Certified GKH Gardening Expert
According to the ASPCA, grape hyacinths are considered non-toxic to both cats and dogs. If your cat is vomiting after eating them, it could be that too much was ingested and simply caused a belly ache. Also, cats (and dogs) commonly eat grass and sometimes other plants in order to induce vomiting when they do not feel well. If you are concerned about your cat and unsure whether it is sick, a call or visit to the vet may be necessary.