Basil is a really popular herb for gardeners everywhere. It’s easy to grow, produces amazing, fragrant leaves and flowers, and goes in a ton of different culinary dishes. My personal favorite way to use basil is in making pesto. But all plants can be susceptible to problems and basil is one of them. One of the more common issues gardeners have is basil leaves curling either up or down.
It is important to remember, especially if you’re new to growing basil, that the leaves of the basil plant are not perfectly flat. They do have a slight downward curl. They don’t tend to curl up, though. Upward curl is a sign of problems. Downward curl associated with discoloration and failure to thrive is also a strong sign of a problem with your basil leaves.
Why basil leaves curl
The tough thing about figuring out why your basil leaves are curling up is due to the number of causes. There are typically six reasons why basil leaves start curling upward or downward. Recognize that this symptom is not a sign of normal health in your basil plant and should be diagnosed and addressed as soon as possible.
Irregular and inadequate watering
This is probably the most common reason that basil leaves begin to curl up. Too much water isn’t a good thing. Not enough water is also not great. Providing a thorough watering once the top inch or two of soil is dry is the correct way to water. If you give the right amount of water too infrequently, this can also cause problems for the basil in your garden. Be sure that you are watering adequately and regularly.
This is the second most common reason for basil leaves curling upward. Basil is a full-sun plant, and when placed in shadier areas that get fewer than 6 hours of sunlight per day, the plant can begin to fail. The good news is that, provided your plant is still alive, it’s not too late to correct this problem. Dig up and move your basil plant to a sunnier location to fix this problem.
Some pests can cause your basil plants to curl upward, but most of the time, the reason your basil leaves are curling down or in on themselves is due to pests. Basil is most susceptible to aphids, spider mites, and scale. Whitefly can also attack your basil plants. Using an insecticidal soap can help ward off these pests and get your plants growing properly again. You can also hand-pick these pests off your plants, but it’s very difficult to get all of them this way.
Diseases are a less common reason for leaf curl, but can still cause this symptom in your basil plants. Fungal diseases, like powdery mildew, downy mildew, and botrytis can harm your basil leaves. This mildew tends to look like white, gray, or ‘dirty’ powder on the underside of leaves. Some gardeners have reported that a mix of 1 teaspoon baking soda in 1 quart of water sprayed onto the leaves of basil takes care of leaf curl caused by fungal diseases.
Sometimes called fertilizer burn, excessive fertilizing can also cause damage to your basil plants that can result in curling, dying basil leaves. This happens when plants receive more fertilizer than they can process and use, which interferes with your basil plant’s ability to process nutrients. Common signs of fertilizer burn include leaf curl, discoloration, and plant death. You can begin to resolve fertilizer burn by removing dead foliage from your plants, removing any excess fertilizer from the top of the soil, and providing thorough watering to help wash the unnecessary fertilizer away.
Suboptimal temperatures can also cause basil leaves to curl, both up and down. Basil is a warm-weather plant that struggles to thrive when nighttime temperatures drop below 50 degrees Fahrenheit. If it’s still getting into the mid to low 40s at night, whether it be spring or summer, it is probably a little too chilly to have basil planted outside. This can cause leaf curl.