Weeds
I'd like to weigh in on the weeds/plant growth question.
Weeds ARE in the eye of the beholder, as I found out from the farm my family has, as well as from a stay in Kansas in my early teens. A crop is a uniform, cultivated field of plants that are meant to be harvested by sentient beings for their purposes. ANY plant not part of that cultivation plan is, at some level, a weed to THAT farmer because it uses resources that he would rather have going to his crop.
So, as my alfalfa farming teacher in Kansas pointed out, the marijuana that grew alongside it symbiotically was a weed.
So, a stalk of corn in a vinyard would be a weed, as would the grapevine in the cornfield.
As for things that many people consider to be weeds outright...Dandelion salads can be quite tasty, as can tea made from that plant. And Kudzu, scourge of the South, has recently been discovered to be appetizing to sheep and goats. If that doesn't detrimentally affect their flavor, we may see an increase in both of those creatures on the menu.
Like the man said- weeds produce seeds, flowers, leaves, etc. They may even produce fruit, like the nightshade plant (a family, which, BTW includes potatoes and tomatoes)- but it may not be of use to THAT person.
So, by a strict reading, the plant growth spell would increase the productivity of both crop and weed.
However, since the first thing a farmer does is remove unwanted plants from his field, and repeats the action on a regular basis, a crop would be
relatively weed free. So, if the farmer & his hands had JUST performed a good weeding before the hired spellcaster laid some carefully targeted spells like plant growth, etc., he would get a much higher level of food production from his crop- which is exactly what a smart farmer would do.
While there may still be weeds present even after the labors of the farmer & his crew, the tradeoff would probably be worth it.
On the other hand, ticking off a spellcaster might also get your crops devoured by creeping doom or a giant locust.
Or turned into a Shambling Mound.
Or just fireballed.
Probable High-Tech vs Magical food production ratio- 1:1
(Besides, if I properly recall, earlier interpretations (1st and 2nd Ed) of plant growth allowed the spell to be used to increase the HD or heal things like Shambling Mounds and other sentient plants.)