There are many such lists are available, but here’s a common list:
January, Carnation
February, Violet
March, Daffodil
April, Sweet Pea
May, Lily of the Valley
June, Rose
July, Larkspur (Delphinium)
August, Gladiolus
September, Aster
October, Calendula
November, Chrysanthemum
December, Narcissus