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Why are Basil Leaves Turning Yellow?

I started growing a basil plant with good intentions, but suddenly their leaves started turning yellow and unattractive, not realizing that something was seriously wrong with it.

If you are experiencing this exact scenario, let us tell you that you’re not the only one. Many gardeners face this dilemma when they first start growing a basil plant.

So, should you be concerned? Umm, Slightly!

Yellowing leaves are the signs of a sick basil plant! And, there are mainly three factors that could turn them yellow; too much water, too little sun, or not enough nutrients.

Yellowing Basil Leaves
Yellowing Basil Leaves (Source: Pixabay.com)

If you’re suspicious that any of these factors, or anything else, are causing the yellowing leaves, then it’s time you take action!

But first, you’d need to determine if it’s usual with all basil plants and what exactly is causing this.

Is it Normal for Basil Leaves to Turn Yellow?

Let’s cut the chase! It isn’t normal for basil leaves to turn yellow, but if they do despite caring for them regularly, then your basil plant may be dying.

Don’t panic yet! There are many reasons for yellowing basil leaves. So, let’s look at the brighter side!

The discolored leaf is naturally the first sign of yellowing, which starts at the petal base. Once the pale yellow shade overtakes the petal, the entire leaf will start yellowing.

Start with determining the yellowish shade (pale or full), the number of yellowing leaves, and the frequency of yellowing.

Then you can begin looking for possible problems with your plant.

What Causes Basil Leaves to Turn Yellow?

Yellowing basil leaves aren’t limited to the problems of overwatering, lack of sunlight, or nutrient deficiency.

Failing to provide the proper growing condition to the basil plant will cause yellowing leaves. Too much or too little of anything will also alter the leaf’s natural color. 

Sometimes, growing these plants indoors will also affect their health because basil needs six to eight hours of sunlight.

Like these, there are varying factors that may affect your basil plant.

Before jumping ahead, let’s take a look at all of these probable causes.

1. Excess watering

Excess watering is the leading cause of yellowing basil leaves. Although regular watering is needful for basil’s healthy growth, too much water will suffocate its roots and cause root rotting.

This is more likely with indoor basil plants grown in a small pot with no proper drainage.

Basil plants require slightly moist soil to help spread their roots, but overwatering them will drain the mud off of roots and hamper their natural growth.

Overwatering house plants
Overwatering House Plants (Source: Unsplash)

You can determine ‘root rot’ by inspecting the yellowing leaves. If it feels soft and limp with new growth failing, it is an indication of overwatering.

Alternatively, slide the plant out of the soil to check for visible root rot signs such as darkening roots.

2. Too Compact Potting Mix

The compact soil is another primary reason for yellowing leaves because the too-tight potting mix will suffocate its roots and hamper its natural growth.

Most potting soil dries out quickly because they contain a significant amount of clay or silt content.

Mainly the clay soil found in most potting mixes and backyard drains very slowly and then hardens, causing compact potting mix.

When the soil doesn’t get enough moisture, it becomes hydrophobic and stops absorbing moisture altogether.

Sometimes, too much nitrogen content will make the soil compact, like the soil found in most gardens or fields.

Materials Used for Repotting
Materials Used for Repotting (Source: Pexels.com)

Check for a visible sign like light brown-colored soil compared to dark, moist soil.

Poke your finger into the ground to further probe into the matter. If the ground underneath feels drier, it is a compact potting problem.

3. Lack of Nutrients

The deficiency of nutrients is a significant cause of yellowing basil leaves. However, like any other herb, Basil dramatically benefits from a nutrient boost.

It needs an adequate amount of NPK -Nitrogen, Phosphorus, potassium, and micronutrients like Iron and Magnesium to grow green, healthy leaves.

Soil lacking the nutrients
Soil lacking the nutrients (Source: Pixabay.com)

Using soil from the land that has experienced erosion, runoff, leaching, and the burning of crop residues will wither your basil plant and turn its leaves yellow.

This kind of soil lacks good bacteria and inorganic salts like calcium and magnesium vital for plant growth.

How to check for soil nutrient deficiency?

  • You can determine nutrient deficiency symptoms by checking for visual signs, problems with new foliage, stunted growth, and yellowing leaves.
  • Young leaves start curling downward with discoloring of leaves, which later turn prominently yellow. You’d also see browning of leaf edges and tips.
  • A soil nutrient analysis is the best way to determine if the soil concentration lacks either of the three major macronutrients; nitrogen, phosphorus, and potassium.

4. Wrong Soil pH

A wrong soil pH causes plant poisoning that affects the growth of your basil plant and healthy green leaves.

Too acidic soil releases aluminum that can poison the plant, whereas too alkaline soil locks the nutrients and makes them unavailable to plant roots.

Start with determining if your soil is lacking the correct pH.

  • Yellowing or browning leaves are one of the first signs. Get a pH check done by a professional, or get yourself a DIY Soil pH test kit.
  • The test kit will read and instantly tell you if the pH is up or down. Remember, a basil plant prefers a soil pH between 6 to 7.5, which is slightly acidic.
Soil pH Test Kit
Soil pH Test Kit (Source: Pixabay.com)

A rapid loss of organic matters such as soil bacteria and minerals can also cause soil pH to alter.

Another probable reason could be using too much nitrogen and sulfur-rich fertilizer.

Check for visual symptoms like yellowing leaves and stalled growth to determine if your soil lacks essential nutrients. Even better, get a plant tissue analysis or soil analysis done by a professional.

Plant-related diseases can also cause yellowish leaves. For example, fungal disease or Fusarium wilt is a common disease that eats up nutrients from plants.

Downy mildew is another fungal disease that is more common in basil plants.

Downy mildew is a quick-spreading fungus identifiable by a fuzzy, gray, or brown-looking substance on the plant leaves.

Milddew on a green leaf
Mildew on a green leaf (Source: Pixabay.com)

Check for these tell-tale symptoms of downy mildew.

  • Appearance of yellow, light green, black, or purple spots on the leaves.
  • The fluffy gray spores grow on the underside of the leaves.
  • The infection generally starts with the bottom leaves and then moves up.
  • Infected leaves first turn yellow around the central veins, then start covering the entire leaves.

If you find two or more of these signs on the basil leaves, you can conclude that your plant suffers from downy mildew.

6. Root Rot

Root rot is a common enemy of basil plants that attacks the roots growing in damp soil.

Although overwatering is one of the major culprits, other prominent factors could also cause root rot, including poor drainage of damp soil and a poorly functioning root system.

The early signs of root rot include wilting and yellowing leaves. Soon after, the foliage and shoots will die, causing the entire plant to wither.

Root rot
Root rot (Source: Unsplash.com)

You can diagnose root rot by sliding the suspicious plant out of the soil and checking for brown and softening roots.

However, waterlogging caused by overwatering isn’t the only culprit. Using the wrong sized pot with no drainage holes, placing the plant at a humid temperature for a longer time, and overfertilizing (high-salt) can also cause root rot.

7. Pests and Predators

Similarly, tiny pests like Aphids, caterpillars, and root-knot nematodes will spread diseases turning basil leaves yellow.

Aphids and Root-knot nematodes are tiny pests that suck the water from basil’s foliage, leaving them high and dry. In addition, caterpillars, especially Cutworms, often attack basil plants.

Cutworm caterpillar that commonly attack basil plants
Cutworm caterpillar commonly attacks basil plants (Source: Unsplash.com)

Cutworms tend to feed on the plant closer to the ground. They are generally an inch long with green or brown skin.

These predators are attracted by green, lush leaves and may travel ten meters from their initial plant to feast on the basil plant.

They feast on the leaves and shoots, only to hatch eggs that later become more plant-eating larvae.

8. A Dying Plant

Excessive yellowing leaves accompanied by flower buds may suggest that your basil plant is dying. However, in some cases, flower buds may appear before the leaves start getting yellow.

Other tell-tale signs include brown or dark leaves, falling petals, wood-like stems, and the looping posture of the plant.

Basil plants commonly grown at home may only live one growing season, typically six to nine months, before dying.

Dead basil plant
Dead basil plant (Source: Unsplash.com)

You can fix it by propagating your plant, cutting few parts, and changing the soil, but trying to fix a dying plant from untreatable root rot, cold, and frost, and plant-related diseases will be useless.

Any of the factors mentioned above that cause yellowing leaves may kill your plant unless you treat them in time.

What to do if your Basil Leaves are Turning Yellow?

Start with determining the causes of yellowing leaves. Once you find the specific problem, you will find a remedy!

Here are the solutions to each situation that may be causing yellowing leaves.

1. Water Your Plant Accordingly

Here are few solutions to minimize yellowing caused by water damage.

  • Keep watering to a minimum because slightly dry soil is healthier for the basil plant.
  • Consider deep watering your outdoor basil plants, preferably every seven to ten days.
  • For indoor plants, water less frequently. In fact, let the topsoil (1 to 2 inches) dry out before watering again.
  • To check whether the soil is too moist, insert your finger into the ground or use a 3-in-1 meter to monitor the moisture level, pH, and sunlight intensity.
  • In case of an overwatered plant, stop all irrigation immediately.
  • If rain is forecasted, bring your plant inside immediately.

If the yellowing doesn’t stop, consider sliding your plant out of the soil to check for possible root rot.

Plants with minimal root rot can be salvaged, but those with worsened root rot must be thrown away.

Quick Tip: Water once every two to three days in hot weather, such as in times of drought.

2. Amend the Soil pH

The wrong soil condition is the worst enemy of basil plants, but you can correct this by amending the soil.

The suitably amended soil with organic mulch has perfect moisture, pH level, and major nutrients required for growing basil seedlings.

Amend your garden soil or potting mix with organic mulch immediately after a frost to improve soil aeration and drainage.

Planting them in the same soil as tomatoes will help gather nutrients quickly because tomato soil has a perfect condition and nutrients required for a basil plant.

Alternatively, you can use an organic feed like Dr. Earth Organic Fertilizer, appropriate for tomatoes and herbs, to amend the soil.

For basils that are already planted, consider amending the soil in another part of the garden or a new potting mix before moving your basil plant to its new position.

Note: You can easily make your soil more acidic by using organic mulches or sphagnum peat, elemental sulfur, aluminum sulfate, and iron sulfate.

3. Provide Enough Sunlight

Amateur gardeners often make the mistake of keeping their basil plant in shadow, mainly indoor basil plants.

Basil thrives in warm temperatures ranging from 80o to 90o Fahrenheit (27o to 32o Celsius); hence consider providing enough direct sunlight, considerably 6-8 hours per day.

Planting basil on the side of the garden that receives the most sunlight will significantly help them thrive.

Consider moving your basil plant by transplanting it to the other side of the garden.

For indoor basil plants, consider keeping them outside during the day.

Alternatively, you can use fluorescent bulbs to provide the much-needed light, at least ten hours a day.

Note: Do not plant your basil seed or sapling until the dangers of frost have passed.

4. Fertilize the Plant

Amending the soil may provide the proper growing conditions, but using fertilizers will help boost plant growth.

Feed your basil plants with a good mix of organic fertilizer every two to three weeks. For indoor plants, consider fertilizing them every four to six weeks.

Consider picking a 10-10-10 organic fertilizer. The equal proportion of N, H, K with essential micronutrients like iron and magnesium will boost your basil plant’s growth, green leaves, and structure.

Basil plant struggles to absorb iron from the soil because it’s an immobile nutrient; hence, most gardeners suggest feeding iron through the foliar spray.

Organic fertilizers like fish emulsion and bone meal are great for plants like basil, but be careful not to use them a lot as they can scorch the leaves and burn roots. Also, don’t mind the pungent stench that it can leave for few days!

Quick Tip: Instead of using granular fertilizer, use a water-soluble feed to instantly provide the nutrients to the plant.

5. Fix Compact Potting Mix

Amending the soil is the only solution for fixing compact potting mix.

The hollow pockets in the aerated soil will help trap water and prevent it from compacting.

Amending the same soil in the potting mix is difficult; hence, you can prepare another potting mix with suitable soil before transplanting your basil plant.

Remember to dress the potting mix with several inches of compost so the nutrients are readily available to plant roots.

Unlike planting in the garden where the friendly earthworms help with conditioning the soil, the potting mix must have perfected soil so the basil plants can immediately start gathering nutrients.

6. Control Plant Diseases

If your plant is badly affected, it’s better to throw it away before the fungus and downy mildew spread to other plants.

Fungicides are effective in controlling downy mildew. Some work as protectants and others as curative solutions.

  • Chlorothalonil and mancozeb are the two best protectant fungicides that alone protect against early infections. You can use them directly onto the fungus.
  • Oxathiapiprolin, Cyazofamid, and fluazinam are curative fungicides mixed with protectants before applying to the plant.

As a preventive measure, increasing airflow and sunlight will help with curbing mildew infection.

Alternatively, you can use fungicides like sulfur, lime-sulfur, and mint oil to prevent mildew.

7. Control Pest Problems

Controlling pest problems is easy when you spot the infestation early. Unfortunately, plants at the last stage of infestation are less likely saved.

  • Start with checking for visible signs of pest infestation, such as cutworms on the shoot and leaves.
  • Check for root-knot nematodes by directly uprooting the plant and inspecting the roots.
  • Check under leaves to find the sticky substances that are the signs of aphids.

Here are few ways to salvage your basil plant.

  • Using nematicide (pesticide) as a foliar spray directly on the soil effectively kills nematodes, but be wary about using it in excess as it can kill most living organisms it comes in contact with.
  • Use insecticidal soap to kill Aphids, but avoid using them when the sun is directly up as it can leave the plant scorched.
  • Pick and remove caterpillars from the plant and drop them into a soapy solution to kill them immediately.
  • Spread Diatomaceous earth (D.E.), a natural powder made from grounded diatoms, to kill cutworms instantaneously when they come in contact with it.
  • To prevent future infestation, spray a homemade solution of Canola oil and liquid dish soap mixed with warm water to flush tiny caterpillar eggs.

How to Prevent Basil Leaves from Turning Yellow?

Unfortunately, you can’t fix the dead plant, but you can save a dying plant with these preventive measures.

  • Before planting your basil, consider getting a pH test and then amend your soil accordingly. For example, add sphagnum peat to make the soil more acidic, or add garden lime or dolomite lime to alkaline.
  • In already yellowing leaves, prune them immediately to ensure it doesn’t spread to other leaves. The yellowing leaves will suck all the nutrients from the plant.
  • Pick flower buds from your basil plant immediately. The flowers are signs of a dying basil plant.
  • Do not crowd plants in a single place and dispose of infected leaves and stems before mildew spreads.
  • Buy a basil pot with suitable drainage holes.
  • For suspicious plants, check for root damage by gently sliding your plant out of its pot and inspecting the roots for any dark color or gall. It’s better to throw away the infected ones!
  • Share the garden space with other similar crops such as tomatoes, lettuce, parsley, and peppers that helps to exchange nutrients among the plants.
  • Water accordingly. Avoid planting where rainwater or irrigation accumulates.
Green Leafy Basil Plant
Green Leafy Basil Plant (Source: Unsplash)

Why are there Black Spots on the Basil Leaves?

Black spots on leaves aren’t uncommon. Some basil leaves turn black after harvesting, so avoid eating them.

Most black spots on the basil leaves are caused by three different reasons; early frost, bacterial infection, and poor nutrition.

Black spots from early frost may be seen only in the tips and edges of the leaf. In contrast, bacterial infection can cause black spots on any part of the leaf.

The bacterial infection called Pseudomonas Cichorii produces visible black spots on the leaves that occur when you accidentally splash infected soil onto the basil leaves.

There is no fix for bacterial spots, but you can minimize the damage by cutting away leaves that have black spots on them and then placing the plant in a sunny area.

For black spots caused by poor nutrition, consider fertilizing the plant with good organic feed.

For preventive measures;

  • Water your plant from the top using a spray to prevent bacteria from splashing onto the leaves.
  • Provide proper air circulation to your plant to prevent bacterial infection.
  • For a potted plant, move it indoors during the winter to avoid frost damages.

Tips to Take Care of Basil Leaves

Basil plants can easily survive when adequately taken care of; otherwise, your plant will hardly survive up to six months.

To ensure your basil leaves remain full throughout the year, consider these handy tips.

  • For basil in pots, water once every three to four days, and for basil in the garden, water at least once a week or two to three days during scorching summer.
  • Check the soil for moisture with fingers and only water if the soil is slightly dry during cool weather and rainfall.
  • For basil seedlings, water once every two days during the morning.
  • Incredibly moist soil is another concern for basil plants. The moisture encourages fungus to grow and spread quickly. So keep the basil pot as dry as possible.
  • Check sun pattern in your location and place your potted plant in the right direction to receive the most sun.
  • Harvest a few leaves from the top down and cut half of the total stem length to ensure healthy growth.
  • Regularly harvest the stem tips to discourage flower growth on the basil plant.
  • When picking leaves, always choose from the top as it encourages healthy new leaf growth.
  • Consider planting basil seedlings indoor for two weeks before moving them outside in a larger pot or garden sunlight.
  • Consider repotting your basil plant in a bigger pot every time it becomes big enough so that the roots can space out easily.
  • For adult basil plants, changing soil can be done once a year or two to ensure they get fresh soil and nutrients.
  • Use all-purpose fertilizer with NPK 10-10-10 to provide a significant boost to your new plant.

To Wrap it up!

You can prevent most plant-related problems by conditioning your soil accordingly for the basil plants.

Use the online resources, tips from seasoned gardeners, and handy guidebooks to learn more about growing and harvesting basil.

With the correct insights, you can prevent basil leaves from yellowing!

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