In The Effect of Gamma Rays on Man-in-the-Moon Marigolds, how does the explanation of the given extract help you understand what Ruth is going through?
In horticulture, dwarfing can be used manually (as with trimming bonsai trees) but it also can be done genetically. This is common with orchards because it makes for easier caring and picking the fruit. Stresses are induced to constrain the plants in order to shrink their normal growth.
Ruth receives the same metaphoric, mental and physical, stresses and this stunts her emotional growth. The mental stresses have everything to do with her mother's absurdly selfish and hostile treatment of Ruth and Tillie. The physical stresses, which also result in emotional stunting, are caused by Ruth's mother literally keeping her in the house and physically guiding where she can and can't go. And all indications are that Ruth is out of high school.
The fact that Ruth has seizures suggests that her epilepsy is, if not caused by, triggered by stressful events in her family life, such as finding the dead rabbit. When epilepsy is uncontrolled, it can result in further psychological damage (further emotional stunting). Yelling, "Stop it, Ruth!" is hardly a caring way of dealing with her disease. The nematodes, viruses and disease are, in a way, cultivated by her mother's behaviour, rather than controlled or cured.