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White Knock Out®

Rosa 'Radwhite'

Rosa ‘White Knock Out’® with first bloom flush in May 2019 in Davis. Photo: SK Reid.
Rosa ‘White Knock Out’® with first bloom flush in May 2019 in Davis. Photo: SK Reid.

Summary

White Knock Out® is a shrub rose bearing simple, white flowers with contrasting yellow stamens and an impressively long and consistent blooming habit. At the UC Davis spring open house, White Knock Out® was one of the highest rated plants, achieving mean overall appearance ratings on all treatments with scores of 4.4 to 4.6 out of 5 (Table 19b). At the summer South Coast REC open house, a similar feat was achieved with two plants ranking in the top 3 of the highest mean overall appearance ratings for their treatment (Table 19d). White Knock Out® flowers fade to tan and they can persist a bit long on the plant before self-cleaning. At UC Davis, this tendency negatively impacted scoring by both open house participants and researchers, but not negatively enough to keep it from receiving our Blue Ribbon award for outstanding low-water performers (Table 19a). At UC Davis, White Knock Out® bloomed heavily in May and June and then continued to be a strong bloomer until the end of the treatment period in October. As with other roses in our trials, White Knock Out® was not as floriferous at South Coast REC as in Davis, though there were constantly some blooms on the plants from April through October (Table 19c).

At UC Davis no difference in relative growth rates between the treatments was observed (Figure 13b). In Irvine the relative growth was noticeably and significantly greater on the high treatment than either the moderate or low treatments, though the moderate treatment had outgrown this difference by September (Figure 13d). That said, none of the health or aesthetic ratings were significantly different between the treatments except for vigor, and large size alone is not necessarily preferable. Therefore, we recommend irrigating White Knock Out® on low water in WUCOLS zones 2 and 3.

Basic Info

Submitted by: Star Roses & Plants
Trial Exposure: Sun
Year evaluated: 2019
Height & Width
(after 2 years):
46" x 26" - UC Davis
44" x 26" - SCREC
Reported Height & Width
(at maturity):
3.5' x 3.5'
WUCOLS plant type: S
Water Needs & WUCOLS Region:
Low - Region 2 
Low - Region 3
Mean Overall 
Appearance rating:
(1-5 Scale, 5 is highest)
4.3 - UC Davis 
3.1 - SCREC
Flowering Months
April-October - UC Davis
April-October - SCREC

Growth and Quality Data

Click Here for Complete Data Set

Rosa ‘White Knock Out’® in Sept. 2019 in Davis showing its tendency to hold onto its old blooms while pushing out new pure white ones. Photo: SK Reid.
Rosa ‘White Knock Out’® in Sept. 2019 in Davis showing its tendency to hold onto its old blooms while pushing out new pure white ones. Photo: SK Reid.