Phlox grows lush green foliage with lovely bountiful blooms, but if you begin to notice yellowing leaves that aren’t part of its regular growth cycle, you should try diagnosing the cause quickly.
Yellowing can be caused by environmental stressors that you should be aware of because they are usually easily treated or prevented!
Why are my phlox leaves turning yellow? Phlox may begin to have yellowing leaves because of various growing stressors or pests and diseases. Yellow leaves may also be caused by nutrient deficiencies, a lack of sunlight, inadequate water, too much water, poor soil health, and incorrect soil pH.
Read on to better understand the 14 most common causes of yellowing leaves on your creeping phlox, garden phlox, or any other variety of phlox that you may have planted in your garden.
Yellow Leaves on Phlox – 14 Causes (& Solutions)
Without an ample supply of necessary nutrients, light, or water, phlox may become stressed or lack the ability to fight off diseases and pests. See the 14 most common causes of yellow leaves below and solutions to fix them!
Overwatering will cause the soil to be saturated and create an anoxic environment for the roots to grow in.
Without a 50/50 mixture of open-air space to water-filled space within the soil, your phlox may become stressed.
Gauge the moisture level of the soil with a moisture meter (like this one that also reads light and pH levels) or by digging 2 inches down into the soil to determine the saturation level.
Avoid watering anymore if oversaturation seems to be the issue. Phlox will thrive with about 1-2 inches of water per week.
2. Drought Stress
Luckily, phlox will become drought tolerant as it matures and establishes, but until then it may show signs of yellowing leaves, dropping them without sufficient water.
In times of excessive drought, it would be best to ensure it is receiving 1-2 inches of water weekly or more during dry spells. Adding mulch to the soil will help to retain moisture and reduce evaporation.
3. Powdery Mildew
Powdery mildew appears as a white powdery coating on the leaf’s top and bottom sides. It will reduce the ability of the leaf to photosynthesize, eventually causing it to yellow and die off.
Water phlox at its soil base, and avoid allowing water to stay on the leaves. The best way to treat powdery mildew is by applying a neem oil-based spray.
It will destroy the powdery spores while preventing them from spreading and taking hold of the leaves again.
4. Lack of Sunlight
Phlox thrives in full sun and requires 6 – 8 hours of direct sunlight per day to grow its best. If the plant’s leaves can’t photosynthesize, they will turn yellow and die due to a lack of energy.
If possible, increase sunlight in any way possible. This can include trimming back other plants that may be creating shade for the phlox or relocating your plant to a sunnier location.
5. Spider Mites
Spider mites will create a thin translucent webbing that coats the leaf’s surface, which reduces the plant’s ability to photosynthesize.
You will notice tiny spiders moving along the webbing. They feed by sucking nutrients from the leaf, causing it to eventually die.
Spider mites prefer to live in an area that is minimally disturbed. Creating more airflow by trimming other plants back as well as washing the leaves off will destroy their webbing and displace the mites.
Neem oil is also a great remedy since it creates a slick surface and unfavorable conditions for them to live in.
6. Root Rot
Root rot is caused by oversaturation of the soil, which leads to anoxic conditions, so the phlox’s roots will begin to rot if they stay in the wet environment too long.
Root rot will lead to an overall decline in health, causing the leaves to yellow and drop.
Allow the soil to dry up if possible. It can be difficult sometimes to reduce the amount of water if it is a rainy season, so planting your phlox in well-draining soil will help to reduce the chances of root rot.
Blight is a viral infection that will show itself through brown or black spots on the leaves along with yellowing leaves.
It comes from infected soil and transfers by water splashing spores on the leaf’s surface.
Blight can be difficult to prevent. Destroy any infected plants to prevent it from spreading to any other plants.
Regularly spraying neem oil or spraying neem on foliage not yet affected will help prevent the spores from taking hold.
8. Tobacco Mosaic Virus
The Tobacco Mosaic virus affects the leaf’s growth, producing curled and misshapen growth along with yellowing leaves.
Unfortunately, this is an incurable viral infection, so the best solution is to remove any infected plants and destroy them to prevent them from spreading further.
9. Impatiens Necrotic Spot Virus
Spread by thrips, this virus will cause necrosis, yellowing, ring spots, molting, and distorted new growth.
This virus is incurable, so it is best to remove any infected plants and destroy them to prevent it from spreading throughout your garden.
Try to prohibit thrips by using insecticidal soap (this one is approved for organic gardening) and neem oil to kill the carriers of the virus.
10. Aster Yellows
With aster yellows, you’ll begin to notice distorted leaves and flowers with an overall stunted growth habit, reducing the plant’s mature size.
The phlox’s leaves will also show pale veins and a yellowing discoloration.
This is a viral infection with no cure, so remove any plants that may be infected and destroy them so it won’t spread throughout your garden.
Spraying an antiviral solution or neem oil can help prevent any virus from taking hold on other plants.
11. Other Virus/Fungal/Bacterial Infections
In most cases, these infections come from a lack of air circulation and wet environments.
They cause yellowing leaves and will distort the plant’s growth habit, eventually disrupting its regular growth pattern and killing it.
Plant phlox with ample space between plants, and avoid allowing water to sit on the leaves’ surface for extended periods of time.
Cut back other plants to allow airflow and reduce the chances of infections occurring.
Since many infections don’t have a cure, it is best to remove the plant at first signs of infection and destroy it to prevent the issue from overtaking all the plants in your garden.
12. Incorrect Soil pH
Phlox prefers a soil pH of 6.0- 8.0 and is able to handle slightly acidic or slightly alkaline soils.
If your soil becomes too acidic or alkaline, it will begin to stress the plant since vital nutrients aren’t available outside of that pH range.
Use a pH meter to test your soil and determine if it falls outside of phlox’s preferred range. Adding a fertilizer specifically formulated to adjust the pH to a balanced level will help balance the growing conditions for your phlox.
13. Nutrient Deficiency
Phlox grows quickly and requires ample nutrients to grow its best. A nutrient deficiency will commonly cause your phlox’s leaves to yellow and most likely reduce the chances of it blooming.
Fertilize your phlox seasonally with a well-balanced fertilizer (I like this slow-release food) right after blooms form to give it everything it needs to thrive.
14. Chemical Drift
Sometimes there are pesky weeds that require an herbicide to kill fully, but when it is sprayed in the air, wind can catch the particles and carry them to plants you don’t want to kill.
Reduce the effects by washing the leaves off to reduce the potency of the spray. Fertilize your plants if they experience burn to help supply extra nutrients for them to grow new foliage.
If you are spraying an herbicide, cover any plants you hope to keep safe.
Phlox Care Tips
Catering your growing area to your phlox plant is the best way to ensure it grows strong and blooms for seasons to come. The list below outlines the most important things to remember to grow the best phlox.
- Plant phlox in sandy or loamy soil that drains well but still retains some moisture.
- Plant new seedlings in the spring so they have time to establish during the growing season.
- Water your phlox regularly until it is established, and then taper off the number of times you water it. This will help harden the plants off and make them less vulnerable to drought.
- Annually fertilize your phlox with a well-balanced fertilizer in the spring after buds form.
While yellowing leaves may cause you to worry, many times there are remedies you can enact to help your phlox get back on the right track!
Use the list above to help identify what the issue may be and try the solution. If one solution doesn’t fix it, the plant may be stressed by multiple causes and needs a few remedies to bring it back to its normal green lushness.