Introduction & History

Building on the unprecedented success of the original Knock Out, William Radler answered the most frequent consumer request — more petals — with the introduction of 'Double Knock Out' in 2004. Registered as 'RADtko,' this sport or seedling selection offered the same legendary disease resistance and self-cleaning habit as its predecessor but with fully double, 18-25 petal blooms in a rich, classic red. Where the original's single flowers charmed with breezy informality, Double Knock Out delivered a more traditional rose aesthetic that appealed to gardeners who wanted the carefree performance of a Knock Out but with flowers that looked like what they imagined a rose should be. The result was another blockbuster: Double Knock Out quickly rivaled the original in sales and became a fixture in landscapes from coast to coast, proving that Radler's breeding breakthrough was not a one-time accident but a replicable formula for excellence.

Breeding History

'Double Knock Out' was bred by William Radler and introduced by Star Roses and Plants in 2004 under the cultivar name 'RADtko.' It was developed from the same breeding program that produced the original Knock Out, sharing its foundational genetics of disease resistance derived from rigorous unsprayed selection. The specific parentage involves crosses within Radler's advanced breeding lines, all developed under his strict no-spray protocol. It received the AARS (All-America Rose Selections) award in 2004, making it one of the few roses to win the award in its introduction year. The double flower form addressed the single most common consumer critique of the original — that its single blooms didn't look 'rosy' enough. Commercially, it matched the original's trajectory, selling tens of millions of plants within its first decade.

Flower Form & Fragrance

'Double Knock Out' produces fully double blooms with 18-25 petals, opening from rounded buds into cupped, rosette-form flowers approximately 6-8 cm across. The color is a rich, classic red — slightly deeper and more saturated than the original Knock Out's cherry tones, with less of the pink undertone. In cool weather, blooms take on darker, more crimson tones; in extreme heat, slight lightening may occur. Flowers appear in clusters of 3-12, creating dense masses of color. The fragrance is mild, with subtle spicy-sweet notes more pronounced in morning hours. Like its parent, Double Knock Out is completely self-cleaning, with spent petals dropping neatly to reveal the next wave of buds.

Garden Design & Uses

'Double Knock Out' is the quintessential landscape rose, equally at home in residential foundation plantings, commercial property borders, and municipal median strips. Its fuller blooms give it a slightly more formal presence than the original Knock Out, making it suitable for more manicured landscape settings. Plant in masses of 5-20 for sweeping color displays. Combine with white-flowered shrubs like Knock Out's own 'White Knock Out' sibling or with dwarf butterfly bush for pollinator-friendly borders. It makes an excellent low hedge at 90-120 cm spacing. Use as anchoring shrubs at entry points, along walkways, or to frame garden focal points like fountains and sculpture.

Growth Habit & Hardiness

'Double Knock Out' forms a rounded, dense shrub reaching 90-120 cm tall with a matching spread of 90-120 cm. Growth habit is nearly identical to the original Knock Out — compact, bushy, and naturally symmetrical without corrective pruning. Vigor is strong, with annual new growth of 30-50 cm. Foliage is dark green with a slight blue undertone, turning rich burgundy-purple in autumn. Leaves have 5-7 serrated leaflets and exhibit the same excellent disease resistance as the original. Thorn density is moderate. The plant is own-root, establishing rapidly and forming a dense, well-branched framework. Canes are sturdy and self-supporting under the weight of double blooms.

Climate & Hardiness

'Double Knock Out' matches the original's broad adaptability, rated for USDA zones 5-10. Cold hardiness extends to approximately -26°C (-15°F), with reliable performance across most of the continental United States. Heat tolerance is outstanding, thriving through humid Southern summers and dry Southwestern heat alike. The rose handles urban pollution, reflected heat from pavement, and poor air circulation better than most roses. Rain tolerance is slightly less than the single-flowered original, as the doubled petals can occasionally trap moisture in prolonged wet weather, though this is rarely a significant issue.

Seasonal Care Calendar

Planting & Container Growing

Plant in full sun (6+ hours daily) in well-drained soil. Tolerates a wide range of soil types from sandy loam to clay, though amended beds produce the best growth. Preferred pH is 6.0-6.5. Space plants 90-120 cm apart for a continuous hedge effect, or 120-150 cm for individual specimens. Dig the planting hole twice the width of the root ball and set at the same depth as the nursery pot. Apply 5-8 cm of organic mulch. Water deeply at planting and maintain regular moisture during the first season of establishment.

Container Growing

'Double Knock Out' adapts well to container culture, maintaining its compact habit and continuous bloom in a large pot. Use a minimum 40-litre container with excellent drainage. Fill with a premium potting mix amended with compost and perlite. Water consistently, checking daily in summer heat, as double-flowered plants in containers are heavy drinkers. Feed every 3-4 weeks with a balanced liquid rose fertilizer. In zones 5-6, insulate containers for winter or move to an unheated garage. The doubled blooms make container specimens particularly showy on patios and decks. Feasibility rating: 4/5.

Seasonal Care & Pruning

Spring: Prune back by one-third to one-half in late winter before bud break, removing dead, diseased, or crossing canes. Apply balanced granular rose fertilizer and 5-8 cm of fresh mulch. Summer: Water deeply once weekly during dry periods. No deadheading required. Apply a mid-season feeding in late June. Inspect regularly for Rose Rosette Disease symptoms. Autumn: Cease fertilizing by mid-August to allow hardening off. Enjoy the autumn foliage color transition to burgundy. Do not prune. Winter: In zones 5-6, mound 8-10 cm of mulch over the root zone after the ground freezes. No other protection needed.

Common Mistakes to Avoid

1. Planting too close together, restricting air circulation and creating conditions that could promote disease in an otherwise resistant rose. 2. Severe annual pruning that removes all old wood, reducing early-season bloom on established canes. 3. Applying heavy fertilizer regimens designed for hybrid teas, which is unnecessary and can promote excessive soft growth. 4. Failing to recognize Rose Rosette Disease symptoms (witches' broom, excessive thorniness, distorted growth) and not removing infected plants promptly. 5. Assuming deer resistance — deer do browse Knock Out roses when hungry.

Propagation

'Double Knock Out' was protected by US Plant Patent PP16202 (now expired). Propagation by softwood cuttings in early summer is straightforward: take 12-15 cm cuttings of semi-ripe wood, remove lower leaves, apply rooting hormone, and root in a moist perlite/peat mix under a humidity dome. Rooting occurs within 4-6 weeks with good success rates. Hardwood cuttings taken in late autumn are also viable. The variety grows vigorously on its own roots and does not benefit from grafting. Tissue culture is used for commercial production. Difficulty rating: Easy.

Diseases & Pests

'Double Knock Out' inherits the outstanding disease resistance of the Knock Out breeding program. Black spot resistance is excellent, matching or exceeding most landscape roses on the market. Powdery mildew is rarely an issue. Cercospora leaf spot may appear in humid conditions but is typically cosmetic. The primary disease threat is Rose Rosette Disease (RRD), which has devastated monoculture Knock Out plantings across the eastern and central United States since the 2010s. Infected plants show distorted, reddish growth, excessive thorniness, and witches' broom; they must be removed and destroyed immediately. Aphids, Japanese beetles, and spider mites are minor pests managed through standard methods.

Cut Flowers & Exhibition

The double blooms of 'Double Knock Out' make more satisfying cut flowers than the single original. Cut stems in early morning when blooms are half to three-quarters open. Select stems with 2-4 open blooms and several buds. Strip lower leaves, recut stems under water, and condition in warm water with preservative. The rich red doubles look excellent in low bowls and informal arrangements. Vase life is 4-6 days. Combine with garden foliage and white or yellow companion flowers for cheerful mixed bouquets.

Collector & Exhibition Notes

'Double Knock Out' has collector significance as the sequel to the most commercially important rose of the 21st century. For exhibition, it competes in shrub and landscape rose classes, where well-groomed sprays of fully double blooms on clean foliage score well. Show preparation involves selecting sprays at peak form, removing any damaged petals, and ensuring foliage is clean and healthy. While ubiquitous in the nursery trade, it is valued by rose historians as a key cultivar in the story of the landscape rose revolution. Its AARS 2004 award adds to its exhibition credentials.

Comparison with Similar Roses

Compared to the original Knock Out, Double Knock Out has more petals (18-25 vs. 5-7), a slightly deeper red color, and a more traditional rose appearance, but shares identical disease resistance and care requirements. Against 'Drift' series roses, Double Knock Out is taller and more upright, better suited as a hedge or foundation shrub. Versus 'Flower Carpet Scarlet,' it offers superior disease resistance and a more refined habit. Compared to 'Home Run' (another Radler rose), Double Knock Out has fuller blooms but a virtually identical growth habit and landscape performance.

Heritage & Conservation

As part of the Knock Out family, 'Double Knock Out' represents the modern era of performance-bred landscape roses. Its legacy lies not in rarity but in impact: it helped normalize the expectation that roses should be tough, self-sufficient landscape plants. The variety is commercially abundant and faces no conservation threat. However, the widespread monoculture planting of Knock Out varieties has raised important ecological discussions about genetic diversity in landscape plantings and the vulnerability of uniform populations to novel diseases like Rose Rosette.

Cultural Significance

'Double Knock Out' cemented the Knock Out brand as the dominant force in American landscape roses. Where the original broke the mold, the Double proved the concept was scalable and refinable. It addressed consumer desire for a more traditional rose aesthetic without sacrificing the low-maintenance ethos that made the original revolutionary. Together, the Knock Out family became the most recognized rose brand in America, appearing in millions of landscapes and fundamentally reshaping the nursery industry's approach to rose marketing and breeding priorities.

Frequently Asked Questions