JMZ, I don't know that anyone is trying to insult you or imply that you are stupid. You've asked a question and people are just trying to figure it out. Sure, tending the garden might seem like a no-brainer, but that doesn't mean that everyone knows that it can have an impact on plant quality as well as, say, harvest quantity.
I read a post over the weeked (might have been an old post, I sometimes dig back when I'm bored) from someone who couldn't figure out why they were not getting some prepare meal options for things the sim definately knew the recipe for. Turns out it they were missing the no-cook meals, and were clicking on the stove to make them rather than the 'fridge. Missing this little detail didn't make them stupid, it was just an oversight. And if you are asking for help with an issue, I hope that you don't get too upset if someone askes about the little oversights people can make.
For what it's worth, I think that getting a plant upgrade from a seed or fruit without the Super Green Thumb reward or trait is just random luck. Since you say this is on a sime that does have that, I honestly don't have a clue.