How are the red tulips symbolic of the handmaids?

The red tulips that Serena Joy is trying to grow in her
garden represent life and fertility -- the key theme of the novel.   Serena Joy is a
wife, but she can't bear her own child.  Because she can't "grow" her own children she
tends her garden.  The red of the tulips is certainly suggestive of blood and life. 
What is notable about her garden though is that even that is failing.  Serena works
diligently, but the garden never flourishes, just as her womb can't flourish.  The
flowers themselves look like receptacles, and that is another connection of them as
symbols of the women, specifically the handmaids that are the "wombs" of the society
-- bearing the children for the wives and the commanders.  Serena Joy is subjected to
the "ceremony" in a similar, and yet very different way from Offred.  Everything in her
life depends on Offred's success in getting pregnant, and Serena Joy has little to no
control over that, so she takes control of her garden and does what she can there, event
though it appears that the garden will fail as well.