From everything I just read it sounds like mostly plants getting harvested early. The OP mentioned letting a a "landrace" plant go to 18 weeks. Was that 18 weeks of 12/12 time, or of actual flower time, because they are different. Also what kind of landrace was it, a Sativa? I've personally ran sativas to 14+ weeks of actual flower time which was about 16-17 weeks of 12/12 time, and they still could have used another couple weeks to mature more, but I had to move so I HAD to chop finally.
Otherwise, the longest flowering time I saw mentioned was 12 weeks, and I pose the same question to that person was it 12 weeks of 12/12 or actual flowering time? The OP seemed surprised you had the patients to go 12 weeks, but 12 weeks isn't crazy long for a hybrid that leans sativa dominant. Especially if your only talking 12 weeks of 12/12 time.
There are NOT MANY strains that are
properly matured in 8 weeks of 12/12. There are plenty of Indica dom strains that are fully matured in 8 weeks of actual flower time. Many of the strains on the market these days though are hybrids or even poly-hybrids which means most have some sativa in the mix someplace. That generally means that 9-12 weeks of actual flower time is VERY common to get a properly matured hybrid strain. Then Full sativas, hell easily 12-14+.
Then after you consider proper flowering time frames, you must consider the fact that every environmental misstep can slow the growth and maturation of these plants.
I currently have clones flowering that I"ve been growing for 4 years. This winter I had some extreme cold issues for a couple of weeks. I dealt with it all and got the plants through it. They have grown great, and are going to yield nicely I believe in spite of the issues. However, they are roughly 3 weeks behind in maturation.
Its VERY easy to harvest plants too early, it's WAY harder to over ripen plants then many people think. There is typically a couple weeks worth of time when you can get a perfect harvest, but if you don't wait long enough it won't happen
.