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.
The tulips in the garden symbolize the handmaids because they are red, like the gowns the handmaids are forced to wear.
Like the tulips Serena Joy grows, the handmaids have no individuality. They are simply part of a mass of handmaids, just as each tulip is part of a mass of tulips in the garden.
The handmaids are even stripped of their names. Instead, they are given the name of the man with whom they are forced to have intercourse in the hopes that they will bear another child. For instance, if a handmaiden is put in the household of Fred, her name becomes Offred. If she is then transferred to another household, she takes on the name of that patriarch, becoming, for instance Ofjohn. She simply functions as an appendage of her master, of little more value as an individual than a flower.
Like Serena Joy's tulips, the handmaids exist merely for the convenience and pleasure of others. They are disposable and expendable. Like the tulips, they are merely objects.
No other women in Offred and Serena Joy's society can get pregnant (with the possible exception of the Econowives, but they are so little discussed in the book that they hardly seem worth mentioning here) aside from the handmaids. Serena Joy seems to cultivate her flowers out of some sense of appreciation for their beauty, and perhaps in order to grow something, since she cannot grow a child.
However, these red tulips are also, whether she is conscious of it or not, symbolic of the handmaids' ability to grow and bloom in a way that few other women in the community, if any, can. The handmaids—clothed in a red that matches the tulips' hue—are thought of as receptacles for their commanders' seeds, which, if they are lucky, will grow into a baby. One of their prescribed phrases is actually "May the Lord open," which parallels how the tulips open.
Everything in regards to the Handmaid is relative to red. In the book The Handmaid's Tale the world as people know it has changed. Inside the country women are nothing and men are in control. The fanatics have revised life so that people live in status roles. Infertility is rampant and the objective is to try and reproduce children into the households of people of authority.
The handmaids are a select group of women who are imprisoned by the society to serve as pregnancy vessels. They are forced to wear red as their attire. The color red symbolizes life's blood. Therefore, the red tulip symbolizes the life blood which nourishes the infant as well.