Why is my verse so barren of new pride,
This poem repeats the theme of Sonnet 38, which examines the issue of the poet's obsession with the Youth as the repeated and sole theme of his poetry.
The poet expresses frustration with his poetry; that it is repetitive and he can't find inspiration. He ponders finding inspiration from other artists. He ends the poem justifying the endless, uninspired, repetition of his love poetry to the endless repetition to the rising and setting sun.
"Noted weed" is usually glossed to mean familiar clothing. The Norton Shakespeare annotates "and keep invention in a noted weed" thus: And keep literary creativity in such familiar clothing. This conforms with the Oxford English Dictionary's definition of Weed, sb2: 1: an article of apparel; a garment, and is consistent with the theme of mending, re-using, etc. ("all my best is dressing old words new").
Although no academics concur, it has been suggested that Shakespeare is referring to the influence of drugs in poetry creation., with the subject phrase "Noted weed" referring to the use of hemp/marijuana, which was common in England at the time. In this interpretation, "Compounds strange" is taken to be a reference to strange chemicals (i.e. drugs), instead of a use of inverted construction, a common poetical device common to Shakespeare. One could argue the poet is thinking he could use drugs to be inspired. He then states he decides not to use such inspiration. (The poet does not "glance aside". Also, he decides to keep the inspirational in the "noted weed" rather than use it.)
This interpretation is based on an assumption about the colloquialism "weed". It was not used in reference to the drug marajuana in the USA until the 1920s, unless one counts the present example as an antecedent.