Paul Zindel's play contains a powerful central metaphor in the title and to what it refers. The experiment Tillie conducts to determine if radiation creates mutations in marigolds symbolizes the effect that negative energy and dysfunction has on her own family. The impact of negative emotions like anger, regret, and cruelty strongly affect the family unit.
The relationship between Beatrice and her daughter Ruth is particularly problematic. The outgoing Ruth shows some promise of being socially accepted—unlike Tillie, whose shyness prevents her from engaging with people. Beatrice thinks of herself as attractive and outgoing, but people in her community mock her behind her back and suggest she is mentally ill. This creates a problem with Ruth seeing her mother as a role model. Like Betty, Ruth is very aware of other peoples' opinions of her and is vulnerable to them.
An example is that people have been known to call Beatrice "Betty the Loon," and this remark is overheard by Ruth at school. Ruth repeats it to insult her mother when the two are arguing over the science fair. Ruth's illness makes her dependent on others, though. This causes her to feel resentment, and she may be worrying she'll turn out like her mother, as her condition makes her vulnerable to taunting like Betty was. Some adjectives to describe their relationship might include problematic, toxic, complicated, dysfunctional, dependent, antagonistic, intense, and volatile.