Growing Radcor Rainbow Knockout Roses

Perhaps no other perennial flower has captivated the world as much as the rose. These plants have been the subject of countless cultivars over the years.

From the crimson red rose representing the Dark Tower and romance to the black rose that symbolizes death, the rose has a language more varied in human culture than any other plant.

Blooms of the Rainbow Knock Out RosePin

But there’s one other thing that shrub roses are known for, and that’s how difficult they can be to grow.

Since the first patent in 2000, William Radler’s Knockout Rose family of hybrids has gained steady attention.

As of 2021, a total of 11 different Knock Out Rose varieties exist:

  • ‘Meibenbino’ (Petite Knock Out Rose) – a bright red dwarf
  • ‘Radcon’ (Pink Knock Out Rose) – bright pink flowers
  • ‘Radcor’ (Rainbow Knock Out Rose) – coral with yellow center
  • ‘Radgor’ (Peachy Knock Out Rose) – shell pink with yellow center
  • ‘Radral’ (Coral Knock Out Rose) – brick red fading to coral
  • ‘Radrazz’ (Knock Out Rose) – the original, red to pink
  • ‘Radsunny’ (Sunny Knock Out Rose) – yellow that fades to cream
  • ‘Radtko’ (Double Knock Out Rose) – red double-flowers
  • ‘Radwhite’ (White Knock Out Rose) – white blooms
  • ‘Radyod’ (Blushing Knock Out Rose) – light pink

One of these, 2007’s Rosa ‘Radcor,’ has become the go-to substitute for more traditional bicolor hybrids such as the peace rose (Rosa ‘Madame A. Meilland’).

More commonly known as the Rainbow Knock Out (or Knockout) Rose, the cultivar name ‘Radcor’ is short for Radler and coral, the flower’s signature margin color.

It won the prestigious 2007 All American Rose Selection award and continues to delight growers worldwide.

‘Radcor’ Care

Size & Growth

As with its siblings, ‘Radcor’ is a fast grower, forming into a rounded adult bush between 3 and 4′ feet in size.

It shares the classic rose foliage of serrated leaves, starting burgundy and fading to a dark, mossy green.

The leaves and woody green to brown stems contrast beautifully with Radcor’s blooms.

Flowering and Fragrance

Speaking of blooms, the flowers of this rose cultivar appear from late spring into fall.

The 2″ inch single blooms feature 7 petals that transition from uniquely colored, coral-pink blooms to a bright orange center.

They lack the typical heady rose scent, but their blooms attract pollinators, notably butterflies.

The resulting rose hips are an attractive orange-red color.

Light & Temperature

Rainbow Knockouts are one of the hardier varieties and enjoy full sun to partial shade.

Unlike many roses, the sunlight won’t cause their blooms to fade, although the plant may prefer a bit of noonday shade in the hotter climates.

It’s hardy enough that blooms can form with as little as 3 hours of full sun per day.

It’s not overly picky about humidity and will readily grow in USDA hardiness zones 4 to 11.

Watering and Feeding

As with many roses, ‘Radcor’ is a heavy drinker.

Drip irrigation is generally best. However, if you choose to water by hand, your plant may need almost daily watering when it’s young.

Mature plants only need watering weekly to every two weeks during the growing season, depending on how hot and dry the weather is.

Note that this plant isn’t known for root rot (when the soil is well-draining). It scorches easily if you leave water on the leaves.

Therefore, it’s best to water in the morning and avoid overhead watering or splashing (rain is the natural exception).

A balanced, granular fertilizer works well for this sport. Make one application in the spring and again in summer, following the labeled instructions.

More on Fertilizing Knockout Roses.

Soil Type & Transplanting

Well-drained, loamy soils are key to a happy and healthy rainbow Knockout.

A good mix of compost and perlite will work wonders, although you can also use a commercial medium such as MiracleGro for roses.

Mulching is important, especially in the summer.

This keeps the roots cool, reduces moisture loss, and helps insulate the plant in winter.

Even more importantly, organic mulches help add to the soil.

And speaking of organic mulches, used coffee grounds (mixed with an equal amount of shredded coffee filters, leaves, or other organic matter) can work wonders on these roses.

While coffee won’t improve the acidity or feed the plant directly, it can provide food for nitrate-loving microorganisms and worms, so your fertilizer is more effective.

If you must transplant your Rainbow Knock Out Rose, you will need to dig a hole at least 1′ foot out from the foliage to safely get the entire root mass.

However, avoid transplanting unless necessary, as all roses tend to be sensitive to transplant shock.

Grooming And Maintenance

Prune your rose in late winter while it’s dormant, removing any dead or damaged branches and shaping it as desired.

Clear any organic waste from around the plant (such as dead leaves) to reduce the risk of attracting infestations.

How To Propagate Rainbow Knockout Rose

As with all Knock Out roses, ‘Radcor’ is patented, meaning it’s illegal to cultivate the plant without a license by the patent holder.

Knock Out Rose’ Radcor’ Pests or Diseases

Mildly drought-tolerant and frost tolerant, ‘Radcor’ has excellent disease resistance (but not immune) to many common rose diseases.

It has exceptional resistance to black spot, common rusts, and mildews, and a lower risk of root rot than many other plants if in the right soil.

Aphids, mealybugs, plant scale, and spider mites are all known to attack Knock Outs, although a regular neem soil soak regimen will help prevent this.

As with all roses, this plant is non-toxic to humans and pets, and the hips are consumable, but it does bear thorns.

Rosa’ Radcor’ Uses

Radcor is ideal for:

  • Hedges and borders, ‘Radcor’ providing beauty while attracting butterflies.
  • Perfect for entryways.
  • Usable to fill partial shade areas.

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