We care about rhododendron leaves, not about their blossoms, so this isn't relevant to the argument.

Unfortunately for them, we don’t care about the people holding the thermometer. We’re only interested in whether rhododendron leaves and crocus blossoms can tell us about the thermometer's accuracy.

It doesn’t matter how these plants grew, because the argument requires them to already be present. We care about a specific instance of using the plants’ biological thermometers to test a mechanical thermometer.

Since the air temperature surrounding the two plants is likely to differ, we can’t use the plants’ reactions to temperature to gauge if the thermometer reading is right. The plants might be reacting to different temperatures. This weakens the argument.

We don’t know what a “moderate” temperature is. 0°C? 20°C? Besides, the accuracy of the thermometer has no bearing on the argument. The conclusion is all using plants to test the accuracy of a thermometer.