“When do I harvest my hemp?” is a common question we’re frequently asked. Hemp is photoperiod dependent, meaning it is signaled by the sun to either be vegetative (non-reproductive) or in flower (reproductive) phase. From the indoor cannabis world, growers often switch their cultivation rooms from “vegetative” to “flowering,” meaning the photoperiod shortens and often the spectrum of light is changed from blue to red. 

Understanding how and when your hemp will flower is paramount to your success as a hemp farmer. Choosing the wrong genetics that have an incorrect photoperiod sensitivity for your region (when your hemp will be induced into flower) will not only be detrimental to your harvest, but can lead to complete failure. Let’s run through a few scenarios… 

If you are growing in Montana and (unknowingly) plant a variety of hemp that has a lower photoperiod sensitivity and therefore will take longer to flower, Mother Nature will likely come through with a heavy snow or deadly frost that will wipe your plants out before they are fully ripe with flower. In Montana, for instance, you would want genetics that induce flower earlier, known as “an early finisher” to avoid fall freezes and snow. 

Another example: if you are planting in the Southeast, perhaps South Carolina, and hemp plant genetics that possess high photoperiod sensitivity (an early flowering variety), this will likely also lead to failure. July, August, and September humidity levels in conjunction with fall rain can result in bacteria and fungus issues in flower, which will destroy a harvest. In these regions, farmers should plant mid-to-late flowering genetics; doing so results in the plant being in its vegetative state while it brunts high humidity and moisture during the wet months. As an added bonus, this planting strategy provides a longer growing period in which the plant will become larger in size before flowering.

Now, let’s discuss exactly how hemp flowers. 

In the Northern Hemisphere, the days grow longer and longer in the spring until the summer solstice, the longest day of the year. This occurs around June 20th and after this date, we begin our trajectory towards fall and winter as the days incrementally grow shorter. This day shortening is what triggers hemp to flower. The shorter days signal that winter is coming and it is time to reproduce (and therefore flower); like all species on Earth, plants have a built-in blueprint to reproduce and pass on genetics to continue the next generation.

As the Northern Hemisphere begins to tit away from the sun, the days grow shorter. In the US, the lower 48 states have a latitude that ranges from 25° N to 49° N. One would expect the days to shorten at drastically different times, but what happens in reality is surprising. Let’s run through an example. 

The sun is strange during the fall equinox and only differs a few minutes from Florida (29.7 degrees N), to Colorado (39.7), to Oregon (44.1 degrees N). On September 20, the day length in FL, CO, and OR is 12 hr 11 min, 12 hr 15 min, and 12 hr 16 min—not much different. You can find daylength for any date and latitude by using this website for calculations. Most hemp starts to flower sometime in August (around the 14-13 hour mark) and will finish flowering in September-October. 

Understanding Photoperiod Sensitivity and Genetics

Genetics are the main differentiator between harvest times of different hemp varieties. Certain varieties are more photoperiod sensitive (sensitive to shorter days) than others. As mentioned above, this is incredibly important when determining which varieties to grow for your region. 

Planting earlier vs. later for certain varieties will not cause your hemp to flower earlier/later; planting times only affect how much time your plants will have in their vegetative state. In other words, the earlier you plant (within reason), the more time you give that plant to grow vegetatively and therefore the larger harvest you will have. If you are planting later (in July, for example), you can still grow beautiful, big flowers but the overall size of plant will be smaller. To compensate for this, we suggest farmers plant at a higher density if planting later in the season. 

In Summary

When selecting varieties, keep three things in mind: 1) Understand your geographic region and the potential weather events you need to work around, 2) Know the genetics you are working with and their photoperiod sensitivity, and 3) Work with at least 2–3 varieties to stagger your harvest to ensure you have time and labor to get all of your plants out of the field. 

That’s all, folks! Happy growing!

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