In Hawai‘i, Verbascum thapsus L. exhibits high rates of fasciation, which could have ecological and evolutionary consequences for spread of this noxious weed. Fasciated plants produce more seed capsules on average; however, the cause of fasciation in V. thapsus is not known. This study investigated whether fasciation in V. thapsus has a simple genetic basis, or whether it is caused by physical damage or pathogenic bacteria. Plants derived from self-pollinated fasciated and normal plants were grown in a field common garden and subjected to mechanical damage (simulated herbivory) and natural herbivory. Bacteria cultured from normal and fasciated plants were compared, and field plants were inoculated with a slurry of fasciated tissue. In the common garden, 31% of plants developed fasciation, but fasciation did not follow a simple monogenic pattern of inheritance. Artificial damage substantially reduced fasciation rates; damaged plants were between 1.3 and 32 times less likely to become fasciated, compared with undamaged plants. Bacterial isolates were similar between normal and fasciated plants and no inoculated plants developed fasciation, suggesting that bacteria do not cause fasciation. Fasciated and normal plants often grow less than 1 m apart, indicating that climatic factors are not inducers of fasciation. Localized combinations of environmental conditions in Hawai‘i may promote frequent and persistent fasciation.