I believe most anything and most everything Greenman says about growing but I think there is some confusion in regards to using the term potency.
Potency and high THC levels do not always go 100% hand in hand. Durban Poison is considered to be a potent pot and is from an equatorial to semi-tropical region but it has a lower percentage/level of THC compared to many strains that are not considered to be as potent.
The right combination of THC, THCV, CBN, CBD, CBC and CBL create potency.
A landrance sativa from an equatorial region is going to have different percentages of THC, THCV, CBN, CBD, CBC and CBL that different types of marijuana.
THC (Tetrahydrocannabinol) gets a user high, a larger THC content will produce a stronger high. Without THC you don't get high.
CBD (Cannabidiol) increases some of the effects of THC and decreases other effects of THC. High levels of THC and low levels of CBD contribute to a strong, clear headed, more energetic high.
Cannabis that has a high level of both THC and CBD will produce a strong head-stone that feels almost dreamlike. Cannabis that has low levels of THC and high levels of CBD produces more of a buzz or stoned feeling. The mind feels dull and the body feels tired.
CBN (Cannabinol) is produced as THC ages and breaks down, this process is known as oxidization. High levels of CBN tend to make the user feel messed up rather than high.
CBN levels can be kept to a minimum by storing cannabis products in a dark, cool, airtight environment. Marijuana should be dry prior to storage, and may have to be dried again after being stored somewhere that is humid.
THCV (Tetrahydrocannabivarin) is found primarily in strains of African and Asian cannabis. THCV increases the speed and intensity of THC effects, but also causes the high to end sooner. Weed that smells strong (prior to smoking) might indicate a high level of THCV.
CBC (Cannabichromene) is probably not psychoactive in pure form but is thought to interact with THC to enhance the high.
CBL (Cannabicyclol) is a degradative product like CBN. Light converts CBC to CBL.
If you are a grower, you can experiment with different strains of cannabis to produce the various qualities you seek.
A medical user looking for something with sleep inducing properties might want to produce a crop that has high levels of CBD.
Another user looking for a more energetic stone will want to grow a strain that has high levels of THC and low levels of CBD.
In general, Cannabis sativa has lower levels of CBD and higher levels of THC. Cannabis indica has higher amounts of CBD and lower amounts of THC than sativa.
So all it takes is altering the percentages of the different active ingredients of marijuana and something with a lower level of THC can have a seemingly more potent effect on someone and other times it will work in an opposite manner.
You can take several different strains with 21% THC content and depending on what the percentages of the other active ingredients in the marijuana is one can stand head and shoulders above the rest and one will lag far behind in their effects because certain active ingredients work against others and others work with and increase the effects of others.
That is why a strain like Durban Poison that came from an equatorial to semi-tropical region can have a THC content, depending on whose strain you pick it can run from 5% to 15%, can give you a much more amazing high than something with a lot of indica in it that has a 21% THC content.
It is not higher in THC content but the percentages of the different active ingredients on the marijuana work to give you a more soaring more impressive more amazing head high so to the smoker they say, wow this stuff is POTENT even though it is not as high in levels of THC as other stains that they will just say, this stuff is pretty good. And it is that sort of soaring more impressive more amazing clear cerebral head high that landrace sativas from equatorial and semi-tropical regions gives you.