Lovely poem (it rings a bell, but I can’t put my finger on it)! I wonder if ‘golden hour’ in the poem has a figurative meaning, otherwise it could refer to the second of these Wikipedia definitions:
Medicine: The first sixty minutes after major traumatic injury. This refers to the importance of timely intervention to the chances of saving life and limb.
Photography: The first and last hours of daylight, during which a specific photographic effect is achieved.
I would interpret “golden hour” as “an hour of great happiness” or “a very good hour”.
(It’s an hour of great happiness because the speaker has created something: “I cast to earth a seed”.)
“And muttering discontent/Cursed me and my flower” means “making negative comments about me and my flower”.
The “crown of light” is the flowerhead (cf. “corolla”, which derives from the Latin for “little crown”, and refers collectively to the petals of a flower).
The thieves are only explicable in terms of metaphor. Thus if the poet intends his flower to represent e.g. his own poetic style, the thieves might be other, lesser poets who had copied his style.
They “steal the seed by night”, i.e. “not openly”.