10 Reasons Why Outdoor Plant Leaves Turn Yellow

When the leaves of outdoor plants turn yellow, it is commonly associated with chlorosis, a symptom caused by insufficient chlorophyll being produced by the foliage. This pigment is essential for photosynthesis and gives leaves their green color. There are several reasons why chlorosis could be occurring. Yellow leaves can also be caused by pests and diseases, or can simply be a sign of the normal aging process of a plant.

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 to occur, 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 inground plants is more involved. If a nearby tree or shrub is casting shade, you might be able to prune it. If that is not possible or a wall or a building is casting the shade on your plant, you might have to transplant it to a sunnier location.

2. Lack of Water

Yellow leaves are a common sign that the plant needs water. Often, it does not take a real drought for that to happen. A few hot days when the plant loses more moisture through its leaves than the roots are able to absorb can be enough for yellowing to appear. 

Water the plant slowly and deeply 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 cannot breathe or, even worse, get damaged or compacted, all of which causes leaves and stems to turn yellow and brown. 

Do not plant in spots with poor drainage or amend the soil before planting. Another option for vegetable gardens is to consider raised beds. 

4. Nutrient Deficiency

Nitrogen is often the first nutrient that comes to mind as the culprit when leaves turn yellow, but it is not the only one. Yellowing of the leaves can also indicate that the soil is lacking other nutrients such as iron, manganese, or zinc.

While the pattern of how the yellowing of the foliage progresses can give a trained eye some clues to which nutrient might be lacking, the most reliable way is to do a soil test. This will provide precise information about the nutrient content and the nutrient additions you need to make.

5. High Soil pH

Yellowing leaves can also be caused by soil that is too alkaline for the plant. Alkaline soil is directly connected to nutrient deficiency. In soil with a pH above 7, the nutrient 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 in which they can absorb sufficient iron.

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 overdid the fertilizer. The damage can occur in two ways: when soluble salts in the fertilizer draw moisture from the roots, or when fertilizer granules or liquid fertilizer is spilled onto the leaves. Not all plants are equally sensitive to foliage fertilizer burn, and hot, dry weather increases the damage.

To prevent this from occurring, if using granular fertilizers, make sure to water the plants immediately afterwards, and that no granules accumulate on the foliage. Or use slow-release organic fertilizer, which is less concentrated than inorganic fertilizer.

7. Pesticide or Herbicide Burn

When you spray pesticides or insecticides in temperatures over 85 degrees F, 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 in order to avoid drifting from the weeds you want to get rid of.

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. These are 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 and practice good garden hygiene and a thorough fall cleanup, as the pathogens overwinter in the soil for many years.

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 you can spot infestations early and take the appropriate control measures before they spread.

10. Normal Ageing

Leaves yellowing and subsequent dropping, especially older leaves at the bottom part of the plant, can simply be part of the ageing process of the foliage. It all depends on the frequency and the number of leaves, but a few occasional yellow leaves on a mature plant are usually nothing to worry about.