Who moving others are themselves as stone,Unmovèd, cold, and to temptation slow— (3-4)
We know what you're probably thinking: what could be more unnatural than these weird, powerful people who can manipulate other people while keeping their own emotions under wraps? But nature has some pretty weird stuff in it—like magnets, for example. In these lines, Shakespeare compares the powerful people to a lodestone (an old word for magnet), which has exactly the same ability as the powerful people: it can attract other things to it (or repel them from it) while remaining motionless itself.
They rightly do inherit heaven's graces,And husband nature's riches from expense; (5-6)
In these lines, we learn that the powerful people are actually in charge of preventing "nature's riches" from being wasted ("expense" means "wastage" in this context). So you could say that they are the original environmentalists, but that might be stretching it a bit. But, as the poem as a whole makes clear, this is probably just a metaphor; it's likely that the powerful people themselves are "nature's riches," which means they take care of nature by taking care of themselves, by preventing themselves from being corrupted. So these lines could be thought of as paving the way for the flower imagery in lines 9-12.
The summer's flow'r is to the summer sweet, (9)
Here begins the most extended comparison of the powerful people to nature. Why do you think Shakespeare chose this particular image from nature—the isolated "summer's flow'r." What does the "summer" represent in this context? (This is assuming, of course, that it represents anything, and isn't just being itself—which it could be.)