Other methods I have read about to increase trichome density outdoors. From my reading; trichomes are a defense mechanism by the plant to protect it from elements. Minor stressors can increase trich production. Such as cold temps, dry air, and letting the soil dry out before harvest.
I’m sure there are other methods and please note, this is not from some scientific research just claims from other growers. I have all these stressors naturally in my climate...so I can’t say otherwise from 1st hand experience outdoors.
Please also keep in mind. I have seen ditch weed gleaming with trichs in KS...but smoke that and nothing but a nasty head buzz. So trichome density is 2nd to genetics. If I had lots of cash, space, and legally able to grow lots of plants, my method of increasing bud density and thrichome density would be to start a large scale breeding program, imho. I’m super glad others have done this leg work for me to enjoy great cannabis.