Factual error: Gallaxhar scans the galaxy to find the missing quantonium. The scanner zooms in on earth, showing it to be very close to the center of the galaxy. Earth is actually nowhere near the center of the galaxy; it's located much closer to the outer edge.
Factual error: The TV station in Modesto, CA had call letters starting with a W. All broadcaster call letters west of the Mississippi start with a K.
Chosen answer: Well, it's a fiction movie; some suspension of disbelief is always needed. There could be a number of explanations: for instance, lifeforms such as insects on the grass, bacteria inside them or even grass itself, being unable to absorb quantonium. It might affect only sentient beings. Or Susan might even be the first being to get in contact with the substance - this would be possible depending on the shape of the meteorite container, the exact means by which quantonium touches her body - which isn't shown... but, first and foremost, this happens because its not happening would leave the filmmakers with a plot issue to be dealt with - so, other beings don't grow simply because they don't.