Phlox can be a wonderful addition to your garden. But it can also be tricky to grow. Finding the right plants to go with it and that work together can be a minefield.
So I’m going to take you through which plants not to plant with Phlox and why so you can make sure you don’t make any mistakes.
Understanding Phlox
Garden phlox (Phlox paniculata) stands tall and proud at 2-4 feet. Creeping phlox (Phlox subulata) on the other hand creates wonderful ground covers. Both types bring incredible color to your garden from mid summer through to fall, with some varieties blooming for weeks on end.
But phlox can be particular about their growing conditions. They generally prefer:
- Rich, well-draining soil
- Consistent moisture without waterlogging
- Good air circulation
- Moderate to full sun (depending on variety)
Plant them with incompatible neighbors, and you’re asking for trouble.
The biggest issues? Competition for resources, conflicting soil needs and susceptibility to disease. Phlox are already somewhat prone to powdery mildew (I battle this every humid summer), so adding plants that increase moisture or shade can spell disaster.
13 Plants NOT to Plant with Phlox
1. Aggressive Mint Varieties
Mint’s roots will absolutely overwhelm phlox, stealing nutrients and taking over the space within a season. It may look innocent enough at first but by mid summer any phlox will be struggling to compete with the mint’s aggressive spread.
Even if you try to remove it the stubborn mint runners will keep popping up everywhere.
Why they clash: Mint’s invasive root system competes too aggressively for nutrients and space.
2. Tall Sunflowers
Sunflowers look spectacular but plant them near phlox and it simply won’t work. I’ve seen a row of sunflowers planted behind phlox and the sunflowers cast too much shade and stole all the nutrients.
Why they clash: Sunflowers are heavy feeders that deplete soil and create too much shade for sun-loving phlox.
3. Black Walnut Trees
Black walnut trees work with very littl. They produce juglone, a natural chemical that’s toxic to many plants including phlox. If you have black walnuts on your property, keep your phlox far away.
Why they clash: The juglone produced by black walnut trees is toxic to phlox and will kill them over time.
4. Russian Sage
Despite both being perennial bloomers Russian sage and phlox make terrible neighbors. Russian sage prefers dry, almost drought like conditions. Phlox however needs consistent moisture. When I tried growing them together my phlox constantly looked wilted as the sage hogged all the water.
Why they clash: Completely different water requirements make it impossible to keep both happy.
5. Aggressive Groundcovers like Vinca
Any aggressive groundcover will compete with phlox for root space. Vinca might look innocent but it will gradually choke out your phlox plants.
Why they clash: Competing root systems and aggressive spreading habits.
6. Daylilies
Both may be popular perennials but daylilies and phlox don’t work well together. Daylilies’ dense clumping habit and extensive root systems compete directly with phlox for nutrients and space.
Why they clash: Competition for underground resources and daylilies’ thick foliage can reduce air circulation around phlox, increasing disease risk.
7. Most Vegetable Plants
Most vegetables have different fertility and water needs than phlox. And the regular harvesting and soil disturbance stresses phlox plants.
Why they clash: Different care requirements and the disturbance from vegetable gardening practices stress phlox.
8. Other Phlox Species (Sometimes)
This might sound counterintuitive but different phlox species don’t always play nice together. I’ve tried mixing tall garden phlox with creeping phlox, thinking they’d create a nice tiered effect. Instead they competed for the same resources while increasing each other’s disease susceptibility.
Why they clash: Similar disease vulnerabilities can create a hotspot for powdery mildew and other issues.
9. Hydrangeas
Hydrangeas are keen on water and that will leave your phlox parched. I learned this the hard way when I planted a hydrangea bush near my phlox border. The constant battle for moisture meant my phlox were always struggling, especially during hot summer days.
Why they clash: Hydrangeas’ extreme water requirements make consistent care difficult for neighboring phlox.
10. Butterfly Bush (Buddleia)
Despite both attracting pollinators butterfly bush and phlox compete too much for the same resources. Butterfly bush also grows much larger than most expect, eventually overshadowing and competing with phlox for light.
Why they clash: Size disparity and resource competition make this pairing problematic.
11. Foxglove
While foxglove and phlox both have vertical growth habits that might seem complementary foxglove’s biennial nature means you’ll be disturbing the soil around your phlox every couple of years when you replace them.
Why they clash: The soil disturbance from foxglove’s biennial lifecycle stresses nearby phlox roots.
12. Rhododendrons and Azaleas
Shrubs like Azaleas and rhododendrons have soil requirements that directly conflict with phlox’s preferences. Phlox generally prefers neutral to slightly alkaline soil, while rhododendrons demand acidity.
Why they clash: Incompatible soil pH requirements mean one or both plants will suffer.
13. Hosta in Full Sun Areas
Though hostas and phlox can sometimes work together in partial shade, placing this combo in full sun is asking for trouble. The hostas will struggle, look burned and phlox won’t receive the air circulation it needs to prevent disease.
Why they clash: In sunny spots, hostas suffer while potentially creating disease-friendly conditions for phlox.
Good Companion Plants for Phlox
Now that we know what to avoid we can focus on what actually works with phlox. I’ve had great success with these combinations:
- Coneflowers (Echinacea) – They bloom concurrently and have similar care needs
- Black-eyed Susans – Their gold tones complement phlox colors beautifully
- Ornamental grasses – Provide contrast and don’t compete heavily for the same resources
- Shasta daisies – Their white flowers make phlox colors pop
- Bee balm – Attracts similar pollinators and has comparable needs
My favorite combination has been purple phlox with white Shasta daisies and some ornamental grasses in the background. The contrast is stunning, and they all seem to thrive together without competing.
Tips for Successful Companion Planting with Phlox
- Ensure all companions have similar water requirements – this is probably the most critical factor
- Provide adequate spacing to promote good air circulation (I plant phlox at least 18-24 inches apart)
- Choose companions that bloom at different times to extend your garden’s show
- Mulch around plants to reduce competition from weeds and maintain soil moisture
- Consider the mature height of companions to prevent unwanted shading
One trick I’ve learned: place lower-growing plants around the base of phlox to hide their sometimes bare lower stems while keeping good air circulation at leaf level.
Final Thoughts
Phlox are worth the effort of finding the right companions – they look great for those few weeks. But you need to understanding their needs and respect their somewhat picky nature when it comes to garden companions.
Keep in mind that garden “rules” are really just guidelines. Your specific conditions, soil, climate and even the particular varieties you choose might bend these guidelines. The best approach is to start with these recommendations, observe carefully and adjust as needed.
What I know for certain is that giving phlox enough space, appropriate companions and consistent care will reward you with one of the most spectacular floral displays a garden can offer.