I haven't played the game, but I'll try to interpret the comic. I'd really hate to be left out.

1st screen:
I'm gonna go out on a limb and assume that the gray chaps with missing teeth are the zombies from the comic's title. They are NOT the plants, although you can say that a zombie is like a plant. You know, like someone who's in a coma. Brain dead, so to speak. It's not scientifically accurate, but it'll have to do for now. We're in a hurry here, we don't have time to read walls of text. They often turn out to be just big piles of nonsense, don't they ?

Plants are green, at least that's how we usually think about them. Unless we're emo and dead plants pop into mind. Zombies have decaying derma that lost it's color, but I'll get to that later. It is obvious that one of these zombies has invited the others to his (it's?) birthday party, as people or in this case the living dead, usually do. If I were to point out the birthday zombie, I'd probably choose the one on the right. He says "thanks, you guys are great !" Now that, along with the beautifully wrapped box in his hands, implies that he received a present from his guests. You can actually see them to his right (our left), as they anxiously await the package's opening. There's probably a very special item inside ;) We'll find out soon enough, I think.

2nd screen:
BANG ! Something unexpected and disturbing. But what is it ? Confusion and terror. Will it last ? Is the danger real, or is it imagined ? Is our zombie suffering from multiple personality disorder ? Is he both the hero and the villain ? Or has the mysterious object triggered memories of a deeply buried childhood trauma ? Perhaps I'm over-analyzing and it's just a plain old bomb. We don't know. Yet. Try as we might, we cannot find the answer in this screen. We see the shock, we see the fear, there's even... but wait ! Green ! The object is green.

By now, you've no doubt made the connection between "green" and "plants". But why in God's name would a zombie be so afraid of a plant, as unusual of an appearance it may have ? Even if the plant is prickly, would it matter to a zombie ? A zombie's derma ought to be devoid of nerve cells. But what am I talking about here ? A zombie's body is lacking in other important departments and their number isn't exactly negligible, amirite ? (Ha-ha)

3rd screen:
Now this is where the comic becomes funny for those of you who've played the game. Me, I'm not sure what to think. It looks like the dangerous plant has been refitted to act as a hairdryer. Or was it the other way around ? Did they paint a hairdryer green, to make the birthday zombie think it was a plant ? Neither of these theories explains his reaction in screen nr.2. Why is the zombie terrified of plants ? Is he revolted by the freakish thought of a hairdryer that blows spores into his hair ? Would tulips grow out of his skull ? Ugh !

Now I have another, completely different explanation for all of this, but it might be a little far fetched: It wasn't the plant that scared the zombie, it was the white stuff in the box. Some of you might say that it's just styrofoam, no big deal. And I would agree. But what if it ain't styrofoam ? What if it's... fudge ?! Fudge in a package ! Fudge... that has been packed. See what I'm getting at ? Pretty fucked up, isn't it ?
The only loose end here is the plant/hairdryer. I can't find it's meaning. What does an item like that have to do with fudge packing ? Any ideas ?