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The Importance of Rose Evaluations

August 10, 2020

By Alex Henderson, Curator of Living Collections, Royal Botanical Gardens.

As beautiful as the rose garden is it also provides RBG with several great educational and interpretive opportunities as regards sustainable horticulture. Each rose cultivar in the garden has been selected for superior genetics in relation to cold and drought tolerance but particularly against the most serious global disease of roses called black spot (Diplocarpon rosae). Black spot is a fungal disease that causes black patches on leaves, sometimes with surrounding yellow foliage, causing leaves to drop and with black lesions occasionally appearing on young stems. Different cultivars have various levels of susceptibility with Hybrid Tea roses being most vulnerable. Black spot is a devasting disease most commonly treated with fungicidal chemicals which was not an option being that that RBG’s goal was to design a chemical free rose garden. With this in mind cultivar section based upon the advice of rosarian Peter Kukielski became critical. All the roses in the garden have been selected for superior black spot resistance and are evaluated on a regular basis. This is a great example of the types of horticultural research work that is carried out behind the scenes of a botanic garden that benefits not just the living collections but the environment generally.

Summer Romance Roses In Rose Garden
peach coloured rose

Evaluation Criteria

From May through to October monthly evaluations are carried out which not only assess black spot resistance but also bloom abundance and scent. In this way crucial baseline data can be gathered and then each cultivar given a score that dictates if the rose is retained in the collection or replaced. This creates objective and not subjective data and each rose can be evaluated on its true merits. The scoring system is as follows:

Blackspot Resistance – Scored out of 6

Score of: 6 Healthy foliage – no sign of disease
Score of: 5 10% leaf drop
Score of: 4 25% leaf drop
Score of: 3 50% leaf drop
Score of: 2 75% leaf drop
Score of: 1 90% or above leaf drop

Fragrance – Scored out of a maximum of 1

Score of: 0.7 – 1 Very strong fragrance
Score of: 0.4 – 0.6 Strong fragrance
Score of: 0.2 – 0.3 Mild fragrance
Score of: 0.1 Faint fragrance
Score of: 0 No fragrance

Bloom Abundance and Landscape Value – Scored out of 3

Score of: 0 Not blooming
Score of: 1 A few blooms, good growth habit
Score of: 2 Significant blooms, good growth habit
Score of: 3 Abundance of blooms, good growth habit

Overall Rating – Scored out of 10

Score of: 10 Superior Rose
Score of: 9 Excellent rose
Score of: 8 Very good rose
Score of: 7 Good rose
Score of: 6 Acceptable rose
Score of: 5 5 or below are candidates for replacement
Shrub Rose 'Fiji' in bloom
red roses surrounded by companion plants in the Rose Garden with visitors in the background

With these numerical goals in place any rose that scores five or below should be considered as a candidate for replacement with another rose cultivar known to have superior black spot resistance. Replacement of inferior cultivars has several advantages for the garden in general including:

  • Increasing disease resistance in the garden
  • Creating a more resilient collection
  • Moving the garden further and further away from chemical use
  • Increasing biodiversity value in the garden
  • Preventing chemical build up in soils
  • Improving visitor experience
  • Improving RBG staff conditions and working environment
  • Preventing chemical drift or run off into natural areas
  • Planting new roses so the garden evolves in its design ethic
  • Collectively through education and interpretation lower pesticide use in the province of Ontario and improve the environment in general

Once enough data has been gathered RBG will share this information in the public domain so that rose enthusiasts, members of flower societies, garden clubs, master gardeners, green industry professionals and amateur gardeners are able to make informed rose choices based on considered and proven horticultural trials and performance evaluation data.

Team of volunteers standing in the rose garden

The Team

One of the most important aspects of rose evaluations is that these are carried out by a dedicated team of volunteers (see picture, featuring from left to right Jackie Riddle, Toni Carson, Alex Henderson, Curator of Living Collections and John Durfey). The team of volunteers who have recently been christened the Rose Assessment Team Supreme (RATS for short) gather once a month to carry out the evaluation process. The team normally start at 8.00am and are finished by 3.00pm. It’s a fun activity and frankly who doesn’t want to smell the intoxicating fragrances of each rose cultivar in the garden. It’s also not a bad location in which to socially isolate whilst being involved in a fun activity. If you would be interested in joining the RATS and in creating and improving the environment please contact Lexi Van Harten, RBG’s Volunteer Program Manager at

WATCH: Update on RBG’s New Rose Garden

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