A few cultivars of roses included in the Knock Out series produce vibrant magenta to fuchsia flowers. In bright sunlight, the petal color almost sears your eyes it's so bold and piercing. Because the Knock Out rose's blooms display such a bold color, companion plants need to complement the rose in both color and texture. Whatever plants you choose, make sure they also tolerate the same sunny conditions and moist, well-drained soil already present around the rose bushes.
Three foliage colors act as the unifying tone across a garden: green, blue and silver/gray. With the Knock Out rose bushes displaying dark green leaves and fuchsia flowers, nearby plants with large or attractive leaves help visually contrast and tone down the bold flower color. The foliage acts as a resting point for your eyes. The foliage recedes visually and doesn't glare and, thereby, accentuates the bold, colorful aspect of the rose flowers. Artemisia, dwarf blue spruce, upright stonecrop, creeping juniper, iris, lavender, rosemary and lamb's ears are plants with blue to silvery gray-green foliage. Any green shrub or perennial would work next to the roses, but choose large-leaved types to contrast the rose foliage texture, such as elephant ears. A dark purple leaf also would look nice, such as from a Loropetalum, Japanese barberry or bugleweed.
Just like with foliage color, nearby plants with contrasting flower colors can make a dynamic display next to Knock Out roses. White flowers go with all other flower colors, including fuchsia. Any dark blue or deep purple flower also looks stunning. Avoid other pink or red flowers near fuchsia. Since fuchsia is a combination of red, violet and pink hues, a true red or pink flower next to it could look strange. Definitely avoid orange flowers and use yellow with caution -- pale yellow flowers near fuchsia would look less jarring that a bold, canary yellow. Conard Pyle, a wholesale grower of Knock Out roses displays garden photos on their website. Among the nice flower companions shown include blue salvia, veronica, white phlox, baby's breath, Shasta daisies and hardy hibiscus.
Keeping in mind good color combinations with fuchsia, another rose may look good next to Knock Out roses. Rather than a rose bush that matures the same size, perhaps plant a lower growing carpet rose with contrasting white or pale pink blossoms. The low, sprawling rose helps hide the bottom branches of the Knock Our rose and complement it with additional rose flowers.
When designing a garden, you need to think dimensionally through time. While the Knock Out rose looks terrific when in bloom from late spring to fall, it becomes deciduous in winter. Consider a companion plant that will look interesting or flower when the rose is dormant. Dwarf evergreen shrubs, hollies, daffodils and ornamental grass clumps provide texture or seasonal flower and seed interest from fall to early spring.