These two plants are considered to be scrub or weeds. A properly kept Southern yard in town would not have "weeds" of this nature in it. The Radley place has fallen into a type of decline or neglect.
During this time period a Southern yard in town was swept...no grass only soft dirt that was literally swept with a broom each day...under the shade of liveoak trees or magnolia trees was considered a proper yard. Short grass lawns were not quite as fashionable during the 1930s and 1940s due labor saving devices not having been invented.
The symbolism is that of neglect. Details are important in this story.
You may have seen Johnson grass. It's tall about 3 feet high and has a big tassel on top sort of like corn. This is the grass that you may see farmers stick in their mouths on television commercials. It is sweet to the taste. Many chemicals have been invented to get rid of Johnson grass. It is a very tenacious weed.
Rabbit tobacco is a type of weed in the lobelia family. Youngsters would chew this or even smoke it during the 1930s or 1940s to pretend to be "big" like the grown-ups. It did not get one high.