Basil is a deliciously sweet and spicy herb used in cuisines around the world. A member of the Lamiaceae, or mint family, it has a wonderful aroma and versatile uses in the kitchen.
Basil is an easy herb to grow, but it’s not immune to problems. If you find yourself trying to make pesto and discovering yellow basil leaves, we’re here to help!
There are lots of reasons basil starts to yellow. It could be bolting (going to seed) due to weather conditions or not harvesting enough.
Your yellowing plants could have a disease or be suffering from nutrient deficiencies, over-fertilizing, or over-watering.
An unbalanced soil ecosystem could also be the culprit, or the growing conditions could simply not have enough sunlight or warmth.
Fortunately, these problems are easy to avoid and fix. The secret to growing great basil is simply paying attention.
Your plant will tell you when it is unhappy and there are easy steps you can take to bring it back to optimal health.
Let’s dig into why your basil is yellowing and a few simple ways to bring it back to life!
1: Your Basil Begins To Bolt
Basil is an annual herb with many different varieties such as Italian basil, Thai basil, and Holy Indian basil.
Regardless of what type of basil you are growing, it typically wants to “bolt” or go to flower quickly in hot weather.
This transition from vegetative (leaf growth) to reproductive (flower and seed growth) signals the plant trying to complete its life cycle. Once it starts putting energy into flowers, the lower leaves may begin to yellow.
Simply pinch basil flowers off 1-2 times per week in the summer, or as they appear. I prefer to harvest my basil by pinching the tips.
This encourages the plant to branch and bush out more while also preventing it from flowering. You kill two birds with one stone- harvest and stop bolting at the same time!
2: Basil Infected With Downy Mildew
Basil is susceptible to a common plant disease called downy mildew. The pathogen is a fungus-like organism called Peronospora belbahrii that thrives in moist, humid conditions.
If there is not enough air flow between your plants, you may start to notice burned yellowing on leaves.
There could also be a blackish grey color on the undersides of basil leaves. These leaves will look dusty and then start to yellow and die.
Prevention is key for downy mildew. Grow basil with enough spacing between plants and plenty of air circulation.
Try to use drip or soaker hose irrigation so that water does not settle on the leaf surfaces. If possible, remove and isolate infected plants as soon as possible.
If downy mildew has already started taking hold, a diluted neem oil spray is the best organic treatment. You can also make a homemade anti-fungal spray using 1 tbsp.
Baking soda, 1 tbsp. Castile soap (like Dr. Bronners), 1 tbsp. Vegetable oil, and 4 liters of water. Mix these thoroughly in a spray bottle and apply liberally once a day on the infected leaves.
3: Nutrient Deficiency
Potassium (K) deficiency is another very common reason for yellowing basil leaves. This mineral is essential for plant growth and may not be present in your soil if you haven’t added compost or microbial inoculants to the garden.
To identify potassium deficiency in basil, check to see if the yellowing is happening close to the leaf veins.
Kelp meal, wood ash, and greensand are excellent organic sources of potassium. You can also use manure-based compost or an all-purpose fertilizer.
Be sure to follow the application instructions to avoid over-fertilizing your basil.
4: You’re Over-Fertilizing
Too much fertilizer can cause “leaf burn” or yellowing and browning of your basil leaves. If the lower leaves are wilting and yellow, then you might have over-fertilized.
You may also notice the fertilizer crusted on the soil surface. These are all bad signs that you may have gone a little overboard on your frequency or dosage of fertility.
To save an over-fertilized basil plant, remove the harmed leaves and try scoop as much of the over-fertilized soil away as possible, without damaging the plant roots.
Replace with fresh potting soil. Stop fertilizing and give the plant a continuous source of water to flush out the excess fertility (but remember, too much fertilizer can also be damaging to the environment or nearby waterways).
It is best to avoid synthetic fertilizers (like Miracle Gro and other nitrates) because they are “instantly available” and very harsh on plants.
Organic fertilizers like compost, worm castings, or diluted fish emulsion are slow-release and less likely to cause fertilizer burn.
5: You Are Overwatering Your Basil
In general, basil is a hot-weather crop that isn’t super thirsty. Excessively wet soil can cause basil roots to start rotting. This will manifest above-ground as sad yellowing leaves.
Over-watering is the most common reason for yellow basil leaves because many people keep basil in pots in their kitchen.
If your pot lacks drainage holes, or if the soil is very compacted inside the pot, water could be pooling up in the root zone. You could also just be watering too frequently.
Check that there is plenty of drainage in the pot and/or potting mix that your basil is planted in. Stop watering for a few days to let the soil dry out a bit. Before watering again, always check the soil moisture with your finger.
It should never be soggy nor bone dry, rather a happy medium of moisture level. Don’t water the basil plant unless the soil is beginning to dry out.
6: Soil Around Your Basil Plants Has Inadequate Aeration
As mentioned above, soil compaction can cause watering issues, but it can also create anaerobic (without oxygen) conditions.
This is an environment where pathogens and diseases thrive. It can also lead to plant stress, rotting roots, and yes- yellow leaves!
Compacted soil is suffocating to any plant. Basil especially prefers an airy oxygenated root zone.
If you are growing in a pot, transplanting the basil into a fluffier soil is an easy fix. Look for mixes with lots of peat moss, compost, and perlite. Sometimes I add some small rocks and pebbles at the bottom of the pot to also aid in drainage.
If you are growing in the ground or in raised garden beds, fixing soil compaction takes a little more effort and needs to be done before planting again.
For soils with lots of clay, adding compost is your best bet. You may also want to invest in a broadfork to loosen the soil at a deeper level.
Mulching and incorporating compost every season will build up the organic matter and increase aeration over time.
7: Unbalanced Soil Ecosystem
Yellowing basil leaves can be a sign of unbalanced soil ecology. Most gardeners don’t realize that soil is both the digestive system and the immune system of our plants.
There are billions of beneficial microbes (like bacteria, fungi, and protozoa) in soil that act like a defense army against pests or pathogens.
They reinforce plant immunity to these threats and make plant food available to our crops.
When those microbes get out of whack, the beneficial army is weakened and more “bad guy” microbes can come in.
Those could include the downy mildew pathogen mentioned above, or a root rot pathogen, or any manner of pests.
Just like an above-ground ecosystem, the soil needs healthy competition to keep everything in check.
If your soil is heavily compacted, treated with chemical herbicides or pesticides, or lacking in organic matter, the soil ecosystem may be unbalanced and harming your basil plants.
Organic matter in the form of vegetable compost, composted manure, compost teas, kelp meal, leaf mulch, or worm castings is the best possible cure for yellowing basil.
The results will not be instant, but the investment will pay off in a month or so as beneficial soil microbes return to your soil.
There are also microbial soil inoculants and compost teas available for purchase at organic gardening stores to help boost the soil microbiome more quickly, resulting in happier healthier plants.
8: There’s Not Enough Sunlight Nearby
Basil is most definitely a full-sun plant. It thrives in areas with warm sunny summers and doesn’t really like shade. It typically requires 6 to 8 hours of full sunlight per day.
If your basil plant is under the shade of tomatoes or tall trees, it may begin to yellow. Your best bet is to transplant to a sunnier area or (if possible) prune the plants that are shading it out.
Only plant basil seedlings in the sunniest parts of the garden. If growing indoors, put pots by south facing windows or use supplemental grow lighting.
9: Cooler Temperatures
Basil loves summer warmth and faithfully produces all season long. It will start to yellow or look unhappy as autumn approaches, and dies at the first frost.
When cool temperatures start to set in, you may notice your basil yellowing, slowing growth, or dying back.
In the spring, always wait until temperatures are reliably above 50 or 60 degrees Farenheit before planting basil outside.
In the later season, use row cover such as Agribon to add additional warmth and protection to your basil as the summer ends.
You can also plant basil in a greenhouse or move indoors to a south facing warm window.
FAQs About Yellowing Basil
Can you eat yellow basil leaves?
Yes, yellowing basil leaves are still safe to eat but they may not have as great of a flavor.
What are the blackish spots on my basil plant?
Blackening areas of basil leaves can be caused by fungal disease, downy mildew, blight, or pest damage. The best course of action is removing infected leaves and using the anti-fungal organic sprays described above.
Why does my basil look wilted even if it has water?
Excessive wilting is actually a sign that you may be overwatering. This creates anaerobic conditions (lacking oxygen) that suffocate the plant’s roots.
Stick your finger in the soil and if it feels soggy, cut back on watering until it dries out.
Before watering again, check at least 4-6” deep in the soil to be sure the water is fully draining through. If growing in a pot, be sure it has enough drainage holes and the potting soil is fluffy and textured enough to prevent compaction.
Cameron Jenkins moved from the city to a small farm where he lives with his wife and daughters. The farm is divided between the garden, pastures, hayfields, the start of an orchard, and 13 times as many pets as people. Their farm vision is to grow produce and raise animals in unison with nature. When Cameron is not farming (or writing about it) he spends his time playing with his children, reading, cooking, and napping with his pet pig.