The sweet-spicy flavor of basil has made it one of the most popular culinary herbs to plant in gardens and containers. It’s generally easy to grow, but from time to time you may encounter challenges like basil leaves turning yellow. This is a common complaint and there are several potential causes including overwatering, cool temperatures, or a disease like basil downy mildew. Keep reading to learn more about why basil leaves turn yellow and what you can do about it.
How are your basil leaves turning yellow?
First, I think that it’s important to point out that the best way to prevent yellowing leaves is to provide ideal growing conditions. Learn more about basil growing basics HERE. However even the most attentive gardeners may find themselves dealing with yellowing leaves from time to time. Before you panic, take a look at the plant. A random yellow leaf is normal and not usually a symptom of a more serious issue. If the entire plant its turning yellow from the bottom up that can indicate disease. In that case I would inspect the plant closely and look at the tops and bottoms of the leaf surfaces for signs of disease like downy basil mildew.
7 reasons for basil leaves turning yellow
Here are the most common reasons for basil leaves turning yellow:
Problem 1: Too much water
Overwatering is the quickest way to kill a basil plant. This is an herb that needs well-draining soil that is kept lightly moist, but not sopping wet. Excess moisture can cause root rot which results in the plant not being able to take up necessary nutrients. In fact, yellowing leaves are one of the first signs of root rot and it’s more common in potted basil plants than garden-grown basil.
The good news is that this is an easy fix. If you’re growing basil in containers reduce the risk of overwatering by planting the seedlings in pots and planters that have drainage holes. No drainage holes = no drainage. When buying containers turn the pot upside-down to take a look at the bottom. Are there drainage holes? Are there enough drainage holes? A single drainage hole doesn’t offer enough drainage and I recommend adding more. For a ten inch diameter pot I would add six drainage holes.
Adding drainage holes is easy to do with a drill and a 1/4” drill bit (and safety goggles!) for plastic, metal, or wooden containers. It’s more difficult to add holes to ceramic or clay pots. No drill? Use a hammer and decent-sized nail to poke holes through the bottom of plastic pots. (Again, wear goggles).
Once you’ve got your container ready to go, fill it with a lightweight potting mix, not garden soil. Garden soil is too dense and compacts down with each watering. Next, water when necessary not from habit. I water my garden basil weekly when the weather has been dry. Be sure to water the plants deeply to promote deep rooting and drought-resistance. Containers need to be watered more often than garden beds, typically every day or two in summer. If you’re not sure whether your basil plants need to be watered, stick your finger into the garden or container soil to gauge moisture levels. If the soil is dry two inches down, grab your watering can.
Problem 2: Not enough light
Basil loves sunshine! Plant basil in a site that receives at least six, preferably eight hours of direct sun each day. This can be a challenge if you’re growing in pots on a balcony or in a garden with less light. If you’ve got four to five hours of sun, it’s worth trying. Just plant the seedlings in your sunniest spot. If your sunniest spot only offers an hour or two of light, opt for shade-tolerant herbs like parsley, cilantro, and mint instead. Basil won’t grow well in shade and the plants will begin to yellow and decline.
Many gardeners also want to grow basil indoors in fall, winter, and early spring. This sounds like a great idea, but growing basil indoors is actually a bit tricky. You’ll need to get the light right or you’ll soon notice basil leaves turning yellow. Start by placing pots of basil in your sunniest window, but know that even a south-facing window may not provide enough light to promote healthy growth. This is especially true if you live in a northern climate and it’s winter. There is a simple solution: a grow light. You can buy tabletop grow lights, under counter grow lights, or even spotlight grow lights. I have a 12” LED grow light tucked beneath one of my kitchen counters and use it to grow my favorite herbs, including basil, all winter long. I leave the grow light on for sixteen hours each day, using an inexpensive timer to turn it on and off.
Problem 3: Cool temperatures
Basil is a tender herb that is sensitive to cold weather. That’s why we don’t plant it outdoors too early in spring. It’s best to wait until the temperatures are reliably above 50 F (10 C) day and night. Waiting until it’s above 60 F (15 C) is even better! The plants are not going to settle in and start to grow if the temperature is too cold. Planting basil too early is a common reason for yellowing leaves.
You may also spot basil leaves turning yellow in late summer and autumn when the weather begins to cool. That’s the normal end to the season and it’s best to harvest while the plants are still in good shape and most of the leaves look healthy. Use pruners or garden snips to clip stems so you can dry the leaves or use them to make pesto.
Problem 4: Overcrowded plants
Years ago when I would to buy basil seedlings from a garden centre they were sold in four or six packs. Lately, I’m seeing a lot of basil seedlings sold in four-inch diameter pots. When I look closely at that pot I often notice that it’s not a single plant, but rather a clump of seedlings all crowded together in the center of the pot. Why is that a problem? If you plant a dense clump of basil seedlings in a pot or garden bed, they’ll grow fine initially, but after a few weeks the seedlings will be too overcrowded and begin to decline. The first symptom would be yellowing leaves. Instead of planting all those little seedlings in one clump, you need to tease them apart and replant them individually. Or grow your own seedlings, giving them proper growing space so you don’t have to deal with packed plants.
It’s also essential to avoid crowding basil plants when transplanting them into gardens and containers. Overcrowded plants compete for light, moisture, and nutrients, and are more prone to disease due to a lack of air circulation. All of these issues can trigger yellowing leaves. Most types of basil should be planted eight to twelve inches apart.
Problem 5: Plant disease
It was only a few short years ago that I first encountered basil downy mildew in my garden. One day my basil plants looked fine, the next day I noticed some yellowing leaves. Soon, the plants were dead. The culprit, basil downy mildew, is a widespread disease that affects the leaves and stems of basil plants. As noted above, the first sign is yellowing leaves. Flip those leaves over and you’ll likely see a gray-purple fuzz on the bottom surface. This fungus-like organism spreads in the wind but can also be introduced to your garden through infected seeds or seedlings. If you’ve diagnosed your basil with downy mildew, harvest all unaffected foliage and pull the plants. There is no treatment.
To reduce the risk of diseases, like basil downy mildew, plant resistant varieties like Rutgers Devotion DMR or Prospera DMR. I’ve switched to growing these resistant-varieties and have had great success! Pairing resistant varieties with ideal growing conditions and proper spacing is a smart way to encourage a bumper crop of basil. Also keep in mind that splashing water can spread disease so avoid wetting the entire plant when you water. I try to direct water to the soil. Basil downy mildew isn’t the only disease of basil. Other common diseases include fusarium wilt and bacterial leaf spot.
Problem 6: Pests
While basil is generally not favored by large pests like deer, rabbits, and groundhogs (its foliage is too strongly scented and flavored), it can be prone to insect pests like Japanese beetles, aphids, or mealy bugs, as well as slugs. Holes in the leaves are obvious indicators of pest damage, but yellowing leaves can also be a sign of insects like aphids. Aphids are small insects that are green, gray, or brown and suck the plant juices from leaves. This damage can cause leaf distortion as well as yellowing.
If you notice yellowing leaves, the first step is always to look closer. If the issue is aphids, they are likely clustered on new growth or hiding beneath leaves. Aphids are preyed upon by ladybugs and lacewings, as well as other beneficial insects so don’t take action if you notice these insects among the aphids. If no beneficials are present, you can knock the aphids off the plant with a jet of water from a hose nozzle. Repeat a couple of times a week as needed.
Problem 7: Lack of nutrients
Yellowing basil leaves in summer could be a sign of nutrient deficiency. Like most plants, basil requires a steady supply of nutrients to grow plenty of flavorful foliage. Before planting basil in my raised beds I top-dress the soil with an inch of compost and add a slow-release organic vegetable fertilizer around the plants. Be sure to follow the application directions on the package.
When prepping containers I fill the pots with a mixture of potting mix and compost at a ratio of two-thirds potting mix and one-third compost. I also add a slow-release organic fertilizer to containers to ensure the plants have all the nutrients they need for the coming months.
By mid to late summer even container grown basil can be prone to nutrient deficiency as frequent watering flushes nutrients from the potting mix. The solution is to fertilize basil plants with a liquid organic fertilizer every two weeks, or as instructed on the product label.
Now that we’ve covered the main reasons for basil leaves turning yellow, here are more articles on growing great basil:
- How to trim basil for big, bushy plants and larger harvests
- Learn how to grow basil from cuttings
- Types of basil to grow
- How to grow a great crop of basil
Are your basil leaves turning yellow?