Ah, flowers: one of the most common symbols in literature. Well, it's no different here, but maybe just a bit creepier. The blood-red rhododendrons and the white azaleas are both cultivated by Rebecca. Both dominate Manderley, at least from Mrs. de Winter's point of view. The rhododendrons are all over the property, and various things owned by Rebecca still hold the scent of azaleas. (We learn that even Rebecca herself always smelled like azaleas).
So, what's red? Let's see: Rudolph's nose, barns, stop signs… oh yeah, and blood. The menacing red rhododendrons definitely make us think of Rebecca's spilled blood and even foreshadow the red flames of Manderley burning at the end of the novel.
The white azalea petals on the ground of the Happy Valley make us think of Rebecca's dead body. She always smelled like azalea, and her presence (in the form of these flowers) is still all over the property. This makes the moment where Maxim rubs a dead azalea petal on Mrs. de Winter's skin decidedly creepy, and it makes us wonder how he really feels about Rebecca and Mrs. de Winter.
Last but not east, the rhododendrons and azaleas might represent wild, uncontrollable nature. They contrast the careful, quiet roses in the rose garden, which are associated with both the current Mrs. de Winter, and another Mrs. de Winter, Maxim's mother. Just one more confirmation that Rebecca and our narrator are very, very different.