Thyme leaves turn yellow because of root rot, too much or too little nitrogen, or due to a spider mite infestation. Root rot is the most common reason for thyme with yellow leaves which is caused by too much moisture around the roots due to overwatering or slow-draining soils.
Thyme is susceptible to root rot because it is a Mediterranean herb with a preference for dry soil conditions and it does not tolerate damp soil.
Thyme plants with yellow leaves often have a wilting or drooping appearance which is another sign of stress due to moist conditions, too much nitrogen, or pot-bound roots.
Keep reading to learn why your thyme is turning yellow and how to solve the problem…
Thyme Turns Yellow (Overwatering and Slow Draining Soils)
The most common reason for thyme plants turning yellow is because of too much soil moisture around the roots which is due to:
- Watering thyme too frequently.
- Slow-draining soils.
Thyme is a Mediterranean herb that has adapted to the hostile conditions of a dry, sunny climate with infrequent rainfall and it thrives in sandy soils that are well-draining and do not hold moisture.
Because of thyme’s adaptation to dry climates, the roots do not tolerate consistently moist soil.
Moist or damp soil around the roots promotes the conditions that cause root rot, which can be a common problem for thyme plants and other Mediterranean herbs such as lavender and rosemary.
The most prevalent symptoms of a thyme plant suffering from root rot are:
- Yellow or brown leaves.
- A drooping or wilting appearance.
- Roots that are dark brown (rather than a lighter somewhat yellow color).
Too much moisture can also encourage fungal pathogens which turn the foliage brown. To learn how to revive brown thyme foliage, read my article Why is my thyme plant turning brown?
How to solve it…
The key to preventing and treating root rot by turning your thyme leaves yellow is to:
- Water thyme only once a week in dry weather or once every two weeks if there has been rainfall. In most circumstances watering thyme once every week with a good soak is the optimal balance of watering in pots, and containers. In garden soil, thyme is unlikely to require any watering in most climates as thyme is drought-resistant and can attain enough moisture from the environment. The watering frequency of thyme does depend on your climate so adjust the frequency of your watering so that the soil around your thyme plant feels somewhat dry to a finger’s depth before each bout of watering, to ensure healthy roots.
- Amend the soil with sand or grit to improve drainage. Adding sand or grit to the potting mix or planting area replicates the sandy soil conditions of the thyme’s native Mediterranean environment. The sand ensures good drainage and the right nutrient balance so that the soil around the thyme can dry between each watering to prevent root rot.
- If the leaves are significantly yellow and you have been watering your thyme too frequently then it is important to snip back any yellow foliage back to healthy growth. Carefully remove the thyme from the ground and inspect the roots. Snip away any dark brown, rotten, or diseased roots back to healthy growth. Wipe the blades of your pruners with a cloth soaked in disinfectant after each snip to prevent the spread of the disease. Transplant the thyme to a pot or container with new soil that is 30% sand or grit and 70% compost, give the soil a good soak to mitigate transplant shock and place it in full sun.
Follow these steps to give the thyme the best prospect for recovery within a few weeks.
If the thyme has extensively rotten roots then I recommend throwing the plant away or burning it to prevent the spread of any fungal pathogens in your garden.
Prevention is better than cure with root rot so buying a new plant and planting it in a pot with the right soil mix and watered appropriately is often the best course of action.
If there are any other problems with your thyme plant read my article How to revive a dying thyme plant.
Thyme’s Pot or Container Too Small, (Nitrogen Deficiency)
Thyme plants can commonly turn yellow because:
- The pot or container is too small (small pots have a limited capacity for soil and therefore nutrients).
- The thyme is planted too close together (thyme has to compete for space, light, nutrients, and water).
Thyme plants (as with all Mediterranean herbs) tend to prefer a size pot of at least 12 inches across so that the roots have space to establish so they can access nutrients.
With smaller pots and containers there is less capacity for soil and therefore a limited amount of water and nutrients and less space for the roots to establish which can affect growth.
Thyme plants do actually prefer low to medium-nutrient, sandy soil as this is the soil profile of their native Mediterranean habitat.
However, if the roots are pot-bound, the soil’s nutrients can become scarce which results in thyme turning yellow due to a lack of nitrogen.
Thyme naturally grows in open areas and it does not like to compete for airflow, nutrients, soil, or sunlight with other plants which is why thyme plants can turn yellow if they are planted too near each other.
How to solve it…
Transplant your thyme to a larger pot of at least 12 inches with drainage holes in the base to allow excess water to escape.
In a larger pot or container thyme has enough space for the roots to develop which allows the plant to grow abundant leaves that exude a powerful aroma with a strong flavor.
A pot this size will have the capacity for all the soil and nutrients a thyme plant requires to grow healthy.
Transplant thyme to a larger pot with multipurpose compost and some added horticultural sand or grit and the thyme can start to recover from a yellow appearance and turn to a healthy green in a few weeks.
Do not add any artificial fertilizer as the compost and sand mixture recreates the thyme plant’s native soil conditions with sufficient nutrients and additional fertilizer causes more harm than good due to thyme’s preference for poor soil.
For thyme that is planted in garden soil or in a raised bed ensure each plant is not casting shade on each other as thyme requires full sun to grow healthy and green.
Planting each thyme at least 18 inches apart (measured from each planting site rather than the outer foliage) ensures each plant’s root system has enough space to establish and the extra distance promotes good airflow around the leaves which lowers the risk of fungal disease.
Plant thyme in full sun, at a distance, and in a pot that is at least 12 inches across, and your thyme plant with yellow leaves should recover and start to look more healthy in a few weeks.
Too Much Nitrogen Turns Thyme Leaves Yellow
The leaves of your thyme plant can turn yellow because of both a deficit and a surplus of nitrogen in the soil. A surplus of Nitrogen is usually caused by the use of fertilizer, so if you have used a fertilizer for your thyme this is most likely the reason for yellow leaves as thyme plants are sensitive to nitrogen in the soil due to their preference for low to medium fertility soils.
Thyme plants have adapted to thrive in sandy soils which do not retain much water or nutrients in the South of Europe.
Therefore, when growing thyme in your garden it does not require any additional fertilizer to grow healthy as a high amount of nutrients would be contrary to the natural conditions to which thyme plants have adapted.
Too much nitrogen in the soil promotes excessive foliage growth which can cause the plant to droop or wilt in appearance and causes the leaves to turn yellow as a sign of stress.
The concentration of essential oils (which affects the aroma and flavor) in the leaves is strongest in plants that are in full sun and in soil that is not too nutrient-dense so it is important to have a good potting mix with the right balance of nutrients for thyme that tastes and smells its best.
How to Treat Yellow Thyme Leaves With Nitrogen Burn
The key to solving thyme leaves that are yellow due to excessive use of fertilizer is to:
- Stop using additional fertilizers as they can cause thyme more harm than good.
- Amend the soil with sand or grit.
Amending the soil with sand or grit helps to dilute the concentration of nutrients (particularly nitrogen) to emulate the sandy, low to medium soil conditions of the thyme’s native Mediterranean environment.
Too much sand is better than not enough as sand also improves soil drainage and increases pore size which helps with root respiration.
Re-pot the thyme (or lift it carefully out of the ground with a garden fork) and add roughly 30% horticultural sand or grit to 70% multi-purpose compost.
This mix of sand and compost ensures that the thyme has the right balance of nutrients to grow and produce fragrant foliage. The thyme may take some time to revive after nitrogen leaf burn but it should show signs of recovery in a few weeks.
Spider Mites Cause Yellow speckles on Leaves
Another potential cause (but less common) is spider mites attacking your thyme plant. Spider mites look like miniature spiders and tend to attack the new succulent growth on the underside of thyme leaves in the Spring.
Even if they are in your garden, they often go unnoticed as they have many insect predators that control their numbers.
It takes an infestation to do any damage, however, regardless of the scale, spider mites are rarely a serious problem and can be easily addressed.
The damage resembles small, yellow spots on the leaves which if left untreated can cause leaf drop.
If spider mites are causing your thyme leaves to turn yellow, the best course of action is to either blast them off with a hose or preferably remove them by hand.
As long as you exercise diligence for a couple of weeks this is usually all it takes to solve the problem. Trim off any affected foliage with pruners to stimulate new growth and the thyme should recover without problems.
Usually, natural garden and insect predators address the imbalance, and spider mites may not be a serious reoccurring problem.
Another option is to address the problem with chemicals.
I tend to avoid the use an insecticidal soap or pesticides on the leaves of herbs if I am using them for culinary purposes, but an insecticidal soap can be used if spider mites are a persistent problem and your thyme plants are ornamental rather than for cooking.
Key Takeaways:
- Thyme plants most commonly turn yellow because of root rot due to damp soil. Thyme prefers well-draining sandy soils and does not tolerate consistent moisture. Symptoms of thyme with root rot are yellow leaves and a drooping or wilting appearance.
- Add horticultural sand or grit to the soil to improve drainage and replicate the sandy soil conditions of the Mediterranean environment to which thyme is adapted.
- Too much nitrogen from fertilizer or too little because of pot-bound roots can cause thyme to turn yellow. Plant thyme in a pot that is at least 12 inches across to prevent a nitrogen deficit and do not add any fertilizer as this simulates leggy growth with less aroma and is contrary to thyme preference for poor soils.
- Spider mites can cause yellow speckles on new growth. Wash spider mites off with a hose or remove them by hand. Snip off any significant damaged growth to stimulate new healthy growth. Do not use pesticides if you plan on using thyme for culinary purposes.