Anthesis: the zero point flowering time is counted from
In short
In botany, anthesis is the opening of the flower, the start of the flowering phase proper. In cannabis trials it is usually recorded once half the plants in a plot show their first distinguishable female flowers. It is an event observed on the plant, not an instruction given by the grower.
Why it is not the same as the flip
In cultivation, flowering time is almost always counted from the moment the lighting is switched, the so-called flip. Between that intervention and actual anthesis lie one to three weeks, depending on cultivar and conditions. For day-neutral cultivars the gap works out differently again, since no switch is needed at all.
Both ways of counting are reasonable, they simply cannot be converted into one another. Comparing a figure given in weeks of flowering with one given in weeks after anthesis, without checking the zero point, means comparing two different time axes.
Where the difference actually bites
At harvest. One field study places the maximum of total cannabigerol in the seventh week after anthesis, while another source reports the cannabigerolic acid maximum around the fifth week of flowering. Much of the apparent contradiction dissolves once one notices that two different zero points are being counted.
Relevance to cannabis
For practice this means week figures from studies transfer to a grower’s own calendar only when the paper states what the counting started from. Without that, the number remains useful as an orientation but not as a date. What follows from this for cannabinoid development is covered in the article on cannabigerol.