This is a poem with many references to rural England. Not unusual for a Londoner like Shakespeare - there were fields of sheep on the Southbank not far from the Globe Theatre, but he was also brought up in the countryside of Stratford. It is a mournful poem - full of warnings for the 'beautiful young boy' he was so enamoured of. The boy seemed to be keeping bad company - possibly drinking and 'wenching' - nothing changes much with youthful vigour. But there is also some evidence that the boy may have been involved in political plotting. Whatever it was the older Shakespeare seems to have taken exception to it.

When I consider everything that grows
Holds in perfection but a little moment,

The idea of these two lines is that everything that grows is perfect for just one little moment. It is either unformed and growing - or declining and dying. Rather a depressing view but factually true nevertheless.

That this huge stage presents nothing but shows

The huge stage of London or England or even the whole planet, he says, is nothing put plays - or vanities - made up conflicts for the amusement of crowds. It is a nice play on the word 'show' meaning a performance and also 'showing-off'. A reflection of the speech "All the world's a stage".

Whereon the stars in secret influence comment;

We look into the sky above London and see the myriad stars like a celestial audience commenting - even influencing the action below. Many in Shakespeare's time believe that the stars could affect life on earth.

When I perceive that men as plants increase,
Cheered and checked even by the self-same sky,

Across that huge stage men and women multiply by the million - just like plants - the same weather being kind and cruel to them all.

Vaunt in their youthful sap, at height decrease,

This is a sort of repeat of the first two lines - the idea of the rise and fall of human fortune.

And wear their brave state out of memory;

A staggeringly beautiful line which is at first opaque but then obvious. A group of old men once virile and courageous now reduced to husks of their former selves unable to remember who they are or what they did - their memories worn out with years of use.

Then the conceit of this inconstant stay

A complex line about life, or 'stay on earth', which is described as 'inconstant' - many ups and downs, starts and stops, happy and sad. The whole thing being a 'conceit' or made-up vanity - rather like the third line - "That this huge stage presents nothing but shows".

Sets you most rich in youth before my sight,

Now we see, or rather see the poet's view of, the boy. He is beautiful, full of promise and at the point of perfection mentioned twice before.

Where wasteful Time debates with Decay,

Time is something that seems to have haunted Shakespeare in the Sonnets - the idea of getting old and useless - losing youth. Here he personifies both Time and Decay saying that they both intend to destroy the "lovely boy" but cannot make up their minds which it will be. Time would take longer but Decay would be there at the end in any case.

To change your day of youth to sullied night;

"Night" here might mean the darkness of death - black being no colour at all - nothingness. Or could mean the blindness of old age.

And all in war with Time for love of you,

Shakespeare believes that he is at war with Time in order to save the boy from destruction - his own love versus Time's erosion.

As he takes from you, I engraft you new.

This is a difficult piece of imagery to the casual reader but it at first fits nicely into the plant images mentioned before. What is being said is that as the sonnet is being written it is preserving the boy's beauty like grafting on a young cutting to an old stock. A strange way of thinking but he believed that other sonnets too would, in some way, enable the boy to live forever.