THC % in a plant is measured by how many % of the total cannabinoids in the plant = THC . This means a plant with very little trichomes can still have 25% thc while not being that potent since its doesnt have a good trichome production. my gues is that a 20% thc with good trichome production will be stronger. find a strong strain, then find a good phenotype
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Also THC is not everything. it depends on what kind of high you want. Do some studying and then some growing + selection and you shall find your special one
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Good luck