How to Plant, Grow and Care For Knockout Roses
Thinking of adding some knockout roses to your garden this season? These flowering shrubs are a popular option for gardens across many different hardiness zones. In this article, gardening expert Danielle Sherwood takes you through everything you need to know about knockout roses and their care.
Roses are undeniably gorgeous and have beautified gardens for millennia. However, by the 1980s, they started to fall out of fashion due to their fussy reputation. Enter Knock Outs, the roses that changed the industry with disease resistance and ease of maintenance never seen before.
These beautiful roses have a different growth profile than more traditional roses, and are extremely versatile. You can use them as a privacy hedge, or just an accent shrub for a perennial garden. The options are endless for these fantastic plants.
If you’re new to roses or simply want guaranteed, easy blooms, Knock Outs might be the perfect roses for you. Follow along to see the varieties available, and how to grow them in your garden.
Knockout Roses Overview
Plant Type Perennial
Native Area none
Hardiness Zone 4-11
Exposure Full sun to part shade
Plant Spacing 3-4 feet
Planting Depth 16-24 inches
Height 18”-7 feet
Watering Requirements Deep, infrequent
Pests Aphids, Japanese Beetles, Mites
Diseases Black Spot, Powdery Mildew, Rust
Soil Type Well-drained, loamy
Soil pH (6) neutral
Plant With Nepeta, Lavender, Coneflower, Sage
Attracts Bees, Butterflies, Birds
History & Cultivation
Knock Out Ⓡ roses got their start in the basement of hobby hybridizer William Radler in the 1970s. Radler loved roses but disliked that the industry relied on intensive chemical spray regimens to keep them healthy.
He spent 15 years working to develop seedlings that might reduce environmental impact and bring gardeners back to roses, focusing on hardiness, disease resistance, ease of care, and consistent bloom.
By 1989, after extensive research and trials in his own Wisconsin garden, Radler achieved success. By prioritizing health over beauty, he produced a durable, hardy rose that bloomed all season, with no need for pampering and chemical sprays like the hybrid teas popular at the time.
In the year 2000, Star Ⓡ Roses and Plants, a wholesale nursery, began selling Radler’s low-maintenance roses, branding them as Knock Out Ⓡ Roses. They became a best-seller. Since then, they have come a long way from the original, bright pink single petaled flower in Radler’s garden.
Today’s gardeners can choose from a whole family of Knock Out Ⓡ roses, in a variety of colors and forms. Radler and the Star Roses team continue working to bring more roses with the easy-care qualities of this species to the market.
All varieties in the Knock Out Ⓡ Rose family have been tested to ensure their success in a variety of climates and conditions. Most grow about 3-4 ft. tall (but can grow much bigger in ideal conditions!) and have a bushy, rounded shape to the shrub. Average bloom size is 3-3.5” in diameter.
The original member of the Knock Out Ⓡ rose family, this rose has single, open flowers of nearly neon cherry pink. Its bloompower and hardiness earned it a place in the World Federation of Rose Society’s Hall of Fame in 2018.
It also earned the Earth Kind Ⓡ designation for being a reliable landscaping plant with limited need for fertilizers and pesticides. The original ‘Knock Out’ does well in zones 4-9, and is drought-tolerant once established.
‘Double Knock Out’
If you love the look of a classic red rose, ‘Double Knock Out’ gets you closer. This raspberry-red version of the original has somewhat full, double blooms.
An American Rose Trials for Sustainability winner in 2019, ‘Double Knock Out’ is winter hardy and has a light, cinnamon scent.
‘Pink Knock Out’
This rose blooms in bright fuschia-pink with single petaled flowers. It blooms in flushes from spring through frost. Landscapers love these roses for their ability to make an impact from a distance, especially in mass plantings.
‘Rainbow Knock Out’
‘Rainbow’ is named for its bright coral pink blooms and yellow centers. The shrub has a nice, bushy habit. Many gardeners warn that the blooms fade fast in the sun, so choose a more vivid colored member of the Knock Out Ⓡ family if you’re after bright shades in the garden.
Rainbow’s average height is 3-4 ft. tall, but it tends to stay more compact than the other Knock Out Ⓡ roses.
‘Coral Knock Out’
Also sold as ‘Carefree Celebration’, this rose has a striking bloom with shades of orange, coral, and pink. The bloom shape is slightly cupped and semi-double to double.
‘Coral Knock Out’ tolerates some shade quite well, and even makes a pretty cut flower. ‘Coral’ won the American Rose Trials for Sustainability in 2021.
‘White Knock Out’
‘White Knock Out’ offers a nice contrast between dark green foliage and crisp white rose petals. The blooms are mostly single, and occasionally semi-double. Like other Knock Outs, this rose is highly floriferous. It usually grows to about 3.5- 4ft. tall and wide.
‘White Knock Out’ was a Blue-Ribbon winner three years in a row in the Water Use Classification of Landscape Species Trials, making it a good choice for gardeners looking for a drought-tolerant rose.
Planting Knock Out Ⓡ roses is the same as any other rose. You will want to get them off to a good start so they can get established and grow into strong, mature plants.
You can purchase Knock Outs as container-planted or bare root roses (shipped as just the defoliated plant and its roots, no soil or container) from local nurseries or online suppliers.
All roses are woody perennial plants, meaning they will bloom, go dormant, and continue growing the following year.
Six Key Parts of a Rose
|The root ball is the mass of roots found right below the stem of the rose. It collects and stores nutrients from surrounding soil.|
|Knock Out Ⓡ roses are grown on their own roots. The bud union refers to the spot just below where the canes flare out from the base of the stem.|
|Canes are the stems growing from the base of your rose that develop the leaves and buds of the rose.|
|Roses have leaflets (primarily in sets of 3-5, but up to 11 in some cases!) that grow from the stems.|
|These are small fleshy nubs on the canes where new growth emerges. Bud eyes appear at the nodes where leaflets attach to the stems.|
The best time to plant your new Knock Out Ⓡ rose depends on your growing zone. To find yours, take a look at the USDA Hardiness Zone Map. The best temperature for planting ranges from 40-60℉, but Knock Outs can also be planted in the higher temperatures of summer if you give them ample watering.
Roses do best when planted in early spring or fall. When planting in spring, wait until all danger of frost has passed. In the fall, plant 6-8 weeks before your first frost to allow your new roses to get established before they go dormant for winter.
The most important step in successful planting of your new rose is to pick a good planting location. You want a site that gets 6-8 hours of direct sunlight daily.
Knock Out Ⓡ roses are known to be somewhat shade tolerant, so you can get away with dappled shade, as long as they get a minimum of 4 hours of direct sunlight per day. Pick a location about 3-4 feet away from other large plants and away from the root competition of trees.
If you have intense, hot summers, a location with morning sun and some afternoon shade is ideal to prevent scorched leaves and blooms. This is particularly important with the light colored roses like ‘Sunny Knock Out’ and ‘White Knock Out’ as the blooms tend to look brown when scorched.
Bare Root vs. Container Grown
You have 2 main options when purchasing your Knock Out Ⓡ rose: bare root or container-grown. Both can thrive in your garden. Container-grown roses are easier for novice gardeners as there’s no additional prep work needed before planting.
Bare root roses tend to cost less to ship and are sometimes more mature than those purchased from a local nursery. They arrive dormant, and look like a bundle of bare roots and canes. Don’t worry- this pack of sticks will leaf out and be a beautiful rose in no time!
Soak bare-root roses by placing the roots in a bucket of water. Do this for 24 hours prior to planting. This will re-hydrate them and get them ready for their new home.
Prepare the Hole
Dig a hole a bit deeper (6-8 inches is a good rule) and as wide as the container your rose came in (For bare roots, go 6-8 inches beyond the depth of your rose’s roots). When digging you don’t want a clean, pretty hole. Aim for an ugly one!
Why? You want an irregular hole with lots of crevices and tunnels that your rose’s roots can expand into. This way, the rose doesn’t have to work as hard when expanding outward to reach nutrients in the soil as it grows.
Bury the Bud Union
Flip the rose container upside-down, and gently slide your rose out. Loosen the outer roots up a bit from the soil by massaging them lightly. Set your rose into the prepared hole, with roots face-down, canes upright.
Bury the bud-union just under the soil. Knock Out Ⓡ roses are hardy, but they can still experience die-back in extreme cold. Burying the spot where the canes meet the roots (bud union) protects the crown and prevents wind-rock which can destabilize your rose.
Prepare Your Soil
Next, prepare your soil. Knock Out Ⓡ roses are known to be tolerant of a variety of conditions, but they still appreciate quality soil to develop strong roots. If you have nutrient rich soil, no need to add any amendments.
If you have poor, rocky or clay soil, consider adding a bit of product containing mycorrhizae (fungus) into the hole with your rose which will aid root growth and help your plants absorb nutrients.
Avoid adding other fertilizers or products to bare-root roses, as the roots are likely to suffer from root burn.
Next, you’ll backfill the planting hole with a 50-50 mix of organic compost and soil. This will help ensure the roots and bud union are thoroughly covered. Tamp the soil down gently and water your rose in.
Finally, to stabilize soil temperature and conserve moisture, you’ll want to add around 3-4 inches of mulch at the base of the plant, leaving a small circle clear around the canes (this prevents rot from excess moisture). Wood chips and straw are good options.
How to Grow
Knock Out Ⓡ roses are famous for being easy to grow! That being said, their needs aren’t all that different from most roses you could choose to plant in your garden. Roses primarily were bred by florists and breeders that were looking to create a perfect plant. This earned roses a reputation as a difficult plant.
The truth is, the perennial plants already in your yard have similar care requirements to most types of garden roses. The same key component for success in growing roses for your garden also apply to Knock Out Ⓡ roses.
Similar to other rose species, Knock Out Ⓡ roses should get at least 6 to 8 hours of direct sunlight per day to encourage bloom production and a healthy plant. They can sometimes thrive in less optimal light conditions, but aim for no less than 4 hours of direct sun.
When it comes to watering, it’s best to water roses deeply, once or twice per week, and Knock Outs are no different. Once mature, use about 2-3 gallons per watering session. Always water at the base of the rose. Overhead watering can invite diseases like black spot and powdery mildew by leaving wet foliage on your rose.
If you have a large garden, you might want to save time by using a drip irrigation system. Otherwise, you can leave your hose to trickle at the base of your rose for 15-20 minutes twice a week to give it a thorough soak.
Roses that have been recently planted will need more water than established plants, so make sure they get off on the right foot by watering 3-4 times a week.
Roses should dry out between watering sessions for best results. Knock Out Ⓡ roses have been bred to tolerate drought conditions once well established, but like all roses, will bloom best when watered regularly. Do an occasional soil check to make sure it’s neither too wet or too dry and adjust your watering schedule accordingly.
Healthy soil leads to healthy plants, Knock Out Ⓡ roses included. Roses can adapt well to a variety of different soils, including heavy clay and sand. They do prefer well-drained, loamy soil.
The ideal soil for your Knock Outs is neutral (neither acidic nor alkaline), with a pH between 5.5-6.5. If you don’t know the characteristics of your soil, don’t worry. These roses will do well in most soils.
If they don’t perform and you’ve tried other solutions, you can test your soil with a kit easily acquired online or at garden supply stores. The information from the kit will help you decide if you need to add amendments to help your rose thrive.
A good bet is to spread rich compost around the soil at the base of your rose. Aged cow manure or organic mushroom composts are always a hit with roses.
Knock OutⓇ roses don’t have to be fertilized to bloom, but if you’d like to give them an extra boost, you can fertilize them three times a year.
- After they leaf-out in the spring.
- After their first flush or blooms.
- Once you enter the midsummer growth cycle.
Don’t fertilize a newly planted rose. This can burn its roots. If you choose to fertilize, begin after the rose has grown a full year in your garden. You’ll want to stop fertilizing your plants around six to eight weeks before the last frost date for your hardiness zone. This prevents stimulating new growth that could be killed off by the cold.
In spring, pick an organic, slow-release granular fertilizer to provide nutrients for your roses all season. Try Espoma Organic Rose Tone, which contains bone meal to give a boost of extra nitrogen for continued growth.
Granular fertilizer application is easy. Follow the instructions on the package for the quantity you need. Sprinkle the fertilizer around your rose, working it into the top layer of soil a bit. Water in generously, and you’re finished!
After the first bloom cycle, you can use the same fertilizer you did in spring. Alternatively, try a Seaweed fertilizer to keep your Knock Outs going strong.
Mid-season, apply another dose of either fertilizer to give your Knock Outs Ⓡ energy for their final fall flush.
Knock Out Ⓡ roses are very forgiving and will tolerate heavy pruning. As they can grow much larger than the average stated 3-4 ft. size, you may want to control their height via an annual heavy prune.
The best time to assess what pruning your Knock Out Ⓡ roses need is early spring when they’re just beginning to wake up. You’ll notice the budeyes (nubs where new growth begins) start to swell.
Some gardeners use the first bloom of the yellow forsythia bush as an indicator of the right time to prune. If you haven’t planted any forsythia, or don’t have any around, then wait until your last hard frost has passed.
Before Your Prune
Look closely at your shrub, and remember the 3 Ds: Anything dead, dying, or diseased needs to go! This will help revive struggling rosebushes. Use sharp bypass pruners to cut out dead canes that didn’t make it through winter. If any canes experienced partial die back, follow them down to where you see healthy, green growth and cut the brown portion off.
Then, examine the overall size and shape of your rose. You want a vase-formation for proper airflow and reduction of foliar diseases.
Pruning Best Practices
As you prune, always try to snip just above an outward facing bud-eye. This will help train your new rose canes to grow outward rather than in toward the center.
If your rose surprised you by growing much larger than you’d planned, consider controlling its size with a hard prune. You can trim down the entire shrub by ⅓ (or as far down as 5-6 inches from the ground if diseased or damaged). Knock Outs will grow vigorously and flush out again soon.
Last, remove all debris from around the plant. This will help prevent diseases from hanging out in the soil that could impact future plants. Cleaning up after pruning is a crucial step!
Throughout the growing season, they will bloom heavily in flushes, with breaks in between. As blooms fade, you might want to remove them by deadheading (removing the spent blooms from the plant).
Knock Out Ⓡ roses are self-cleaning, meaning they will continue to bloom even if you don’t deadhead. Many gardeners don’t like the messy look of old, faded blooms, so they choose to remove them for aesthetic purposes only. The choice is yours and won’t affect the health of your rose.
In the fall, give your roses a light pruning. This will help prevent diseases sticking around through the overwintering process before winter dormancy.
Traditional Rose propagation is mainly done via stem cuttings. It can be a fun activity for gardeners, and is typically only taken on by gardeners looking for a challenge.
However, all roses in the Knock Out Ⓡ collection are currently under patent, making them illegal to propagate for home gardeners. They are widely available to buy online and easy to find at nurseries.
Knock Out Ⓡ roses are not generally known for making good cut flowers, as they are low on fragrance and have loose form that doesn’t last long in a vase. That doesn’t mean you can’t enjoy them in a bouquet if you’d like to!
To cut the freshest roses for enjoying inside, follow these tips:
- Bring a bucket of water outside while you are cutting.
- This will allow you to keep cut stems fresh.
- Harvest on a cool morning, only choosing buds that are just beginning to open.
- This will help ensure a long vase life.
- Cut just above a set of five leaves with sharp bypass pruning shears.
- Place the cut flower into your bucket of water.
- indoors, fill your vase with cold water.
- Cut off any leaves sitting below water level.
- Put your cut roses in the vase.
- Change water out every 1-2 days, or when the water is cloudy.
Now you can display your bouquet out of bright sunlight, and enjoy! They will usually last 3-4 days depending on conditions, but you can make them last longer by using a floral preservative and giving the stem ends a fresh cut whenever you change the water.
Pests & Diseases
Knock Out Ⓡ roses are resilient and easy to grow, but they are still susceptible to a few pests and diseases. The good news is that when struck by pests or disease, these hardy plants are likely to survive the attack.
Here, we will look at some of the most common problems that you may encounter when growing Knock Out Ⓡ roses and how to respond to them.
Aphids are often the first pest to appear in spring. These small, green soft-bodied insects enjoy sucking the juices of the tender new plant growth.
Don’t worry too much if you see them. Aphid infestations usually clear up in a week or two when the beneficial predators (lacewings, birds, ladybugs) show up to take them out. You can also spray them off with a steady stream of water from the hose.
Gardeners who aren’t squeamish can simply squish them with their fingers. Resisting the use of chemical sprays is important. They kill the predators as well as the pests, causing you to rely on more severe chemical interventions once the balance is disturbed.
Spider mites are difficult to spot. They are teensy-tiny. You probably have them on your Knock Outs if the leaves have turned brown and dull. You may also notice sticky, white webs on their undersides.
Similar to aphid removal, you can knock spider mites off with a steady stream of water from your hose.
These invasive copper and green scarab beetles grow as grubs in the soil, and love feasting on rose blooms (and other plants). Chemical sprays are ineffective in controlling them, so your best course of action is physical removal.
Start with some tweezers and a jar of soapy water. Go out early in morning during the season and pluck the beetles off your blooms, plunging them directly into the soapy water. Dispose of them. Repeat until the population subsides.
If you deformed, brown buds that fail to open, thrips are the likely cause. Brown distorted leaves are another sign of these small winged insects.
Approach a thrip infestation by pruning back and disposing of any visible affected plant matter, then wait for beneficial predators to arrive. The thirps will start to go away on their own at this point. Organic neem oil is also an option, but it can harm beneficial insects so we don’t recommend it unless there are no alternatives.
“Rose Slugs”, larvae of the Sawfly, like to dine on the leaves of your Knock Out Ⓡ shrubs. They are fairly easy to spot, with the appearance of small green caterpillars. Symptoms of their damage are leaves with holes or tannish blotches. Sometimes foliage is completely skeletonized!
Less is more when treating for sawflies. You can physically remove them, or spray them with the hose. Or, you can take the equally effective route of waiting for their predators to show up to make a meal of them for you.
Black Spot is a prevalent fungal disease causing oddly shaped, black and brown spotted leaves. You may also see the spots surrounded by yellow halos. Your rose canes may develop rusty purplish-brown splotches. Black spot can weaken your Knock Out Ⓡ rose, but with treatment, it will come through fine.
To treat, remove all infected leaves and dispose of them. Black spot spreads via spores that thrive in wet environments, so help your roses get adequate airflow via pruning and spacing. Allow them to dry out thoroughly between waterings. Clean up carefully to prevent spores from overwintering in the soil.
Powdery mildew is another moisture, humid loving disease. It appears as a white, powdery coating on your plant. The leaves might curl up. Mildew thrives during cool nights with hot days. It gets worse in humid conditions.
Prevention is the key. Avoid wetting the foliage of your roses by watering at the base rather than overhead. Water in the mornings so your Knock Out Ⓡ roses can dry out during the day, and make sure they get adequate sun.
If your plants have been struck by Powdery Mildew, remove all infected parts of the shrub and dispose of them. Try this homemade baking soda spray: add one tablespoon of baking soda to one gallon water and mix thoroughly. Coat your roses, repeating once a week as needed.
Rose Rosette Disease
Rose Rosette Disease, also called Witches’ Broom, is a virus that spreads via a tiny mite. Rose Rosette is deadly, and can kill your roses. It has no effective treatments at this time.
Knock Out Ⓡ roses have been falsely blamed for the spread of Rose Rosette disease. The truth is that they are no more susceptible to this disease than any other rose.
Knock Outs are extremely popular with landscapers and growers who plant them because they are low-maintenance flowering shrubs. The trouble is that gardeners sometimes fail to monitor them for abnormal growth or signs of disease after planting.
Rose Rosette in Knock Out Ⓡ roses planted in shopping centers or parking strips is less likely to be noticed and will therefore spread faster than among treasured roses in a home garden.
So check regularly for Rose Rosette, but don’t be worried that it’s more prevalent in this species. If you’ve diagnosed your plant with Rose Rosette, you’ll need to remove the entire plant from your garden and take some of the surrounding soil with it. Do not re-plant anything in that location.
Frequently Asked Questions
Why are my Knock Out Ⓡ roses dying?
Knock Out Ⓡ roses are known to be very hardy and disease resistant, but like all plants, they need proper care to keep growing. Your rose may need more water in drought conditions, more sunlight, or protection from temperatures that are too hot or cold. If attacked by disease, remove affected plant material promptly and dispose of it away from your garden.
When do they bloom?
Knock Outs will bloom in cycles, with the first large flush of blooms in spring. They generally take a break of 2-3 weeks between bloom cycles. You can expect 3 main flushes (spring, midsummer, and fall) with 2-4 smaller bloom cycles in between.
Are they deer resistant?
No, deer love all roses, including Knock Outs! Protect them with a barrier in deer country.
Can they be planted in containers?
Yes. Knock Out Ⓡ roses can be u003ca href=u0022https://www.epicgardening.com/container-roses/u0022u003egrown successfully in containersu003c/au003e. The most natural choice would be a compact variety like ‘Petite Knock Out’, but all versions can thrive in a container if pruned to control their size.
The Knock Out Ⓡ collection of roses provides a good introduction to roses for beginning gardeners. Hardy and tolerant of a wide variety of garden conditions, you’re likely to find a Knock Out Ⓡ rose that suits your climate and needs.
With options in bright red to sunny yellow and creamy white, there’s a Knock Out Ⓡ rose to fit your garden palette. Give them a good start in your garden with the same basic care you provide other perennials, and you’ll soon have blooms throughout the season!