Kathy Zuzek, UMN Extension Educator
The number of rose cultivars in the world defies logic. If you open a copy of Modern Roses 12, the most recent edition of the American Rose Society's rose cultivar list, you will find thousands of rose cultivars or varieties listed along with each rose's class, year of release, the breeder who developed the cultivar, parentage, and descriptions of each cultivar's floral and foliage traits, plant habit and thorns.
There are 18-20 roses native to the United States. Four of these (R. acicularis, R. arkansana, R. blanda, and R.woodsii) are native to Minnesota. All 4 of these species are single-flowered, pink, and bloom in spring (Photos 1 & 2).
Given the thousands of rose cultivars in the world, you might think that many of the rose species were used in developing all of these cultivars. With few exceptions, only 8 species are ancestors to our rose cultivars and all 8 are from Asia.
As rose cultivars are developed, they are placed into classes. There are 36 classes of roses. Classes that Minnesota gardeners might be familiar with are the Hybrid Teas, the Shrub Roses, or the Hybrid Rugosas. Every rose cultivar is placed in a class with other roses who share common ancestors and/or similar floral, foliage, or plant habit traits.
Which classes and which cultivars can be grown in Minnesota? That depends on a gardener's taste in rose appearance, the choice of how much time he or she wants to devote to maintaining their roses to insure good performance and long term survivability, and a willingness or reluctance to spray pesticides. Some of the biggest factors that impact these decisions are choosing between repeat-blooming and spring-blooming cultivars, cold hardiness and pest tolerance of individual rose cultivars, and the pH of your soil.
What is important in Minnesota is a plant's ability to re-grow vigorously during the following growing season after experiencing some winter injury. Because repeat-blooming roses produce flowers on the current year's wood, a repeat-blooming plant that grows vigorously in spring and summer after some winter injury can perform beautifully in spite of our harsh winters. There are also entire classes of roses that are not hardy in Minnesota. Unless a gardener is 1) willing to consider a rose as an annual plant or 2) tip and bury roses or provide some other measure of winter protection, these roses should not be grown in Minnesota.
Roses are hosts to many pests. The pests that most impact rose survival in Minnesota are blackspot, the rose stem borer and a wasp that causes mossy rose gall.
Blackspot is a fungus that results in defoliation of rose plants across all classes (Photo 5). Roses without leaves cannot photosynthesize to produce and store the energy reserves that a plant lives on. Many roses that defoliate from blackspot in early or mid-summer also try to produce a new second set of leaves in late summer. This depletes the energy resources of the plant even more. Plants that are susceptible to blackspot are severely weakened by repeated rounds of defoliation and have little ability to survive our harsh winters. Plant size and vigor is diminished with each year of blackspot incidence until finally the rose is too weak to survive over winter. Along with blackspot's impact on winter survivability, few gardeners are willing to tolerate a defoliated rose in their garden. This leaves two options: planting blackspot-tolerant roses or repeated fungicidal sprays during the growing season. Some classes of roses have a higher percentage of blackspot-tolerant cultivars than other classes.
Roses prefer a soil pH of 6.0 to 7.0. As pH increases above these levels, iron chlorosis becomes a problem and can lead to plant stress, low vigor or mortality. Cultivars within the Hybrid Rugosa class are particularly susceptible to iron chlorosis on high pH soils (Photo 8).
Coming August 1 in Rose Classes and their Performance in Minnesota: Part 2: Descriptions of some common rose classes planted in Minnesota gardens and a look at some attractive, low maintenance cultivars within classes.