[adriand]

But wouldn’t that be true for all periods in the dataset? You see ups and downs over the centuries, and in each of those centuries I’m sure humans heavily influenced their evolution. Then you see a pronounced upward trend that just happens to also coincide with what we know to be serious, sustained and highly unusual planetary-wide warming.

[timr]

Sure, but it doesn’t matter. Confounding means that there’s an uncontrollable factor in the analysis. You can’t just assume that the uncontrollable factor is the same over all measurements in the dataset.

Just to illustrate the point, the trend you’re describing (which you haven’t correctly described, but I digress) could be due to warming, or it could be due to selection in favor of earlier blooming varieties in the last generation. You can’t know which explanation is true, so you can’t draw conclusions.