Hemp Resources

Beginner's Guide to Hemp Genetic Selection

by Andy Hopkins / January 12, 2021

In farming, risk management tends to be more closely associated with crop insurance versus seed selection. However, the added risk of crops accumulating too much THC (tetrahydrocannabinol) and exceeding the regulatory limit of 0.3% by dry weight should be a legitimate concern for any grower interested in hemp. You want to go into a season knowing what you're growing.

Since the hemp industry is relatively new in the United States, growers are at a disadvantage when seeking out new genetics. Breeding stable genetics takes time, time that this industry hasn't had. Due to this reality we are meeting a lot of growers that have purchased unstable genetics. From the same lot of seeds you may see 8 foot tall plants and 3 foot tall plants. Some plants will be fully green, some may be purple. This differentiation is directly due to the instability of the seed's genetics.

The offseason is the perfect time to take this guesswork out of your next harvest. This process will allow you to take your seeds and determine which performed the best. Ultimately, you will know which plant to use as a mother for next season's crops, what your flower to whole plant ratio will be, and most importantly, a better idea of when to harvest. Growing indoors and growing outdoors are two totally different methods. While you'll be able to know the highest potency of the plants you sample, you won't be able to know what the potency would be outdoors.


Andys Blog - Hemp Genetic Selection

  • SEED 1 > GERMINATE SEED > ALLOW 3-4 WEEKS OF GROWTH (LABEL PLANT 1): Now that you've purchased some seeds, let's help you pick a winner. There's no reason to go into a new season not knowing what you're growing. Take 10 seeds and run them through the above process. Clearly label each of the germinating seeds so that you can differentiate them. Allow several weeks of growth. I'm recommending 3-4 weeks but those numbers aren't entirely necessary. You really just need enough growth time in order to take 1-2 clones at the end of the process. If you're trying to get a head start on clones for the season, you might want to wait until the plant is larger. 
  • IDENTIFY SEX IF SEEDS ARE NOT FEMINIZED (3-4 weeks into veg.): At this time, you should be able to determine the sex of the plant. Unless you plan on breeding yourself, you will most likely want to discard any plants showing male sex at this time. If you want male pollen, you can flower one of these plants in a small tent away from the rest of your plants. 
  • TAKE CLONES > LABEL CLONE WITH PARENT ID > ALLOW 3-4 WEEKS OF GROWTH: If you've never taken clones before, it is a good idea to do a little research on different cloning methods. For a small-scale test, the easiest thing to do is use a humidity dome and some clone plugs. If you are ready to invest in the idea of cloning now, a good place to start is an aeroponic cloner. Before you do anything else, you need to make sure your plants are healthy. If they aren't, you should not include them in this test. Deficiencies at this stage can be passed on to your clones and will ruin the experiment. Once you take the clone, you will want to make sure you can track it back to its parent. The idea of this whole experiment is to know what the plant you are keeping in a vegetative state is capable of. Once you know that, you will have identified the hemp genetic you would best move forward with. Keep notes on which plants are growing the fastest and are the earliest with trichomes.


Flowering State

  • SWITCH PLANT TO FLOWER: When growing indoors, you have the ability to force the plant into a flowering state. This is accomplished by adjusting the light cycle to 12 hours of daylight and 12 hours of light deprivation.
  • AFTER 4 WEEKS OF FLOWER, IT'S TIME TO TEST: This is a good baseline to record by kicking off your testing sequence. You would never harvest at this time unless there was an extreme weather situation, but this test will tell you which plants are producing elevated levels of cannabinoids. Be wary of plants at this stage that are already producing THC or THCa. It is important to send in a dry sample or request that the lab performs a cannabinoid potency with a moisture content test. A dry sample is the best option for testing. 
  • AFTER 6 WEEKS OF FLOWER, TEST AGAIN: Testing will tell you how much your cannabinoid potency is increasing over time. We recommend testing every two weeks to track these changes.
  • AFTER 8 WEEKS OF FLOWER, TEST AGAIN: This would be a realistic harvest time depending on your region and weather patterns.
  • AFTER 10 WEEKS OF FLOWER, PERFORM YOUR FINAL PRE-HARVEST TEST: This would be a realistic harvest time depending on your region and weather patterns.


  • HARVEST: When you harvest, you will want to record the plant's total wet weight. You will also want to dry this plant and separate the flower from the leaf and stalk. Once the flower is dry, you will want to weigh it and record which plants had the most weight. You don't need to test all of them, just the plants that weighed the most and had the best-looking flower. Depending on if you're growing for bio-mass or smokable flower, you might have different criteria to determine which are more desirable.

Farmers in this industry are at the mercy of genetics. The worst-case scenario can be devastating. Outright destruction of crops and potential criminal charges are real consequences. If you've had plants that show very different characteristics, you might be dealing with unstable genetics. Hemp cultivation is all about genetic selection and stability. Removing variables is the name of the game. You already have mother nature to deal with. Why complicate the matter?

Important note: The time frames referenced above should be considered as general, flexible suggestions vs. rigid requirements. This is designed to give you a high-level idea of what to expect from your plants. Each plant and grow setup will be slightly different and that affects these timeframes.

Tags: Guides Cultivation

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Andy Hopkins

Andy Hopkins

Andrew is the Director of Operations for Zera Hemp Labs. He makes sure clients feel knowledgeable and comfortable about their lab results being accurate and timely.

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