Flower form, color and size, along with inflorescence size and fragrance are highly variable traits among roses. These traits in combination have led to the infinite variation in flower appearance and fragrance found across the thousands of rose cultivars.
All roses were originally single, composed of five petals, with the exception of a single four-petaled species. As mutations occurred that replaced stamens and pistils with additional petals, semi-double (6 to 14 petals) and double (15 or more petals) roses appeared. These oddities were saved and propagated by gardeners. Today, a double rose is the norm, although there are also rose cultivars with single and semi-double flowers. Petal counts among rose cultivars range from five into the hundreds.
Flower form is a function of petal count and petal length. Petal length is dependent on a rose cultivars ancestry. The petals of Old Garden Roses are typically short and numerous. The mature flower is usually very double and when fully open is flat or cupped in appearance. The arrangement of petals sometimes gives the flower a quartered appearance, and often there is a button eye in the flower center. Sometimes, as seen among the Centifolias, the outer petals are larger and curve up and around the short, inner petals, resulting in a mature flower with a globular form. Flower buds of Old Garden Roses are not considered very attractive.
Flowers of Hybrid Teas, Floribundas, and other rose classes with China and Tea Roses prevalent in their ancestry have a lower petal count, but their petals are larger and longer. These mature flowers have a much looser appearance than those of the Old Garden Roses. It is the partially open, high-centered buds of the Hybrid Teas and Floribundas that are considered attractive, rather than the mature flowers.
Shrub Roses, more than any other group of roses, are genetically diverse. Old Garden, China, Tea, and Species Roses are all ancestors of Shrub Roses. As a result, flower forms representative of all of these groups can be found among Shrub Rose cultivars.
Colors and Color Patterns
Pink is the predominant flower color among rose species, but there are also red, white, and yellow-flowered species. The color range among Old Garden Roses is limited to white, pink, mauve, maroon, purple, and crimson. Gallicas are the most richly colored of the Old Garden Roses, with many mauve, maroon, purple, and crimson cultivars. It is only in modern roses that yellow, orange, and true red become common.
Rosa Mundi' is a striped or variegated form of the deep pink Gallica 'Apothecary's Rose'. The semi-double, fragrant blooms appear once each year in June.
Photo editing was by Dave Hansen of Communication and Educational Technology Services. Photography was by Dave Hansen, Kathy Zuzek, Marcia Richards, Steve McNamara and Jody Fetzer. Publication design was by Larry Etkin and John Molstad of Communication and Educational Technology Services. Illustrations and artwork were by John Molstad and Jeff Davel. Publication manuscript editing and product management for the Minnesota Agricultural Experiment Station was by Larry Etkin.
Production of this publication was supported with funds from the Minnesota Agricultural Experiment Station.
'Morden Centennial' is a Shrub Rose developed by Agriculture Canada in Morden, Manitoba. It is a modern, hardy Shrub Rose whose high-centered flowers appear in spring and fall. Hip production is abundant.
There is also a wide variety in color pattern among roses. Some cultivars have flowers that are shaded with a subtle blending of two or more colors. Other cultivars have petals that are striped or stippled, or petals whose top and reverse are different colors. It is also common for flower color to change, intensify, or lighten as buds open and expand into mature flowers. Flower diameter among rose cultivars varies from 1 to 6 inches. The trend in rose cultivar development has been away from smaller flower size and towards larger-flowered types, but a shrub covered with masses of small flowers can be as effective in the landscape as a larger-flowered cultivar. Inflorescence size measures the number of blooms in a single cluster on a plant. The flowers of some cultivars are borne singly while other cultivars produce flowers in clusters of several dozen. Roses with large inflorescences have become popular as bedding plants because of their visual impact in the landscape. Of the roses grown in the Arboretums Shrub Rose Garden, the Floribundas and Hybrid Musks are known for their large inflorescence size, while the other classes have clusters of smaller and more variable size.
Rosa 'Alba semi-plena' has luminescent white, semi-double flowers with bright yellow stamens and green styles. It is one of the hardiest Albas at the Minnesota Landscape Arboretum. It blooms once in spring. It also produces a spectacular hip display in the fall.
The fragrances found among rose cultivars vary both in type and intensity. Many of the Species Roses are fragrant; a few are unpleasantly scented. The Old Garden Roses and Hybrid Rugosas are typically very fragrant. During the development of modern roses, fragrance was often diminished or lost, as breeders selected for flower form and color over fragrance.
R. foetida bicolor, commonly called 'Austrian Copper', is a Species rose. The orange petals have a yellow reverse. This rose is very susceptible to blackspot. Without fungicide protection it is a very short-lived rose at the Minnesota Landscape Arboretum.
Comparison of Floral Traits
Floral traits for the cultivars and species evaluated in this study are listed in Table 2. Flower color, size, fragrance, and form were evaluated on mature flowers, using American Rose Society2 color categories. Inflorescence size was also measured.
There are a number of texts with written descriptions and accompanying floral photographs of rose cultivars. In combination, Roses by Roger Phillips and Martyn Rix,3 Roses by Peter Beales,4 and Hardy Roses by Robert Osborne5 cover many of the cultivars evaluated in this study.
2 Modern Roses 10, T. Cairns, ed. American Rose Society, Shreveport, Louisiana. 1993.
3 R. Philips. and M. Rix. Roses. Random House, Inc., New York. 1988.
4 P. Beales. Roses. Henry Holt and Company, Inc., New York.1992.
5 R. Osborne. Hardy Roses. Garden Way Publishing, Pownal, Vermont. 1991.