16 square feet? 25 square feet? How much indoor garden do you get per 1000-watt grow light? Well, unsurprisingly, it depends on the species of plant you want to grow. Leafy greens like lettuce, rocket, cilantro and basil are more than happy at μMol 300. (That's 300 micromoles per square meter, per second.) Whereas if you want to grow fruiting and flowering crops like chillies or tomatoes indoors under grow lights, you probably want to be shooting for 500 - 700 μMols.
Light intensity decreases exponentially the further plants are from the artificial light source. However, while your plants can probably take all the light you can throw at them indoors, the last thing you want to do is position your grow lights too close—causing localized heat and low humidity stress. That's a no no. Your plants' leaves—those all-important light harvesters—cease to do their job in environmentally stressful conditions. So the key to growing plants successfully indoors is delivering sufficient light while maintaining temperature, humidity and carbon dioxide levels within acceptable ranges. I like my indoor garden to be in the 70s, not the 80s and relative humidity at at moderate 50 - 60 percent. If this sounds like your garden, I bet you're doing well! Hot gardens, on the other hand, tend to be inhabited by leggy, stressed out, and low yielding plants. Maybe it's time to upgrade your ventilation system?
Anyway—check out this video of me working out what area I can cover with my two Magnum XXXL 1000 watt grow lights:
How Much Space Does a 1000-Watt HID Grow Light Cover?
We look at different plants' preferred lighting intensities and how to achieve them with HID grow lights.
One thing I didn't have time to describe in the video is the concept of Moles per Day. Every plant species has a minimum light requirement over a given 24 hour period. This formula, kindly provided to me by my Facebook buddy Darryl Cotton, sums it up nicely:
μMol/square meter x 3600 x photoperiod(hrs/day) ÷ 1,000,000 μMole/Mole = Mol/M2Day
Let's work through this formula with an example:
Greenhouse growers tell me that commercially grown lettuce thrives at 20 mol/day.
If I have my lights on for 18 hours per day, and I measure 300 μMol at the leaf surface with my quantum (PAR) meter, then:
300 x 3600 x 18 = 19,440,000 μMol or 19.4 mol/day
Just short! (I could increase my lights on period to 18.5 hours and I'd be hitting the target of 20 mol/day.)