us to the first rule of thumb when lighting your garden – the proper distance from light source to plant canopy (top layer of leaf sets of your plants = canopy). Generally, the range of distance between a lamp and the plant canopy is between one and two feet. Of course, the goal is to keep your lamps as close to the garden canopy as possible so as to maximize the efficiency of your lighting and the absorption of light by your plants. It is also important to remember the Inverse Square Law, which dictates that for every foot of distance that is between the light source and the plant, the intensity of light is inversely proportional to the square of the distance from the source of that light. In simpler terms, this means that the light’s strength diminishes exponentially with each foot. Less strength means less light energy, which means less plant food (photosynthesis) which means less yield and less quality.
Two air-cooled lamps for a 10′ grow tent.
The next rule of thumb when it comes to lighting is to make sure you have the correct amount of light for your size plant or garden. This works both ways, meaning you shouldn’t have too little light for big plants or many plants, but you also should not have too much light for small (young) plants or just a couple plants. If your plants are under a 400-watt MH (metal halide) bulb and getting burned, they are either to close to the light source, or too young for such a strong lamp. Young seedlings and clones should be raised under softer, cooler lighting such as fluorescent bulbs. Only after they reach 6-10 inches in height with several leaf sets should they be moved under HID lighting, and even then safer distance of 3-feet should suffice for the first week or two.
The last rule of thumb when growing indoors is to maintain adequate temperatures and watering schedule for your plants. Under watered plants are much more susceptible to burning, wilting and dry rot. HID lighting over 400-watts should be have their bulbs air-cooled or have the heat being whisked away by ceiling exhaust fans that can suck heat up and out of the growroom. These additional techniques will go a long way to ensuring your plants do not burn or dry out and remain happy and healthy through to harvest time!
Exhaust ports venting heat from lamps near ceiling.
Thanks for reading everyone and remember: Grow… And help the world grow, too!
Got questions? Email ’em over to Nico at NicosNuggets@hightimes.com and be sure to put “Nico’s Nuggets” in the subject line!
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