Companion Plants for Cosmos | Good and The bad

Cosmos (cosmos bipinnatus) is an ethereal, flowering plant, native to wild-growing meadows throughout the Americas.

A half-hardy annual, cosmos attracts bees, butterflies, and other pollinators to their colorful blooms while being conveniently low-maintenance. 

These features, among others, make the cosmos a fantastic companion plant for other bloomers. But what makes good companion plants for the cosmos, and what should be avoided? Keep reading to find out.

Understanding Companion Planting

Companion planting simply entails positioning specific plants in close proximity to each other to allow their unique features to benefit the entire grouping. 

For example, one could be especially adept at attracting pollinators that feast on destructive bugs, like aphids. Another could have a sub-surface web of roots that keep weeds from establishing themselves and increases moisture retention in the soil.

For thousands of years, from North America to Europe to China, farmers have been utilizing this highly natural method of plant and crop cultivation as a means to improve their yield and better feed their communities.

Anecdotal documentation left behind by these early cultures allows modern botanists and horticulturalists to study this practice from a scientific perspective. A study that is producing its own bountiful harvest of new information every day.

So, how does companion planting apply to cosmos plants? 

The visual beauty of the cosmos pairs well with other blooming plants to create a diverse, polyculture garden space. 

These include tall coneflowers (echinacea) and black-eyes susans (Rudbeckia hirta) as well as low-growing plants and vegetables that we’ll discuss more in detail, in a moment.

Since the cosmos is known to attract aphids and thrips as well as pollinators, pairing them with other bloomers that don’t will do well to mitigate potential damage.

Plants that share a preference for less frequent watering with cosmos will also cut down on bacterial and powdery mildew build-up.

Companion Planting Benefits

When scrolling for scientific findings on companion planting, the terms you’ll most often come across are “polyculture planting”, “intercropping” and “interplanting”.

Each of these offers similar benefits to traditional companion planting. Yet, with a more specific focus.

Polyculture planting – develops a stable growing environment that mimics a natural ecosystem in its ability to effectively and organically ward off pests and disease.

  • Intercropping – a system of planting mutually beneficial crops together to reduce disease, limit pests and weeds and improve soil quality.
  • Interplanting – a smaller-scale version of intercropping to include flowering, non-fruiting, plants like cosmos.

Let’s take a closer look at the range of benefits that the right companion plants afford you.

Pest Control

Nothing improves the overall health of plants faster than keeping damaging pests off of them. With the cosmos, the most prevalent are lygus plant bugs, aphids, and thrips.

These can cause damage ranging from the distortion or prevention of new growth to the absorption of nutrients.

The right companion plants can disrupt pest receptors with otherwise pleasant fragrances by masking the biochemicals within cosmos plants that would enable pests to recognize them as desirable.

Brightly blooming companions will also work to attract the kind of beneficial pollinators that like to snack on these detrimental insects, as well.

Weed Control

Low-growing plants act as living mulches around your cosmos. Their sprawling habits help retain soil moisture in hot summers and deter weed seeds from establishing and germinating.

In the event that a few do, the entire companion plant grouping will eventually starve germinated weed seeds of vital nutrients and moisture, rendering them unviable. 

Improved Soil Health

By employing these biological deterrents for pests and weeds, you’re allowing the soil around your cosmos to become richer in its biodiversity and fertility.

Communities of highly beneficial microorganisms flourish and facilitate improved nutrient absorption.

Moisture retention and soil drainage are also improved when plants are able to grow healthy root systems, unforced and unimpeded by synthetic products.

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Consideration When Selecting Companion Plants

With the cosmos being such a low-maintenance and easy plant to grow, it would be natural to surmise that just about any plant would pair well with them. But, not so.

In order to gain all the above benefits, there are certain important factors that need to be considered.

The first is the type of cosmos you’re growing. Most varieties are considered annuals as they don’t do well in cold temperatures. They are easy to re-seed in the spring but will end their cycle toward the end of autumn.

Cosmos atrosanguineus or chocolate cosmos is the only perennial type and thrives in temperate climates. This variety is well-known for its fragrant notes of vanilla and chocolate and its rich, auburn blooms.

Secondly, consider the types of pests that are common in your hardiness zone and what companion plants may help deter them. These should be small enough to stagger throughout your growing bed for maximum benefit.

The mature size of each plant option is critical. Not only to a cohesive design, but would they outgrow your cosmos and end up competing with it for resources, rather than being of benefit?

In this same vein, plant spacing is vital to ensuring sufficient airflow between plants, to protect against disease and powdery mildew.

The root structure is another important factor. Creating a mix of plants with both horizontal reaching roots and singular tap roots will enable a sharing of resources like moisture and nutrients. While protecting against weed infestation.

Finally and most importantly, the plants you choose as companions for your cosmos should thrive in the same hardiness zone range and similar environmental conditions. These include light, moisture, NPK combinations, soil pH, and temperature.

Best Companion Plants for Cosmos

To date, there are 30 known annual and perennial varieties of cosmos. Most do well in beds and containers and annuals even self-sow, achieving the illusion of a perennial. One aspect that makes them easy to grow is that they actually prefer a bit of neglect.

Infrequent watering is not a problem due to the cosmos being so conveniently drought-tolerant. Poor soil isn’t a challenge either. These plants put on quite a spectacular show with little to no supplemental nutrients.

These are some pretty big shoes to fill when it comes to making the grade as companions. But luckily, knowing what they need makes choosing the best companion plants for them an easy task.

Here are a few of the best performers:

Vegetables

Like most wildflowers, cosmos are highly beneficial when strategically placed in vegetable gardens.

They add a touch of elegant softness and color to a mostly structured and green space. While contributing to organic pest and weed control, as the roots of all weave together.

Vegetables share a common love of the full sun and warm temperatures with the cosmos and benefit from the harsh wind and rain protection that tall cosmos plants offer. 

Tomatoes are indebted to the cosmos for attracting aphid-loving ladybugs while offending foes like hornworms.

Melons and squash depend on pollinators to produce fruit. Cosmos will keep them coming with their brightly colored and easily accessible blooms.

Beans, peas, and other legumes are notorious for attracting aphids. Cosmos, while mildly prone themselves, can actually work with legume plants to deter these tiny marauders.

Herbs and Flowers

Herbs offer lovely fragrances, variant shades of green foliage, and delicious culinary ingredients. As cosmos companions, they will naturally disrupt the receptors (by way of scent) of detrimental pests that are attracted to vibrant, sometimes infestation-prone cosmos flowers.

Using companion plants such as Dill, Rosemary, lavender, borage, and fennel is fantastic as they all share similar environmental preferences with cosmos plants and thrive right alongside them.

Annual and perennial bloomers can contribute all the practical benefits of companion plants while increasing the overall beauty of a polyculture flowerbed or meadow. These include:

  • Marigolds
  • Zinnias
  • Dahlias
  • Verbena bonariensis
  • Sweet Peas

In larger spaces, a few woody shrubs like Nicotiana sylvestris or tropical canna lilies work well. Even warm-season, ornamental grasses like Cheyenne Sky switchgrass provide stunning contrast in color and form to eye-catching cosmos.

Bad Companion Plants for Cosmos

To attain all the benefits that effective companion plantings can provide, it’s imperative that you take your time and make your selections carefully. All the information offered here can help you do that.

Now, let’s touch on the kind of plants that should be left off your list of choices completely.

There are a number of reasons why certain plants would make bad companions. The first of which would be those that aren’t recommended for your hardiness zone.

For example, if you live in a region, like the American Pacific Northwest, that gets above-average rainfall per year, Mediterranean herbs like rosemary and lavender wouldn’t be successful choices. These prefer lots of sun and infrequent watering.

Shrubs and/or flowering plants with mature sizes that are too large for your intended planting space should also be ruled out. They may look really pretty and nicely contribute to a mutually beneficial plant grouping while small. But, once they reach maturity, they often begin to starve the surrounding plants of vital resources.

Considering that cosmos plants prefer full sun, pairing them with those that favor more shade would not be a fruitful endeavor. Hostas, ferns, certain hydrangea varieties, heuchera, and coleus should be avoided.

Lastly, despite being partial to full sun and infrequent watering, heavy feeders shouldn’t be matched with the cosmos. Roses, sunflowers, and astilbe fall under this category. As well as quite a few vegetables, like eggplant, broccoli and squash, and corn.

References 

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.