If you grow basil on the kitchen windowsill, you know the leaves are great for a morning gargle. They also add zest to soups, salads, desserts, meat, or seafood dishes. Basil also helps treat head colds, stomach gas, and insect bites.
The most common reasons why a basil plants dies are:
- Aggressive Pruning
- End of life
- Inadequate light (usually too low)
- Inadequate temperature (usually too low)
- Inadequate watering (usually too much)
- Inadequate humidity (usually too high)
- Unbalanced nutrients (usually too little)
- Salt buildup
- Lack of root space
- Six common pathogens and bacterial infections
If your basil stems turn dark and you feel puzzled, concerned, or even alarmed, this article is for you. The good thing is this: if you know why it’s happening, then you know what you can (and can’t) do about it.
With enough water and sunshine, you can grow basil in a pot on your kitchen window sill. You can pluck leaves for an immediate cool touch in iced tea, salads, or in comfort soups and dishes.
When basil stems or leaves turn brown or black, you may be looking at a natural process or it may be an indication of a serious problem. Here’s a summary of what you should know.
To begin on a bright note… in many cases, when basil stems are turning darker in color, it’s a natural process that is not a cause for worry. For example
1 – Aggressive Pruning
When you pluck leaves or cut off leaves, branches, or flowers from your basil plant, the “wound” is a trauma that tries to heal itself. The trauma seals itself and shows a darker brown color.
However, if the basil is pruned way too much leaving less than a third of the original stems or, even worse, no leaves are left, then the basil will probably not make it.
If this is your case check out the guide on how to prune basil the right way.
2 – End of Life
Basil cannot survive low temperatures. In winter, leaves and branches turn dark and die off, to conserve resources and leave the seeds with water and nutrition to grow in the spring.
Moreover, basil is an annual plant that lasts at most 10 months in ideal conditions despite all the attention and cares it might receive.
3 – Inadequate Temperaure
Low temperature is a typical cause of basil dying, especially when outdoor. When temperatures drop to less than 50°F or 10°C, basil leaves turn dark and die.
Basil originally comes from the tropics of Southeast Asia and Central Africa. It can thrive in temperatures as high as 90°F (32°C). Some types of basil can grow at 43°F or about 6°C. The ideal temperature for your basil is between 72.5° and 82.4°F or from 24° to 28°C. Indoor gardeners agree that this temperature range gives you maximum harvest for indoor basil plants.
4 – Inadequate Light
Sunshine hours: Indoors, basil plants need an average of 7 hours of direct sunshine. However; in hot climates such as in the South and Southwest parts of the USA, basil plants should be shaded in the afternoons when it’s too hot otherwise you will end up with sunburn.
Solutions: In case of low light level a good grow light is recommended. The 60-Pc 10W Full Spectrum LED Grow Light for Indoor Plants (Amazon) is a good choice.
A cool fact!
Basil needs different night and day temperatures (thermoperiods). You see, if you grow basil with a temperature that is constant all the time, it won’t grow well. That’s because, when day turns to night and temperatures drop to between 64.5° and 71.5 °F (or 18° to 22°C), basil plants manufacture highly flavored and aromatic oils.
6 – Inadequate Watering
Under-watering can cause drooping, wilting, and stems drying out and turning darker in color. Severe underwatering can cause basil plants to die.
Too much water & root rot – When basil gets too much water – or poorly drained soil – the fungal disease can cause root decay and root rot, also known as “damping off”, which kills the plant. Symptoms include wilting leaves, darkening stems, and spongy, smelly roots. One effective strategy is to let the soil dry out (but not bone-dry) between waterings.
7 – Inadequate Humidity
Basil plants grow best in the air that is about 60% to 65% humid and, as mentioned, a temperature level between 70 to 80 degrees Fahrenheit (about 24° to 28°C). More often than not, the problem is a too high humid environment, especially if you leave the basil plant close to the sink or any warm source of water.
The solution can go from simply placing the planter in a drier location (avoid the kitchen or bathroom, even if you have a large window) and place it in front of a large windowsill far from a kettle or any other water source that can evaporate.
8 – Lack of Nutrients
One reason why your basil plant can turn dark brown or black is a lack of phosphorus. You can use a rock phosphate fertilizer like this one or a bone meal feed for plants as this good brand. Remember that bone meal might smell quite strongly, so avoid it indoors.
If you want to know the details on nutrient deficiency/excess, check our full guide on nutrients.
Too much fertilizer – Sometimes, confusion or absent-mindedness can result in gardeners feeding their plants too much fertilizer, which can permanently damage your basil plants. You should always and carefully follow the directions on the label.
For more details on the causes and effects of too much fertilizer, click here.
8 – Salt Buildup
The mineral salt ions in inorganic and synthetic fertilizers are not all absorbed by plants. Instead, they remain in the soil and these fertilizer salts (no, they’re different from table salt) slowly accumulate in the soil. More fertilizer means more salinity in the soil, which can kill plants.
A solution for salt buildup is flushing or leaching the soil with running water can help remove salt buildup.
9 – Lack of Root Space
Basil is often sold as a bunch of small plants that need to be repotted as they grow. Failure to report, using too-small pots, or planting too many in a pot can severely restrict the space for root growth and development. When roots can’t draw in enough water and nutrients, or when roots are unable to spread, plants become unhealthy and the plant can struggle to grow.
Just a larger pot for your basil (plastic or ceramic) will do the trick!
Fusarium Wilt – A fungus in the soil or seeds of infected basil plants can infect your basil with fusarium wilt. Symptoms include brown spots or streaks on the stem, severely twisted stems, stunted growth, wilted and yellowing leaves, leaf drop, and the plants eventually die.
Downy mildew or gray mold – When basil turns black, it may be caused by the Peronospora belbahrii pathogen found in contaminated seeds, infected transplants, or wind-borne spores. Green leaves turn yellow, with dark gray or purple fuzzy growth underneath.
Mint rust fungus – For those who grow mint, rust fungus is a disease caused by the Puccinia menthae fungus.
Southern blight – Also known as southern root, southern stem rot, or southern wilt, this disease causes basil to wilt and die. When soil is moist and warm in summer, the Sclerotium rolfsii fungus spreads a network of white filaments (hyphae or mycelia) around the lower stem and roots of basil. You will see leaves wilting, lower leaves are discolored, and the plant dies.
Bacterial leaf spot or Basil shoot blight – Caused by the fungus Pseudomonas cichorii, this disease causes dark streaks to appear on the stems of older basil plants as well as leaves spotting and dropping off.
Cerospora leaf spot or Circular leaf spot – This disease shows as dark brown spots and is caused by the fungus Cercospora ocimicola often due to too much moisture on the leaves. The problem often appears in the rainy season, or in plants with too high moisture and poor air circulation.
These pathogens can be avoided by spacing plants far apart to provide good air circulation, and low moisture to keep leaves dry.
Some gardeners recommend the use of products containing thiophanate-methyl, myclobutanil or chlorothalonil. Others prefer to use baking soda mixed with light horticultural oil, while a few others prefer to use sulfur or neem oil.
How long will it take for my basil to regrow? Regrowing basil can take anytime from 2 up to 4 weeks. The secret is this: trim, cut, prune your basil so that one cut stem grows to new stems. Cut those, and double the number of stems again and again.
How do I know if my basil plant is overwatered? The signs of an overwatered basil plant include roots that are black or brown and roots that are soft (mush) and smelly. For a full guide on how to water basil check the full guide on basil watering.
Fungus attacks can cause stems to discolor or show dark streaks or blotches while bacteria attacks cause dark spots.
If you water your basil too much, the roots will rot and the leaves will droop. Too little water and the soil will dry up and the plant will die. Take time to learn just how much water is right for your plant.
Prevention is a primary strategy. Buy disease-free plants. Use clean soil and pots when potting, repotting, or transplanting.
Also, take time to provide your indoor basil plants with the correct conditions for healthy growth, and check them often for any of the problems summarized here.
Do these, and you’ll be a happy gardener with happy houseplants.
“Antifungal activities of basil (Ocimum basilicum L.) extract on Fusarium species” by S. Kocić-Tanackov, et al in African Journal of Biotechnology
“Reduction of fusarium wilt of hydroponically grown basil by Fusarium oxysporum strain CS-20” by D.R. Fravela, and R.P. Larkin in Crop Protection
“Basil Problem – Fusarium Wilt” by D. Roos, North Carolina State University
“The fragrant mint family dominates the herb world” by B. P. Lawton in The Christian Science Monitor
“Growing Mint” by S. Albert in Harvest to Table
“MInt in the garden” by K. Buckland & D. Drost, Utah State Cooperative Extension
“Solving Black Spots on Basil Leaves” by Gardening Channel
“Basil Plant Leaves & Stem Turning Black – What to Do” by Organic Lesson
“Basil: A Source Of Essential Oils” by J. E. Simon, et al in Advances In New Crops
“Diseases Of Basil And Their Management” by A. Garibaldi, et al in Plant Disease
“Basil: a source of aroma compounds and a popular culinary and ornamental herb” J. E. Simon et al in Perspectives On New Crops And New Uses
“Ocimum sp. (Basil): Botany, Cultivation, Pharmaceutical Properties, and Biotechnology” by O Makri & S. Kintzios in Journal of Herbs, Spices & Medicinal Plants
“Effects Of Nitrogen Fertilization On The Phenolic Composition And Antioxidant Properties Of Basil (Ocimum basilicum)” by P. M., Nguyen & E. D. Niemeyer in Journal Of Agricultural And Food Chemistry
“Basil in the Garden” by L. Pearson & D. Drost, Utah State University
“Robotic Vehicle for Automated Detection of Leaf Diseases” by A. Nooraiyeen in 2020 IEEE International Conference on Electronics, Computing and Communication Technologies (CONECCT)
“Daytime Solar Heating Controls Downy Mildew Peronospora Belbahrii In Sweet Basil” by Y. Cohen & A. E. Rubin in PLoS One
“Suppression Of Basil Downy Mildew Caused By Peronospora Belbahrii Using Resistance Inducers, Mineral Salts And Anti-transpirants Combined With Different Rates Of Nitrogen Fertilizer Under Field Conditions” by E. Ghebrial & M. Nada in Egyptian Journal of Phytopathology
“Sclerotinia Rot on Basil Caused by Sclerotinia sclerotiorum in Korea” by S. S. Hahm, et al. in Research in Plant Disease
“Managing Air Temperatures For Basil Growth And Development” by K. J. Walters and C. J. Currey in Greenhouse Grower
“The Perils of Over-Fertilizing Plants and Trees” by K. Smith, UCCE El Dorado County
“Circular leaf spot of sweet basil caused by Cercospora guatemalensis new to Japan” by J. Nishikawa, et al in Journal of General Plant Pathology