Why won't Miss Maudie "permit a geranium on her premises in To Kill a Mockingbird?
Context: Describing the Ewell Property
“One corner of the yard, though, bewildered Maycomb. Against the fence, in a line, were six chipped-enamel slop jars holding brilliant red geraniums, cared for as tenderly as if they had belonged to Miss Maudie Atkinson, had Miss Maudie deigned to permit a geranium on her premises. People said they were Mayella Ewell’s.”
The answer to your question is never specifically answered, and Miss Maudie does not actually comment about geraniums anywhere in the story. The statement by Scout, "had Miss Maudie deigned to permit a geranium on her premises," probably refers to Maudie's passionate obsession with azaleas, and that she has no time for other plant species. We do know that Maudie has some "house plants" and that she grows scuppernong grapes, but her garden seems to consist primarily of azaleas, of which she is immensely proud. After her house burns down, she looks forward to building a smaller house so she will have more room for her azaleas.
"I'll have the finest yard in Alabama. Those Bellingraths'll look plain puny when I get started!" (Chapter 7)
Scout could also have been comparing Miss Maudie's grand garden and tidy home with the Ewell house, a shack built next to the town dump. Although Scout can see that Mayella "tenderly" cares for her "brilliant red geraniums"--the only spot of beauty on the Ewell property--it could be that Scout simply figured that Maudie would not be interested in growing a plant grown by the lowly Ewells. It could also be that Scout was aware of the geranium's
... distinctive mechanism for seed dispersal. This consists of a beak-like column which springs open when ripe and casts the seeds some distance. (Wikipedia, Geranium)
Such a plant could quickly spread, and Scout may have figured that Maudie would consider the geranium more like the dreaded nut grass:
If she found a blade of nut in her yard, it was like the Second Battle of the Marne. She swooped down on it... (Chapter 5)