10 Reasons Why Outdoor Plant Leaves Turn Yellow

There are several reasons why plants may struggle with chlorophyll production, making the leaves more yellow than green. Yellow leaves can also be caused by pests and diseases, or they can simply be a sign of normal aging.

Read on to understand more about what could be causing your outdoor plant's leaves to turn yellow and if there is anything you can do about it.

1. Lack of Light

Light is crucial for photosynthesis, so if a plant is not getting enough sunlight, its leaves will look faded. Research the light requirements of your ailing plant to understand if it needs full sun or can tolerate partial shade.

For potted plants, the remedy can be as easy as moving it to a sunnier location. Altering the light situation of in-ground plants is more difficult. If a nearby tree or shrub is casting shade, you might be able to prune it. If that is not possible, you may have to dig up and move the yellowing plant.

2. Lack of Water

Yellow leaves are a common sign that the plant needs water. A few hot days without water can be enough for yellowing to occur.

If your plant has yellow leaves and appears to be suffering fro drought, water it slowly and repeat as necessary to keep the soil evenly moist.

3. Excess Water

Too much water can also be the cause of yellow leaves. Soils with poor drainage, such as clay soils, can become tightly packed and waterlogged when it rains, and this shuts out oxygen. In this environment, the plant roots can rot. 

Do not plant in spots with poor drainage or amend the soil before planting. Another option when you have poor-quality soil is to consider raised beds. 

4. Nutrient Deficiency

Nitrogen is often the first nutrient that comes to mind as lacking when leaves turn yellow, but yellowing of the leaves can also indicate that the soil is low in other nutrients such as iron, manganese, or zinc.

The most reliable way to assess the problem is to do a soil test. This will provide precise information about the soil's nutrient content so that you can fertilize appropriately.

5. High Soil pH

Yellowing leaves can also be caused by soil that is too alkaline for the plant. In soil with a pH above 7, iron is less soluble and therefore less available. That is why plants with high iron needs, such as blueberries, rhododendrons, and azaleas, need acidic soil.

Combine a soil test with a pH test. That way you can add any nutrients required and simultaneously lower the pH by adding sulfur to make those nutrients more available to the plants.

6. Fertilizer Burn

If leaves look burned or scorched, it can also indicate that you fertilized too much. The damage can occur in two ways.

One, soluble salts in the fertilizer can draw moisture from the roots, causing yellowing. Or two, fertilizer granules or liquid fertilizer can be spilled directly on the leaves, altering the color.

Not all plants are equally sensitive to foliage fertilizer burn, and hot, dry weather increases the damage. Be careful when applying fertilizer. Don't let any get on the leaves, and water the soil right away after application.

If you're unsure about whether or not you've fertilized enough already, you can conduct a soil test.

7. Pesticide or Herbicide Burn

When you spray pesticides or insecticides in temperatures over 85 degrees, in high humidity, or on an overcast day, leaves can easily get burned. The same happens when you spray plants that are already stressed by drought, frost, pests, or diseases.

After the damage has occurred, there is not much else to do other than removing the affected plant parts.

Regardless of whether you are spraying organic or inorganic pesticides, such as insecticidal soaps, select a day that is dry, cool, and calm. These are conditions in which the pesticide will dry quickly on the plant, and the vapors won’t drift due to hot air and wind.

The same applies to herbicides.

8. Fungal or Bacterial Diseases

If you are growing tomatoes, you might want to monitor your plants closely for two fungal diseases that both start with yellow leaves and eventually kill the plant: early blight and Fusarium wilt.

Early blight begins with spots that have a yellow halo which later merge. In tomatoes affected by Fusarium wilt, the leaves turn yellow, often only on one side of the plant or one side of a branch.  

Once the disease has hit, there is not much you can do other than removing and destroying the plants. To avoid these diseases, select resistant varieties, rotate related crops each season, and practice good garden hygiene.

9. Insect Damage

The damage from several insects can cause the yellowing of leaves, especially in vegetables. Common culprits are harlequin bugs, spider mites, squash bugs, and whiteflies. 

While they don’t necessarily kill the plants, it is crucial to monitor your vegetable garden so that you can spot infestations early and take the appropriate control measures before they spread.

10. Normal Aging

Leaves yellowing and subsequent dropping can simply be part of the aging process of the foliage. This is especially true for the oldest leaves of plants, which are usually near the base. If your mature plant has a yellow leaf or two near the base—but the rest of the plant looks fine—it's probably nothing to worry about.