Basic Considerations for Indoor Hemp Growing

Written by Nick Congleton

As demand for hemp grows, cultivators are always looking to increase yield. Since most regions aren’t ideal for year-round growing, and environmental conditions can be unpredictable at best, cultivators may turn to indoor growing solutions. Indoor growing can boost hemp yields, mitigate losses, and provide tighter control over product quality. However, unlike outdoor cultivation, growers are responsible for everything their crops need, making it imperative to get conditions right.



Light is an essential factors in growing any crop. Cannabis plants need specific light conditions to trigger different developmental phases and influence the cannabinoids they produce.

For example, research indicates that red light helps plants develop longer stalks, while blue light promotes internode growth, meaning the plants will have more branches and leaves with fewer stretches of bare stalk. [1] The ratio of blue to red light provides an important metric to consider that portends how a plant will grow.

UV-A radiation is theorized to help prevent and repair DNA damage caused by UV-B radiation. UV-B is also thought to trigger higher tetrahydrocannabinol (THC) production, which provides a natural sunscreen to limit damage caused by UV-B.

Light-emitting diode (LED) lighting provides a range of options, allowing cultivators to utilize each piece of the spectrum in the ideal proportions. Some studies report high-pressure sodium lights as producing the highest biomass with the lowest THC concentration, which may be important for hemp cultivation versus THC-rich cannabis.

Hemp plants also need specific amounts of light for each stage of their life cycle. Cannabis plants naturally flower in the late summer, when the days have started to get shorter. That means, to promote the plant’s natural growth cycles, you need to simulate the day/night patterns of spring leading through summer.

Ideally, plants are exposed to as much light as possible during the growth and vegetative parts of their life cycles. This should be upwards of 16 hours of light. However, when it’s time to trigger flowering, research suggests that dropping that amount to 12 hours is best. The environment needs to be controlled to eliminate as much light as possible during the off hours to truly simulate night.


Temperature and Humidity

Temperatures are important for plant growth as well. It’s obvious that cold temperatures inhibit growth and often kill plants, but excessive heat isn’t a cultivator’s friend either. Research suggests that the ideal range for cannabis is between 25°C and 30°C (77°F and 86°F). [2]

Humidity is another important environmental factor that’s influenced by temperature, but research on cannabis growth suggests that, at a temperature of 25°C, the ideal relative humidity is around 75% for immature plants and around 60% for vegetative growth and flowering. [3]


Growth Medium

Both soil and hydroponics are popular for cannabis cultivation, including hemp. Hydroponic systems provide greater degree of control, but they are costlier and harder to set up and manage. Soil is simple and effective. Studies suggest that the correct concentration of nutrients coupled with adequate irrigation can produce great results.

The ideal pH for hemp is between 6.5 and 7.2. Soil with excess magnesium led to lower levels of both cannabidiol (CBD) and THC. [4] At the same time, additional phosphorus in soil matched with lower CBD concentrations.

One study compared two soils based on their capacity to hold water. [5] The study found that the soil with the lower ability to hold onto water resulted in both greater weight of the dried plants and higher cannabinoid concentrations, indicating that better irrigation is idea for increasing cannabis yield.

Whichever options you choose, ensure that nutrient levels are ideal for cannabis plants and there is adequate irrigation.



[1] Jin D, Jin S, Chen J. Cannabis indoor growing conditions, management practices, and post-harvest treatment: A review. American Journal of Plant Sciences. 2019;10(06):925-946. doi:10.4236/ajps.2019.106067 [journal impact factor = 1.17; times cited = 15]


[2] Chandra S, Lata H, Khan IA, Elsohly MA. Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. Physiol Mol Biol Plants. 2011;17(3):297-303. [journal impact factor = 3.08; times cited = 28]


[3] Chandra S, Lata H, Khan IA, and ElSohly M.A. The role of biotechnology in Cannabis sativa propagation for the production of phytocannabinoids. In: Chandra, S., Lata, H. and Varma, A., Eds., Biotechnology for Medicinal Plants, Springer, Berlin, Heidelberg, 123-148, 2013. [times cited = 10]


[4] Coffman CB, and Gentner WA. Cannabinoid profile and elemental uptake of Cannabis sativa L. as influenced by soil characteristics. Agronomy Journal. 1975;67:491-497. [journal impact factor = 2.650; times cited = 27]


[5] Caplan D, Dixon M, and Zheng Y. Optimal rate of organic fertilizer during the vegetative-Stage for Cannabis grown in two coir-based substrates. HortScience. 2017;52:1307-1312. [journal impact factor = 1.455; times cited = 33]


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Nick Congleton

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