The reason thyme plants turn brown is because of fungal disease caused by excess moisture around the roots. Thyme is a Mediterranean herb that prefers somewhat dry conditions. Over watering and slow draining soils promote the conditions for fungal pathogens that cause thyme plants to turn brown.
Thyme can also turn brown due to age as thyme (like a lot of Mediterranean herbs) is not particularly long lived plant and may naturally turn brown and die after 4 years even with good care.
It is possible that thyme plants can turn brown due to a lack of water but this is unusual as thyme is drought resistant, prefers dry conditions and tends to wilt rather then turn brown due to dehydration.
Therefore the cause of thyme turning brown (often with a drooping or wilting appearance) is far more likely to be because of moisture stress due to over watering, slow draining soils or pots without drainage holes in the base.
Thyme can also turn brown as a result of humidity, particularly if they are planted too close together.
Keep reading to learn what is causing your thyme plants turn brown and how to resolve the problem…
Thyme Turning Brown due to Moist Soil
Thyme plants originate in the Mediterranean region of Europe where they enjoy full sun, sandy well draining soils with infrequent rainfall.
Thyme is a drought resistant herb that is suited to dry conditions with full sun and can suffer as a result of over watering and cold wet soils which is why thyme often turns brown during Winter.
If thyme plants are planted or are cared for in conditions that are contrary to their preferred somewhat dry soil conditions then the thyme can turn brown as a sign of stress due to various fungal pathogens such as root rot which are more prevalent in damp, cold soils.
There are three main reasons for thyme turning brown due to excess moisture around the roots:
- Over watering. Thyme is a perennial herb that has adapted to the arid conditions of the Mediterranean climate, with full sun, relatively low humidity and infrequent rainfall. Thyme is a drought resistant plant, so if you are watering your thyme before the soil has a chance to be somewhat dry (rather then consistently damp) then you are over watering thyme which can cause it to turn brown.
- Slow draining soils. Thyme plants will grow best if your replicate the sandy soil conditions of the Mediterranean. Thyme prefers porous, well draining soil that has an high sand or grit content which allows water to drain away from the roots efficiently after bouts of watering or high rainfall.
- Pots without drainage holes in the base. A common mistake when plant thyme in pots is to plant it in pots or containers that do not have drainage holes in the base of the pot. Excess water can then not escape and the soil becomes boggy which causes root rot or promotes other fungal pathogens in the soil that turn thyme brown.
Save Thyme That is Turning Brown Due to Moist Soil
One of the most common mistakes (and the most common reason why thyme plants turn brown) of caring for Mediterranean herbs is over watering but slow draining soils can play a significant role in the demise of thyme as well as pots without proper drainage all of which have the affect of creating consistently damp soil.
The most important steps to revive a thyme plant that is turning brown is to:
- Scale back the watering to around once a week. Thyme prefers the soil to dry out somewhat between bouts of watering. If there has been significant rainfall, wait till the soil feels dry to a fingers depth before watering. This may seem harsh but these are the dry conditions in which thyme plants thrive! Adjust the frequency of your watering to suit the climate and conditions so that the soil always has a chance to dry out rather then remain consistently moist.
- Transplant thyme if it is planted in slow draining soils or boggy ground. Slow draining soils that retain too much moisture (such as clay soils) can promote the conditions for fungal disease pathogens that turn thyme plants brown. The best thing to do is to transplant your thyme to a pot as you can control the soil profile to ensure that the soil is porous and well draining. Amend the potting mix with around 30% horticultural sand (or grit) to ensure that the compost is porous and does not retain too much moisture which recreates the preferred conditions for thyme plants in their native environment.
- Ensure that the thyme is in a large enough pot with good drainage. Clay or terracotta pots are both good for growing thyme as they are more porous then materials such as plastic or metal and allow the soil to dry out more efficiently. Plant thyme in a pot that is at least 12 inches across and ensure there the pot has several drainage holes in the base to allow excess water to escape.
It is also advisable to transplant the thyme (if it is turning brown significantly) to a pot as the fungal pathogens can live in the soil for years and potentially reinfect new plants.
- When you are transplanting the thyme, inspect the roots. If any of the roots look brown, soft or rotten then snip off the infected section of root back to healthy growth.
- Use a sterile pair of pruners and wipe the blades after each snip with a cloth soaked in alcohol disinfectant to prevent spreading the fungus to otherwise healthy parts of the plant.
- Snip off any brown foliage and stems back to healthy growth and burn or discard any infected growth (rather then place in a compost heap).
- Discard the potting soil as this will be host to the fungal pathogen that is affecting your thyme plant.
- Replant you thyme in new soil, that is around 70% multipurpose compost and 30% sand or grit to improve the drainage. This ratio recreates the native well draining soil conditions of thyme plants which improves the chance of recovery.
- Shelter the thyme plant from excessive rainfall if possible and place the pot in full sun. After 2 weeks water the thyme with a generous soak at the roots with enough water so there is a trickle out the base of the pot.
- If your thyme plants are planted in garden boarders then treat the soil with an organic fungicide so that the fungal pathogens that live in the soil cannot reinfect new plants.
If you snip off all the infected plant tissue and plant thyme in new soil (use multipurpose compost from the garden center as it does not host fungal disease) then the thyme plant can recover in a few weeks as long as it is planted in full sun and watered only once per week at the most.
If you follow the steps to give the thyme the best chance of recovery although if the plant is most brown and badly infected it can be difficult to revive the plant back to full health.
Thyme plants (as with all Mediterranean herbs) grow very well in pots or containers as they typically have more favourable drainage then garden soil so consider growing thyme in pots, container or raised beds to prevent them from turning brown due to water stress.
High Humidity Causes Leaves to Turn Brown
Thyme plants thrive in arid climates with low humidity so it is important to emulate these conditions to prevent thyme from turning brown.
If Thyme plants are planted too close together then this can create a humid micro-climate which can encourage the disease Alternaria blight which tends to affect the lower leaves with brown spots.
Snip off any infected foliage and ensure the pruners are cleaned with alcohol disinfectant after each snip so that the fungus does not spread to the healthy parts of the plant. Burn or discard the diseased leaves and stems.
Ensure that you plant each thyme around 2-3 feet apart if you live in a humid climate to allow for airflow which reduces the risk of disease.
Planting thyme in pots is also a great way to increase the airflow and keep your plants free of disease.
Thyme Turning Brown Because of Age
Thyme is a perennial plant that can live for a few years and can survive frost in Winter (hardy to USDA zone 5) however, the leaves of thyme plants have their best flavour for only around 3 or 4 years after which the flavour and aroma is not as pronounced and the plant can turn woody and less productive in terms of growing tasty leaves.
It is not uncommon for thyme plants to begin to turn brown after around 4 or 5 years even with good care after which you may have to replace your plants.
Here is a YouTube video which shows how easy it is to grow Thyme from cuttings to replace your old plants:
- Thyme plants turn brown due to excessively moist soil around the roots. Damp soil increases the chance of fungal diseases such as root rot which turn thyme leaves and stems brown as a sign of infection.
- The damp soils that promote the conditions for fungal diseases are often caused by over watering, slow draining soils and because of pots and containers without proper drainage holes in the base.
- Save thyme plants that are turning brown by scaling back the watering and transplanting the Thyme to a pot with well draining soil. cut away any infected tissue back to healthy growth. Wipe the blades of the pruners with alcohol disinfectant after each snip to prevent spreading the disease.
- Plant thyme around 2-3 feet apart to ensure good airflow.
- Thyme can turn brown due to age as they often do not live for more then 4 or 5 years. Propagating thyme can be a cost effective solution for more thyme plants to keep a supply of plants with more leaves for cooking.