Why Basil Leaves Turn Yellow: Discover the Secrets Behind Yellowing Basil

Basil, with its delightful sweet-spicy flavor, has become a beloved culinary herb in gardens and containers everywhere. While it’s generally easy to grow, there are times when you might encounter the frustrating issue of basil leaves turning yellow. Don’t worry, you’re not alone! This is a common problem, and there are several potential causes, including overwatering, cool temperatures, or even a pesky disease like basil downy mildew. In this article, we’ll explore the reasons behind basil leaves turning yellow and provide you with practical solutions to tackle this issue.

The Mystery of Yellowing Basil Leaves

Before we dive into the main causes of yellowing basil leaves, it’s essential to note that the best prevention is to provide your basil plants with ideal growing conditions. However, even the most attentive gardeners may find themselves confronted with yellowing leaves from time to time. Don’t panic just yet! Take a closer look at the plant. If it’s only one random yellow leaf, that’s perfectly normal and not usually a cause for concern. However, if the entire plant seems to be turning yellow from the bottom up, that could indicate a disease. In such cases, inspect the leaves carefully for signs of issues like downy basil mildew, paying attention to both the upper and lower surfaces of the leaves.

Encourage healthy growth of basil by planting in a site with full sun and well-draining, fertile soil.
Encourage healthy growth of basil by planting in a site with full sun and well-draining, fertile soil.

Unveiling the 7 Main Culprits Behind Yellowing Basil Leaves

Let’s delve into the seven most common reasons why basil leaves turn yellow:

Problem 1: Water Woes

One of the quickest ways to kill a basil plant is through overwatering. This herb thrives in well-draining soil that is kept lightly moist, but not saturated. Excess moisture can lead to root rot, causing the plant to be unable to absorb essential nutrients. Yellowing leaves are often the first sign of root rot, which is more prevalent in potted basil plants than in those grown in gardens.

Fortunately, this issue is easily fixable. If you’re growing basil in containers, reduce the risk of overwatering by planting the seedlings in pots with proper drainage holes. Inspect your containers before purchasing to ensure they have adequate drainage. If you find only a single drainage hole, consider adding more. Adding drainage holes can be done with a drill and a 1/4” drill bit for plastic, metal, or wooden containers. For ceramic or clay pots, use a hammer and a decent-sized nail. Once your container is ready, fill it with lightweight potting mix, avoiding garden soil, which is too dense. Water your basil plants only when necessary, not out of habit. Deep watering promotes deep rooting and drought resistance. Container-grown basil generally needs more frequent watering, typically every day or two during the summer. To determine if your basil plants need watering, insert your finger into the soil. If the soil is dry two inches down, it’s time to water.

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A random yellow leaf from time to time is normal, but does merit a closer look to make sure there isn't a disease or pest issue.
A random yellow leaf from time to time is normal, but does merit a closer look to make sure there isn’t a disease or pest issue.

Problem 2: Lack of Sunshine

Basil thrives on sunshine! It craves at least six to eight hours of direct sunlight daily. If you’re growing basil in pots on a balcony or in a garden with limited sunlight, it can be a challenge. However, if you have four to five hours of sun, it’s worth a try. Plant your seedlings in the sunniest spot available. If your sunniest spot provides only one or two hours of light, consider growing shade-tolerant herbs like parsley, cilantro, or mint instead. Basil simply won’t thrive in the shade, and its leaves will start to yellow and wither.

Many gardeners love the idea of growing basil indoors during fall, winter, and early spring. While it may seem like a great plan, growing basil indoors can be tricky. You need to get the lighting just right, or else you’ll soon notice those dreaded yellowing leaves. Start by placing your pots of basil in the sunniest window available. However, even a south-facing window may not provide enough light, especially during winter in northern climates. The solution? Invest in a grow light. There are various types available, from tabletop grow lights to under-counter or spotlight versions. For example, a 12” LED grow light placed beneath a kitchen counter can work wonders. Leave the light on for sixteen hours each day, using a timer to handle the on/off schedule.

Problem 3: Chilly Temperatures

Basil is a tender herb that doesn’t appreciate cold weather. That’s why we avoid planting it outdoors too early in the spring. Wait until the temperatures are consistently above 50°F (10°C) day and night, or better yet, above 60°F (15°C). If the temperature is too cold, the plants won’t establish themselves and start growing properly. Planting basil too early is a common culprit behind yellowing leaves.

You might also notice basil leaves turning yellow in late summer and autumn as the weather cools down. This is a natural occurrence signaling the end of the season. It’s best to harvest the remaining healthy leaves before they decline. You can dry them or use them to make pesto.

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When you buy basil seedlings, try to avoid those that are overcrowded in the pot. They can be difficult to untangle and don't transplant well.
When you buy basil seedlings, try to avoid those that are overcrowded in the pot. They can be difficult to untangle and don’t transplant well.

Problem 4: Overcrowding Issues

In the past, basil seedlings were sold in four or six packs. Lately, however, it’s increasingly common to find basil seedlings crammed into four-inch diameter pots. Upon closer inspection, you’ll notice that what appears to be a single plant is actually a clump of seedlings tightly packed together in the center of the pot. Why is this a problem? If you plant such a dense clump of seedlings in a pot or garden bed, they’ll initially grow fine but eventually become overcrowded and begin to deteriorate. The first sign of this issue is usually yellowing leaves. Instead of planting all those seedlings in one clump, separate them and replant them individually. Alternatively, you can grow your own seedlings, ensuring they have sufficient space to grow without becoming crowded.

It’s also crucial to avoid overcrowding when transplanting basil into gardens or containers. Overcrowded plants compete for light, moisture, and nutrients, and they’re more prone to disease due to poor air circulation. All of these factors can lead to yellowing leaves. Most types of basil should be planted eight to twelve inches apart to avoid overcrowding.

A lack of air circulation between basil plants can encourage disease and yellowing leaves.
A lack of air circulation between basil plants can encourage disease and yellowing leaves.

Problem 5: The Disease Factor

Basil downy mildew, a widespread disease that affects basil leaves and stems, was the bane of my garden a few years ago. One day, my basil plants looked healthy, and the next day, yellowing leaves appeared. Soon after, the plants withered and died. Basil downy mildew often manifests as yellowing leaves. If you flip these leaves over, you’ll likely notice a gray-purple fuzz on their undersides. This fungus-like organism spreads through wind dispersal or via infected seeds or seedlings. Unfortunately, once your basil is affected by downy mildew, there is no treatment. You’ll need to harvest any unaffected foliage and remove the entire plant.

To reduce the risk of diseases like basil downy mildew, considering planting resistant varieties such as Rutgers Devotion DMR or Prospera DMR. I switched to growing these resistant varieties and achieved great success. Pairing resistant varieties with ideal growing conditions and appropriate spacing is a smart way to encourage a robust basil crop. Additionally, be cautious not to splash water onto the entire plant when watering, as this can contribute to disease spread. Aim to water the soil directly. Basil downy mildew isn’t the only disease that can affect basil; other common diseases include fusarium wilt and bacterial leaf spot.

The first sign of basil downy mildew is yellowing leaves, accompanied by a purple-gray fuzz on the bottom surface.
The first sign of basil downy mildew is yellowing leaves, accompanied by a purple-gray fuzz on the bottom surface.

Problem 6: Pesky Pests

While larger pests like deer, rabbits, and groundhogs typically leave basil alone due to its strong scent and flavor, the herb is still susceptible to smaller insect pests such as Japanese beetles, aphids, mealybugs, and slugs. Holes in the leaves are a telltale sign of pest infestation, and yellowing leaves can also indicate an aphid problem. Aphids, small insects that come in various shades of green, gray, or brown, suck the plant juices from the leaves, causing distortion and yellowing.

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To tackle aphids, closely inspect your plants. If you spot aphids clustered on new growth or hiding beneath leaves, take a closer look for beneficial insects like ladybugs or lacewings that prey on aphids. If beneficial insects are present, hold off on intervention. However, if no beneficial insects are around, you can simply knock the aphids off the plant with a strong jet of water from a hose nozzle. Repeat this a couple of times a week as needed.

To promote healthy growth and flavorful foliage, fertilize your basil plants every few weeks with a liquid organic fertilizer like Neptune's Harvest.
To promote healthy growth and flavorful foliage, fertilize your basil plants every few weeks with a liquid organic fertilizer like Neptune’s Harvest.

Problem 7: Nutrient Deficiency

During the summer months, yellowing basil leaves can be a sign of nutrient deficiency. Like all plants, basil requires a steady supply of nutrients to produce an abundance of flavorful foliage. Before planting basil in raised beds, top-dress the soil with an inch of compost and add a slow-release organic vegetable fertilizer around the plants. Follow the instructions on the fertilizer package for proper application.

When preparing containers, combine potting mix and compost in a ratio of two-thirds potting mix to one-third compost. Additionally, incorporate a slow-release organic fertilizer into the containers to provide your plants with the necessary nutrients for the months ahead.

As mid to late summer approaches, even container-grown basil can suffer from nutrient deficiency due to frequent watering that flushes nutrients from the potting mix. The solution is to fertilize your basil plants every two weeks with a liquid organic fertilizer, or as directed on the product label.

“Ames Farm Center”: Your Solution for Basil Brilliance

Now that we’ve uncovered the main causes behind yellowing basil leaves, it’s crucial to have a reliable source for basil plants and supplies. Look no further than “Ames Farm Center” for all your basil-growing needs. Their wide range of high-quality seeds, fertilizers, and gardening tools will ensure your basil plants thrive and produce an abundance of luscious leaves. Don’t let yellowing leaves deter you; with proper care and the right resources, your basil garden will flourish!

Note: All images in this article were sourced from Savvy Gardening.