The joke in the teaser is humorous. The one in the tag falls flat. And it's because we're getting two jokes at once. The joke is that Jerri thinks she is doing something good for Arbor Day, but her efforts are a failure (Audience: "That's not where you plant a tree!"). OR the joke is that Jerri is doing something good for Arbor Day, but then she takes it back once the holiday is over (Audience: "But you're supposed to let the tree grow!"). But the joke is not both of these.
A joke can't be funny for two reasons. It just can't. The trick to a good joke is that it's simple. Expectation, then subversion. If a joke has two punchlines, the result is confusion. And while a confused character can be funny, a confused audience is no laughing matter.
Compare to this type of scene: As two characters have a conversation in the foreground, something wild and unexpected is happening behind them. The joke is that the characters don't notice the crazy antics in the background. But if the dialogue in the foreground is also supposed to be funny, then nothing in the scene gets a laugh. The jokes choke each other out.
This is an old rule of comedy, but I just realized that Aeschylus uses it to his advantage in Agamemnon, of all things. Queen Clytemnestra speaks to the chorus after she (spoiler alert) kills Agamemnon. She gives a lot of good excuses for why she did it. She did it because Agamemnon killed their daughter, Iphigenia. She did it because Agamemnon brought a concubine home from the war. She did it because of the curse on the House of Atreus.
Just as a joke can only be funny for one reason, you can only kill someone for one reason. But the error here is Clytemnestra's, not Aeschylus's. If Clytemnestra had protested with any one of these excuses, the audience might sympathize with her. But the combination of reasons results in no reason at all. She seems unreliable and the audience doesn't trust her, just as Aeschylus intends it.
Be on the look-out for when one-plus-one equals zero.
