Growing varieties that produce high CBD will virtually always yield finished flower that has more than 0.3% total THC – if accurately tested. The gene that is responsible for CBD production in plants, the CBDA synthase, also produces THCA throughout the flowering cycle. So as it makes CBD, it also makes THC – and at a consistent ratio of around 25-28 parts CBD to 1 part THC. This ratio cannot be altered by growing conditions, harvest date, or any other factors.
For example, if you have a finished and dried flower with 28% CBD, you can reasonably expect it to have around 1% total THC. If you have a flower that has 14% CBD, you can expect it to have around 0.5% total THC.
Plants sampled throughout flower show the same ratio, just differing quantities of both CBD and THC. With this in mind, a CBD flowers on a crop wide average, cannot have more than around 7% CBD before it also has more than 0.3% total THC. This is why sampling 30 days before harvest is important as both the CBD and THC numbers raise quickly in the plants final weeks of flower