Rosemary is one of my favorite herbs because of its pleasant aroma. Sadly, that scent can weaken and disappear once its needle-like gray-green leaves start turning black. What even causes rosemary leaves to become black? Can you prevent it?
The leaves of rosemary shrubs can turn black because of 1) overwatering, 2) poor drainage, 3) high humidity, 4) insufficient light, 5) low temperatures, 6) pest infestation, and 7) plant diseases. Black rosemary leaves are, regardless of cause, generally not recommended for cooking and eating.
For foliage plants, too much light is more likely to cause black leaves than too little light. For rosemary, the opposite is true. Find out why this is the case by reading until the very end!
Water accumulating in the soil will cause rosemary leaves to turn soft and black before falling off. This often happens due to overwatering and frequent heavy rainfall.
One thing to keep in mind with growing rosemary is that it’s native to the Mediterranean region. In other words, it deals better with drought than it does with torrential downpours.
So it’s not that surprising that overwatering—whether by man or Mother Nature—is the number one cause of blackened rosemary bushes.
After being kept in waterlogged soil, a woody herb like rosemary which thrives in rather dry climates will inevitably start deteriorating.
These very same conditions also serve as favorable breeding grounds for microorganisms that cause root rot such as Berkeleyomyces, Pythium, and Rhizoctonia.
In the earlier stages of root rot due to overwatering, rosemary leaves will first turn yellow. Then, it will begin turning black from the leaf tips until the damage extends to the rest of the plant and it dies.
Avoid watering rosemary too frequently as it is sensitive to excess moisture. Never water it while the top 1–3 inches of the soil is still moist.
When growing it outdoors, it’s best to grow rosemary subshrubs in containers for easy transfer in case of heavy rains. This will allow gardeners to freely move and protect their rosemary plants from the often-unpredictable elements.
2. Poor Drainage
Poor drainage will cause rosemary leaves to darken and drop. Rosemary plants kept in plastic pots with little to no drainage and with easily-compacting clay soil are likely to retain too much water.
In terms of planter material, clay is indeed ideal for rosemary as it can wick away excess water from the soil. Hence, the root system and the plant as a whole are kept dry.
Read more about this in our article on terra cotta pots!
Clay soil as a growing medium, however, serves as a great detriment for cultivating a lush and healthy rosemary shrub. They soak up lots of water but become compacted quickly.
Moreover, rosemary prefers well-draining rocky soils over any growing medium that holds too much water over long periods.
But even with the best soil mix and growing container, your rosemary will eventually go black and die if it can’t readily drain itself of too much water. Its roots and crown are bound to rot.
When clay pots are out of your budget or hard to come by, be sure to check the drainage holes of your rosemary’s plastic pot. Remove any debris that could cause clogging.
It’s also advisable to add holes along the sides of your plastic container—much like air-pruning pots—to improve drainage. For mine, I simply used a soldering iron to poke a few.
Amend your potting mix as well if you notice that it gets too compacted, as is to be expected of clay-rich soil. Mix in some perlite (here on Amazon) or pumice into your packaged mix.
3. High Humidity
High humidity, often exacerbated by rain and overhead watering, will lead to black patches on rosemary leaves and stems as a result of mold development.
Needless to say, overly humid conditions often lead to mold growth.
Indoors, humidity levels consistently raised above 60% can lead to significant molding which can lead to the death of rosemary and other non-tropical plant species.
Because rosemary hails from a naturally arid region, it’s especially prone to developing fungal diseases. (I’ll talk more about such conditions in greater detail later on.)
This is to be expected when home gardeners water their rosemary shrubs from above late in the day, boosting the humidity around them and not allowing enough time for them to dry.
To add, while rosemary is a great companion for other herbs and vegetables, such as onions and carrots, it can suffer if it’s packed too closely with other plants—cutting off air circulation.
Check out which plants are perfect companions for rosemary in the garden!
Water rosemary early in the morning directly through its soil to allow it to efficiently absorb the moisture it needs through its roots without raising the humidity too much.
If you’re growing it indoors, make sure to place it away from other plants that need more water and humidity such as calatheas. When necessary, place a dehumidifier or fan near your pots of rosemary and other Mediterranean herbs to keep them relatively dry.
In the event that rosemary leaves do form patches of mold due to high humidity, simply trim off all affected parts with a clean pair of garden shears. Disinfect it with alcohol after each cut to prevent further spread of mold spores.
4. Insufficient Light
The needle-like leaves of rosemary bushes can turn black and dry if it receives less than 6 hours of direct light. Leaves die and fall off the stems due to the lack of light.
Rosemary, when grown as an annual, naturally slowly turns black before completely dying off during the colder months of the year.
Etiolation isn’t the only problem to be expected in such growing conditions. You see, insufficient light can also worsen issues with drainage and humidity.
With less direct light exposure, rosemary can’t dry up as efficiently which boosts the humidity around it for a longer time—which, as I’ve previously explained, are both greatly detrimental.
Simply put, for a sun-loving herb like rosemary, a lack of light can easily prove to be a death sentence.
Similar to other culinary herbs, rosemary is a high-light plant. Meaning, it grows best in at least 6–8 hours of full sun.
But when they’re grown indoors and even the natural light from south and southwestern windows is no longer enough, they can be supplemented with full-spectrum grow lights.
A 24W grow light like the one below from Amazon is perfect when growing rosemary indoors.
Hang it about 6–12 inches (15–30 cm) above your rosemary plants. Keep it on for about 12–14 hours each day to encourage prolific growth.
5. Low Temperatures
Freezing temperatures below 27–30°F (-3–-1°C) injures rosemary leaves, causing them to turn black. As a warm-season plant, this woody herb prefers mild winters.
Pair with over-saturated soil, low temperatures can quickly lead to dark brown and black leaves and weak development before the rosemary finally dies.
Such damage becomes even more noticeable on more tender varieties of rosemary like Tuscan Blue. They simply can’t tolerate freezing weather as well as frost.
Cold dark winters will kill off rosemary plants grown outdoors. This is also why rosemary turns black in the fridge.
In the Mediterranean, hard freezes are not a common occurrence. Due to this, rosemary and similar herbs are not accustomed to harsh winters.
It’s because of this that rosemary bushes typically die around winter when they are grown in northern states like Michigan and New York.
Otherwise, they can be grown as evergreen perennials in hotter states down south including California and Texas.
Choose a more winter-hardy variety such as Arp if you’re planning to grow your rosemary outdoors, despite experiencing very cold winters.
If you’re able to, overwinter your rosemary indoors so that you won’t have to start from scratch again once spring comes around. Make sure they don’t dry out too much indoors to ensure their survival and provide them with enough light exposure.
When you’ve run out of space in the house, or you simply want to risk killing your rosemary by pulling them from the ground, use a season extender like this one from Amazon.
6. Pest Infestation
Dark-colored pests, their poop, and their damage result in rosemary leaves turning black from a severe infestation. This includes aphids, scales, spider mites, slugs, and rosemary beetles.
Although rosemary isn’t as commonly affected by pest problems as other edible and fruiting plants, it can rapidly deteriorate in serious cases of infestation.
Common pests that attack rosemary are
- Rosemary beetles
- Spider mites
In the beginning, it can be hard to even see that they’re taking refuge within the leaves and stems of your rosemary. So it’s not unusual for gardeners to only realize they’ve got a pest problem once the damage has been done.
These insects will feed on your rosemary plant, causing necrotic black spots and patches in the long run.
Such pesky insects may also bring with them bacteria, viruses, and mold spores which could further worsen the state of your already-infested rosemary plants.
Regularly inspect your rosemary plants to see if any pests have taken shelter in them and remove them. Cut off affected leaves and stems and discard them properly.
If you do see any, spray them with soapy water. Make it with cattle soap or regular dish soap. Spray them either early in the morning or in the evening to prevent leaf burns.
Pesticides containing Bacillus thuringiensis, carbaryl, capsaicin, malathion, or pyrethrum are also great for keeping edible plants such as rosemary pest-free and safe to eat.
7. Plant Diseases
Various bacterial and fungal diseases can cause rosemary leaves, stems, and roots to turn black due. Often, they are aggravated by pest damage and excessive humidity.
Tiny black fuzzy spots popping all around rosemary plants are often caused by such plant diseases. In some cases, clumps of leaves or the entire plant may also get darkly discolored.
Below are common plant diseases that cause rosemary leaves to turn black:
- Botrytis blight
- Cottony soft rot
- Crown gall
- Downy mildew
- Root rot
Now, these diseases don’t usually appear incredibly dark straight from the onset.
More often than not, the damage they inflict on rosemary plants starts out with light discoloration—usually a pale yellow or tan color.
Already damaged rosemary leaves are more likely to succumb to such diseases as harmful microbes are readily able to enter the plant and wreak havoc on it from the inside too.
Closely monitor and manage the growing condition of rosemary plants to prevent them from suffering from any kind of plant disease. Proper care is the number one preventive measure.
Don’t overwater your plants. Avoid making unnecessary lesions and wounds on their leaves and roots as much as possible as well. Use sharp pruning shears for clean pruning cuts.
Water rosemary directly through the soil, keep the humidity under 60%, give it at least 6 hours of direct light, and make sure that the temperature of the air around them stays above 30°F (-1°C).
Can you eat rosemary with black spots?
It’s best to avoid eating rosemary sprigs and leaves with black spots on them as those could be caused by harmful pests or signs of serious mold development and plant disease. Though there are no specific dangers associated with eating rosemary leaves with black spots, it’s best to avoid them altogether, especially when their cause is unknown.
Is rosemary bad when it turns black?
Harvested rosemary sprigs, whether fresh or old, are generally considered to have gone bad if they have already turned partially or completely black. These sprigs have likely already lost most of their essential oils which are responsible for their distinct fragrance and flavor profile. Hence, it is not advisable to use black rosemary for cooking.
Summary of Why Rosemary Leaves Turn Black
Too much water in the soil, not providing good drainage, excessively high humidity levels, lacking light exposure, unfavorably low temperatures, serious pest infestation, and several bacterial and fungal diseases can cause rosemary leaves to turn black.
Avoid having black rosemary leaves by holding back on watering too much or too often, ensuring proper drainage of the soil, lowering humidity levels, providing supplemental lighting, protecting the plant from low temperatures, regularly monitoring for potential pests, and ensuring proper overall care for the herb.