Roses have represented garden elegance for centuries. Yet, some gardeners are hesitant to consider them due to their fussy reputation.

Knockout roses present a much higher resistance to common ailments that previously made roses so labor-intensive to grow. But, they’re not foolproof.

The best companion plants for knockout roses protect and defend against any remaining vulnerabilities.

What Is Companion Planting

Companion planting, in a nutshell, is the practice of harnessing the inherent properties and growing habits of specific plants and positioning them in close proximity to create a synergistic relationship that’s mutually advantageous.

In other words, each plant contributes something to that grouping that every plant in it will benefit from. Sometimes, it’s a pest-deterring fragrance or a web of sub-surface roots that choke out weeds. Other times, it’s a sharing of vital nutrients and moisture.

The reliability of companion planting and the drive to scientifically discover more about it are fueled by thousands of years of observation and documentation by gardeners and farmers across the globe.

In North America, this synergy between plants was first realized when certain crop seeds (the three sisters) were inadvertently sown in the same space. 

The resulting plants appeared to thrive and produce a much higher yield than when planted separately.

Once this idea caught on, horticulturalists began to wonder if it could be applied to ornamental plants, like knockout roses, as well. Luckily for us, their curiosity proved fruitful. 

We now have a long list of plants, trees, and shrubs that make perfect companion plants in a number of highly beneficial ways. 

It’s also important to note that the benefits of companion planting aren’t limited to practical applications. Yes, some plants focus on shared resources for healthy growth, hardiness, and productivity. While others specialize in pest and weed control. 

But, some plant species contribute in a more visual way. Offering complimentary and contrasting color, form, and texture through deciduous or evergreen, flowering, or non-flowering growth habits.

The mature height and width of certain plant species can be their contribution, as they protect your roses from the harsh summer sun or the biting wind in cold winter regions.

Knockout roses, in particular, are prolific bloomers and bred to be hardier against common rose afflictions than heirloom varieties. But again, they’re not infallible. 

To a certain degree, these hybrids can still be susceptible to pests, disease, powdery mildew, black spot, and rust. Companion plants can make up for the deficit between resistance and vulnerability. Let’s take a look at how.

What Are the Benefits

First, we’ll touch on aesthetics. If you’ve ever grown roses, you’ll be familiar with the fact that leaves and buds typically grow more densely on the highest, most sun-exposed parts of each branch. Often leaving the lower halves bare. This can give an otherwise luxurious shrub an awkward appearance. 

Low-growing companion plants that thrive in similar conditions can provide cover for those bare bits, leaving only the pretty parts visible. Dwarf evergreens, small flowering perennials, and even flower carpet roses serve this purpose, magnificently.

The most successful partner plants will be those that don’t compete for root space. Knockout roses develop a web of roots that spread outward and delve deeper into the soil the older the plants get. 

Companion plants that have a fairly shallow root system or a single tap root are usually the best options and this is where strategies for the best use of space come in.

Improving the use of space

Growing mutually beneficial plants together in a small space not only creates a beautiful, low-maintenance garden for you but heightens the potential of the available square footage.

Even in sizable gardens where individual beds are typically long and narrow, this idea is highly advantageous and time-tested. It’s been applied to crop cultivation for decades, as farmers seek out creative ways to increase crop yields on the same plot of land.

Controlling Pests

Inviting all the beneficial pollinators to your yard that feast on rose-damaging pests is one way that companion planting works as pest control. 

Yet, since hybridized knockout roses produce very little pollen or nectar, flowering plants that do are critical to attracting those pollinators. 

Highly scented herbs are also great choices because they’ve been shown to deter aphids, mites, et al. by disorienting their sensors.

Improving Soil Health

With the right companion plants in place around your roses, there will be little to no need for supplemental pesticides. Herbicides to ward off weeds will most likely not be necessary either as the combined roots of these will starve any nested weed seedlings of nutrients and moisture and eliminate them.

In the absence of any toxic chemicals, your soil will quickly see increased fertility as microorganisms, earthworms, and other soil life multiply and rapidly turn applied nutrients (fertilizer) into a form that your plants can easily absorb.

Increasing Flower Yields

In turn, this organic method of pest and weed control, coupled with increased soil fertility will lead to healthier, more robust, and carefree rose bushes.

How will you be able to tell this is happening? Vigorous, vibrant growth will be visible in spring, followed by fuller, more luxurious foliage formation, and finally an explosion of bright, colorful blooms that repeat right through the first autumn frost. 

Consideration When Selecting Companion Plants

Now that you understand what companion planting is and how it works, your first consideration before implementing it is whether or not you actually need it. 

Keep in mind this does mean purchasing new plants or starting new ones from seed. More plants in your garden also mean increased watering/fertilizing usage and cost. 

That said, in my experience, the benefits far outweigh these factors. Your next step is to assess the size of your intended planting area. 

In a small space, your companion plant choices should be shorter than your roses and come in varying heights and widths. Yet, with reasonably mature sizes so as not to outgrow the space.

The third point of deliberation should be which plants share common care needs while offering some or all of the benefits we’ve discussed. Let’s examine the requirements for Knockout roses for comparison:

• A hardiness zone recommendation of 5b-9
• 6-8 hours of full sun
• Weekly watering
• While drought-tolerant, consistently moist, well-draining soil is preferred
• A 5.5 and 6.5 soil pH

Since plant diversity is the key to a visually appealing garden, bloom times for your chosen companion plants are important. 

Do you want your plants to bloom at the same time or would you prefer some early-spring bloomers to start the show before your knockout roses steal it? 

And since deadheading these self-cleaning hybrids isn’t necessary, some equally easy companions may be just the ticket.

Once you’ve read through the following list and chosen the best companion plants for your roses, the final consideration should be plant spacing. This will dictate how many partner plants will fit around your shrubs.

Without regular pruning, a knockout rose can potentially reach 6 ft in height. While typically reaching 4 ft wide, at maturity. This is your spacing marker. 

Companion plants should be positioned no closer than 4 ft from the main trunk of your rose bush. Any closer and the growth of all your plants will be impeded and they may start competing for resources, instead of sharing them.

Best Companion Plants for Knockout Roses

Now it’s time to talk about what these plants are! With knockout roses being so hardy already, the list of companion plants you have to choose from is far-reaching. 

Here, I’ll suggest specimens that match their low-maintenance character, while providing that extra bit of pest and weed control to keep them thriving.

Evergreen shrubs like sweet box (Sarcococca hookeriana var. humilis) or small Holly varieties (Ilex) can carry that experience through to the following spring. Their resilient foliage is barely affected by seasonal change and offers a pleasing and fixed structure around your knockout roses.

These examples are but two on a long list of flowering shrubs that attract the beneficial pollinators that ward off harmful pests. They can also provide your roses with some protection against harsh winds and weather.

If your preference is a non-flowering shrub that’s beautifully complementary, I suggest low-growing boxwoods that come with solid green or variegated foliage.

Vegetables

Roses have been used to decorate kitchen gardens for centuries and it’s no wonder. Beets, radishes, carrots and parsnips, kale, cabbage, swiss chard, and a variety of lettuces all thrive in the shade provided by rose shrubs, and being smaller plants they won’t compete for moisture and nutrients.

Since aphids, thrips, red spiders, and leaf-eating beetles are common on both roses and vegetables, plus the lack of nectar-filled flowers on either, combining these with some pollen-heavy bloomers would help to relieve pest problems.

Herbs and Flowers

Roses, in general, may prefer consistently moist soil but Knockout varieties were bred to be more drought-tolerant. This means you have a wider range of low-growing plants you can pair with them. 

Scented herbs are superb choices for knockout roses. These emit fragrances that are pleasing to us but scramble the senses of bugs seeking to chew on leaves and flowers. These include catmint, oregano, lavender, rosemary, Mexican sage, and thyme.

Showy, flowering plants, like yarrow, alliums, and butterfly bush, attract beneficial pollinators, keeping pests away from your prized roses. Other plants that serve this purpose are scented geraniums and foxgloves.

These all share a love of the full sun and contribute vibrant color, form, and blooming habits to a gorgeous garden tapestry.

Bad Companion Plants for Knockout Roses

It’s easy to get carried away by all the beautiful possibilities. While the list of advantageous companion plants is long, there is also a list of plants that should be avoided. 

While just as beautiful, there are lots of plants that prefer a growing environment opposite to Knockout roses and would slowly deteriorate.

Since roses prefer full sun, any plant that requires partial or full shade should be removed from consideration. Examples are fuschias, astilbe, begonias, hydrangeas, and the like. 

Deep shade plants also not recommended include hostas, ferns, bleeding hearts, coleus, and heuchera. 

In addition to being adverse to full sun, these and similar plants lack the drought-tolerance of Knockout roses and require more moisture in the soil for easy metabolism of nutrients and photosynthesized food.

Aside from care needs, the growing habits of potential companion plants need to be considered, as well. Popular landscaping plants such as bee balm and canna lilies, wildflowers like evening primrose, and even some herbs like mint, should not be grown around knockout roses. 

These plants are fantastic for attracting pollinators but they are aggressive spreaders and in time could end up choking the roots of your roses or simply taking over the bed.

I did recommend lavender as a beneficial companion plant for knockout roses in a previous section. The caveat to that recommendation is the variety of lavender you choose. Slower growing types make far better partners than faster-spreading ones which can end up in the aggressive grower category.

I also mentioned that large, shade trees should also be removed from consideration as good companion plants. If chosen, the resulting lack of sunlight would cause few pollen-filled blooms to develop. Inviting harmful pests to feed on your roses unimpeded.

Now, that’s not to say that all trees are off-limits. If your knockout roses are planted in a north or west-facing bed, you can plant small trees behind them to create a dramatic backdrop for splashy knockout roses without blocking any sunlight.

Dwarf evergreen varieties like a ‘Blue Wonder’ spruce, ‘Nana’ balsam fit, or ‘Chalet’ Swiss Stone Pine work well for this. They don’t take up as much room as full-sized varieties or fight with your roses for resources. These are just a few suggestions. The best ones for you will be those recommended for your hardiness zone.

Citations

• UC Master Gardeners of San Mateo & San Francisco Counties – Better Together: The New Science of “Companion Planting”