Earlier quoted context omitted.
It disappoints me to see this kind of article so uncritically received here. This is on par with, say, a "scientific" article claiming that the lack of a changing trend in the date of the Canadian wheat crop harvest is indisputable evidence that climate change does not exist. As well as the possible influences you list there are many more - changes in local water or air chemical content, changes in cultivation techni…
It's easier to see nuance in an area you're an expert in.
In this case though it's not nuance but blatant gaps and bad logic. In this case it is because it's easy to shut down your rational thinking when confronted with something that seems to affirm your beliefs.
An equivalent article that used the flowering date of a different plant to as "indisputable evidence" for the non-existence of global warming would have been rightly torn to shreds on here.