There are lots of flowers all over the text of Uncle Tom’s Cabin, from the "scarlet bignonia and a native multiflora rose" that grow over the front of the cabin itself (4.1), to the "rare bouquets" that Tom buys for Eva (22.60), to the "cape jessamines" and "wreath of roses" that Eva uses to dress Tom (16.92). It’s a flowery novel, both in terms of the prose and content. But the most important flowers are definitely the "brilliant scarlet geranium, and one single white japonica" that Topsy picks and brings to Eva on her deathbed (26.13).
Why are Topsy’s flowers so important? Well, first of all, because bringing them is the first thing Topsy ever does for someone else out of kindness. Of course, she’s picked them out of the St. Clare garden against the rules, but this is still the first sign that Topsy is trying to reform and return Eva’s love. It’s not insignificant that the flowers are red and white – a contrast of suffering and purity, perhaps, that evokes Topsy’s inner state. She has the purity of a child, but she’s been distorted by the beatings and physical punishments that she endured with her previous master. The contrasting colors also vaguely suggest the contrasting skin colors of Topsy and Eva themselves.
You might also notice that Topsy has arranged the flowers very carefully; the bouquet "was tied up with an evident eye to the contrast of color, and the arrangement of every leaf had been carefully studied" (26.13). This suggests that Topsy has an innate artistic and even moral sense; once she’s been given even the slightest suggestion of affection, her abilities can blossom. (Flower pun intended.) At this moment, the reader knows that Topsy is capable of complete reform, so it’s no surprise that she becomes a pious Christian missionary in the end.