Hardy Shrub Rose Research Trials

Dr. Laura G. Jull, Assistant Professor and Extension Specialist, University of Wisconsin-Madison

Background and Rationale
Roses are among the most popular plants in the United States among amateur and professional gardeners. Traditionally garden roses (floribunda, grandiflora, and hybrid tea) have attracted the majority of market attention, however, the care and attention required to grow them successfully has spawned a demand for rose species that require less intensive management, i.e., hardy shrub roses. The climatic conditions of the upper Midwest can make garden rose culture a challenge. High summer humidity and sub-zero winter temperatures can cause numerous disease problems and lack of winter hardiness. In addition, homeowners and green industry professionals would like roses that have a reduced reliance upon pesticides without sacrificing plant vigor, health, and flowering.

Characteristics of hardy shrub roses such as cold hardiness, repeat flowering, and pest resistance make them attractive choices for modern landscapes, yet not all cultivars exhibit these desirable traits. In a review of 30 common rugosa rose (Rosa rugosa) cultivars, Epping and Hasselkus (1989) found that only 10 of the selections could be recommended for Midwestern landscapes with the remaining cultivars considered inferior due to inadequate pest resistance, lack of cold hardiness, or poor flowering characteristics. Similarly, Hawke (1997) evaluated 51 English and Canadian (Explorer and Parkland Series) roses and found only three of the English and seven Canadian cultivars to be acceptable for the Midwest. With new cultivars making their way to the market each year, independent rose evaluations need to be conducted in order to help Midwestern nurserymen and landscapers increase their profitability by providing high quality, low maintenance roses to consumers. A 3-year hardy shrub rose research trial was conducted in Wisconsin to evaluate 20 cultivars of hardy shrub roses. The roses selected for the study have not been completely evaluated in the Upper Midwest for pest resistance, ornamental value, and cold hardiness.

Materials and Methods
The hardy shrub rose evaluation trials were planted at three locations in Wisconsin (zones 3, 4, and 5) in spring, 2000. The locations include the West Madison Agricultural Research Station, Middleton, WI (cold hardiness zone 4b-5a, clay loam soil); Brown County Extension Office, Green Bay, WI (zone 4b, silt loam soil); and Spooner Agricultural Research Station, Spooner, WI (zone 3b, sandy loam soil). The three locations were chosen to represent three hardiness zones, each with its own representative soil type. There were nine replications (plants) of each of the 20 cultivars located at each location for a total of 180 roses at each research station/office. However, due to a number of factors including vole and deer damage, winter kill, and heavy disease pressure, a number of the original 180 plants at each location died. These rose plants were not replaced in the trial, as hardiness and vigor are part of the three-year evaluation trials’ data collection. Three years worth of data is considered a sufficient amount of time to successfully evaluate hardy shrub roses, considering yearly temperature fluctuations and amount of insect and disease pressure.

Nineteen of the twenty shrub rose cultivars were obtained mainly as a donation from three large scale rose growers in the U.S. (Bailey Nurseries, St. Paul, MN; Weeks Roses, Upland, CA; and Star Roses, West Grove, PA). The dwarf shrub rose ‘Scrivluv’ (Baby Love™) was generously donated by Schroeder’s Flowers, Green Bay, WI. These shrub roses cultivars were selected because most were either new cultivars just introduced to the market, have been on the market for less than three years or are underused in the landscape, or the roses are still in evaluation by the nurseries in hopes for future release. Two cultivars ‘Bucbi’ (Carefree Beauty™) and ‘Meipotal’ (Carefree Delight™) are used as “industry standards” in the trial to compare the other rose cultivars. These two cultivars are commonly used by the landscape industry in the Midwest. The roses used in the trial are #1 to 1 1/2 grade, bareroot roses, each on their own root systems (nonbudded). There were some obvious differences between various roses within a cultivar, however, the roses were randomly selected at each research location to ensure proper experimental randomization. The cultivars evaluated in this study for three years include:

  1. ‘Robusta’: single, medium-red flowers, 6’ tall, everblooming, robust, thorny stems
  2. Carefree Beauty™ (‘Bucbi’): Buck rose with semi-double, coral pink flowers, 3’ tall, everblooming
  3. Carefree Delight™ (‘Meipotal’): single, pink flowers, 2.5-5’ tall, everblooming
  4. ‘Wisconsin Cheese’: new, experimental rose from Bailey’s, not on the market, light yellow, fragrant flowers, long stems, recurrent
  5. ‘Serendipity’: Buck rose with double, orange blend flowers, 4’ tall, everblooming
  6. ‘Marie-Victorin’: new Explorer series rose with double, pale peach flowers fading to pink, recurrent
  7. Livin Easy™ (‘Harwelcome’): floribunda rose with double, apricot/orange flowers, 3’ tall
  8. Magic Meidiland® (‘Meibonrib’): semi-double, medium pink flowers, 2’ tall, everblooming
  9. Kaleidoscope™ (‘JACbow’): shrublet with double, tan to mauve blend flowers, 3’ tall, recurrent
  10. Cambridge™ (‘Poulrust’): Town and Country Series with double, lavender flowers, 2’ tall, everblooming
  11. Madison™ (‘Poulrijk’ ): Town and Country Series with double, pink flowers, 2’ tall, everblooming
  12. ‘Paloma Blanca’: Buck rose with double, white fragrant flowers, 3’ tall, everblooming
  13. Flower Girl™ (‘Fryyeoman’): small, soft pink flowers in large clusters, recurrent
  14. Red Fairy™ (‘Morredfar’): polyantha rose with small, cherry-red flowers, recurrent
  15. Betty Boop™ (‘WEKplapic’): floribunda, semi-double, ivory flowers w/ bright red edges, everblooming
  16. Mix ‘n’ Match™ (‘CHEwily’): semi-double, pastel pink flowers, reddish new growth, recurrent
  17. Baby Love™ ‘(Scrivluv’): shrublet with single, bright yellow, slightly fragrant flowers, recurrent
  18. Knockout™ (‘RADrazz’): single, deep cerise-red flowers, 3’ tall, everblooming
  19. Fire Meidiland™ (‘Meipsidue’): double, deep red flowers in large terminal clusters, everblooming
  20. Mystic Meidiland™ (‘Meialate’): single, light copper with yellow flowers fading to pink, everblooming
*Everblooming: continuous, or repeat flowering habit, rarely not in bloom
*Recurrent: flowers produced in succeeding cycles throughout a flowering season, bloom, rest, then rebloom at a later date

The roses were received bareroot during the second week in March 2000. The roses were potted into 2 gal., black plastic containers using a soilless mix (5 pine bark : 1 peat : 1 sand) and a slow release fertilizer, Osmocote 16-16-16 was applied to the top of the media after the roses were potted. Plants were potted up at McKay’s Nursery (Waterloo, WI) and placed into miniature poly houses with no heat (temperatures maintained at around 40-45° F to enhance root development). The plants were watered as needed and allowed to break bud slowly. The plants were then transported to the three locations for planting during early to late May 2000 (date is location dependent). Roses were planted into the soil at each location and shredded bark mulch was applied at a 3” thickness to the entire soil surface to control weeds and retain moisture. Hand weeding was performed weekly at all three locations throughout the growing season.

To allow for proper evaluation for insect and disease resistance, no insecticides or fungicides were applied during the duration of the experiment. A 3-month, 14-14-14, slow release fertilizer was top dressed around each rose plant yearly in June. Watering occurred at planting time and after fertilizer application. Plants received 1” of additional irrigation if rain had not occurred during a 10-20 day period, depending on location. During the first and second year of the trails, removal of spent flowers occurred weekly until July at which time rose hip production initiated. During the remaining growing seasons, deadheading of flowers occurred once in June. The ability of each cultivar to shed its spent flowers was evaluated. In addition, the absence of deadheading during the final seasons resulted in only a slight increase in colorful hip production, particularly in the northernmost location of Spooner, WI.

Results and Discussion
Evaluation of the roses occurs monthly at all three locations throughout the growing season and into late fall (May-Nov.) and continued for 3 years. Qualities evaluated included: height and width of the plant, overall plant habit, foliage color, especially the new growth, flowers (color, amount, size, single or double, fragrance, and duration), hips (production, size and color), insect injury (rose sawfly, leaf cutter bee, aphids, Japanese beetle, stem girdler, spider mites, etc.), disease susceptibility (blackspot, anthracnose, powdery mildew, botrytis, canker, etc.), and winter hardiness (amount of dieback that occurred in winter). Deer fencing was installed at the Spooner location but not at the West Madison or Brown County sites as historically, deer pressure at these latter two locations are negligible. However, deer pressure, and especially vole injury, was eventually considered a problem at the Brown County location despite its urban surroundings.

No winter protection was used on the roses at any of the three locations in order to provide successful analysis of cultivar hardiness. During hardiness evaluation in early May, removal of dead wood was completed each year. After the first year, roses that completely died to the ground were left planted to allow for potential regrowth from their root system. Even if the tops of plants died back to the ground, the resulting new growth from the root system is the same cultivar as none of these roses were budded onto a rootstock. Rose plants that completely died back were allowed to remain in the trial until July to see if regrowth occurred from the base of the plant. If regrowth did not occur, these plants were not replaced in the trial, as hardiness and vigor are part of the three-year evaluation trials’ data collection. However, Knockout™ (‘RADrazz’) plants were purchased and replanted in each location for analysis due to vole and winter injury. Only six plants of this cultivar were replanted at West Madison and Green Bay and only three were replanted at Spooner (less winter injury). Knockout™ (‘RADrazz’) was replanted at these locations to see if winter hardiness results are the same for each year. These rose plants were not used in the final evaluation as they were not part of the original plantings, but they were evaluated separately for flowers, pests and hardiness.

First year winter hardiness data proved a number of these cultivars are not particularly hardy to Wisconsin unless winter protected. For the first year’s results, many of the Knockout™ (‘RADrazz’), Baby Love™ ‘(Scrivluv’), Livin Easy™ (‘Harwelcome’), ‘Wisconsin Cheese’, Betty Boop™ (‘WEKplapic’), and Carefree Beauty™ (‘Bucbi’) were killed to the ground at the West Madison and Green Bay locations. Surprisingly, winter survival was best at the far northern location of Spooner with only a few plants from the above cultivars dying, except for Knockout™ (‘RADrazz’), which suffered death of six out of the nine plants used at this location. The hardiness results at Spooner were greater than for some of the same cultivars at more southern locations, perhaps due to complete snow cover for most of the winter in Spooner. In addition, Fall, 2000 had unusually warm temperatures in Green Bay and Madison. This prolonged the warm fall season in the central and southern parts of Wisconsin, which may have affected the winter hardening of many of the rose cultivars. The above listed cultivars, in particular, Knockout™ (‘RADrazz’), were still flowering in November. In contrast, plants in Spooner finished flowering in early October and experienced gradual decreases in fall temperatures, which lead to sufficient hardening off of the plants. December of that year came with a vengeance all over the state with very cold temperatures, which killed unhardened tissue in the roses, particularly in Madison and Green Bay. However, high snowfall led to protection of voles from predators in the winter, particularly at the Green Bay location, which suffered tremendous vole injury and death of 30% of the rose plants. Vole baits were then installed at this location. Two more years worth of winter hardiness data showed similar results as the first year’s hardiness data. Unseasonably high winter temperatures, fluctuating temperatures, and low snowfall amounts provided interesting results in 2002 and 2003.

In terms of pests, the most common insect damage seen at each location during the first two years was rose sawfly (rose slug) and leaf cutter bee. Neither insect causes death of the plant but severe enough damage can significantly reduce a rose’s ornamental value. These insects commonly attack the leaves with chewing of the foliage and eventual defoliation of severely infected leaves. Other pests seen at the three locations include: aphids, gypsy moth (Green Bay only), forest tent caterpillar (Spooner only), tussock moth, and false Japanese beetle otherwise known as spring rose beetle. There was no preference to a specific cultivar for any of the insects except for aphids, which seemed to prefer the larger flowers of ‘Robusta’ and ‘Wisconsin Cheese’. ‘Marie-Victorin’ was the most severely infected rose for rose sawfly, forest tent caterpillar and other chewing insects’ activity.

Diseases of the roses mainly appeared as black spot and anthracnose on the leaves. Both of these diseases can be hard to distinguish from each other. Black spot appears as black, roundish to obtuse spots on the leaves sometimes with a yellow halo surrounding the black spots. Lesions (spots) often have feathery edges. Leaf anthracnose can also have similar symptoms as black spot, however, the spots often do not have the feathery margin, instead a more distinct edge to the spot is apparent. Both diseases can cause severe defoliation. Blackspot and anthracnose were severe (> 60% of plant infected) during the first two summers on ‘Robusta’, Red Fairy™ (‘Morredfar’), Kaleidoscope™ (‘JACbow’), Cambridge™ (‘Poulrust’), and Madison™ (‘Poulrijk’).

Flower amount and production varied between cultivar but was fairly consistent among locations. The cultivars that were continuously in flower for most of the growing season include: Carefree Delight™ (‘Meipotal’), Cambridge™ (‘Poulrust’), Madison™ (‘Poulrijk’), Knockout™ (‘RADrazz’), Fire Meidiland™ (‘Meipsidue’), Mystic Meidiland™ (‘Meialate’), and Red Fairy™ (‘Morredfar’). However, many of the above cultivars are not recommended due to their high disease susceptibility (Cambridge™ (‘Poulrust’), Madison™ (‘Poulrijk’), Red Fairy™ (‘Morredfar’), and ‘Robusta’). Unfortunately, the roses, which produced the best flower display with high pest resistance often, were not winter hardy. Hip production occurred on many of the cultivars, however, only Carefree Beauty™, 'Marie-Victorin', and Mystic Meidiland™ produced colorful orange-red hips. Hip production in the northern location of Spooner was poor as many of the cultivars, including Carefree Beauty™, ‘Marie-Victorin’, and Mystic Meidiland™ only produced green hips and did not have sufficient enough time to change colors to orange or red before a severe frost occurred thereby turning the hips black.

Literature Cited Epping, J.E. and E. R. Hasselkus. 1989. Spotlight on Shrub Roses: Rosa rugosa tops a shrubby list of Midwestern landscape performers. American Nurseryman 170(2):27-39.

Hawke, R.G. 1997. Plant Evaluation Notes: An Evaluation Report of Shrub Roses. Chicago Botanic Garden Plant Evaluation Notes, Issue 11, 6 pgs., published by Chicago Horticultural Society