Updated: October 18, 2022

Cosmos plants are a popular choice for gardeners due to their beautiful daisy-like flowers that come in a range of colors. They are easy to grow and care for, but sometimes the leaves may start turning yellow, which can be distressing for plant owners. In this article, we will discuss the reasons why your cosmos plant leaves may turn yellow and how to fix the issue.

Lack of Water

One of the most common reasons for yellowing cosmos leaves is a lack of water. These plants require moist soil to thrive, and if the soil becomes too dry, it can cause stress on the plant, resulting in yellowing leaves. It is important to water your cosmos regularly and ensure that the soil is not too dry.


While it is important to keep your cosmos plant hydrated, overwatering can also lead to yellowing leaves. Overwatering can cause the soil to become waterlogged, which can lead to root rot and other problems that ultimately affect the plant’s overall health. To avoid overwatering, make sure you only water your cosmos plant when the top inch of soil feels dry to the touch.

Nutrient Deficiency

A nutrient deficiency can also be a reason for yellowing cosmos leaves. If your plant is not getting enough nutrients like nitrogen, magnesium, or iron from the soil or fertilizer, it may start showing signs of yellowing leaves. To correct this issue, you can try adding a balanced fertilizer to the soil or providing compost or other organic matter to improve soil nutrient levels.

Pests and Diseases

Pests and diseases can also cause yellowing of cosmos leaves. Common pests that attack cosmos plants include aphids, spider mites, and whiteflies. These pests suck sap from the plant’s leaves and cause them to turn yellow and eventually wilt. Diseases like powdery mildew and leaf spot can also cause yellowing of cosmos leaves. To prevent these issues, you can try using insecticidal soap or neem oil for pests and fungicides for diseases.

Environmental Factors

Lastly, environmental factors like extreme temperatures, too much or too little sunlight, and exposure to harsh winds or chemicals can also cause yellowing of cosmos leaves. It is important to plant your cosmos in a location that provides the right amount of sunlight and protection from harsh weather conditions.


Can I save my cosmos plant with yellow leaves?

Yes, it is possible to save your cosmos plant with yellow leaves by addressing the underlying issue causing the yellowing. This can include providing proper watering, fertilization, and pest control.

Should I remove yellow leaves from my cosmos plant?

Yes, it is recommended to remove yellow leaves from your cosmos plant as they will not recover and can attract pests or diseases. Make sure to use clean pruning shears to avoid spreading any infections.

How often should I fertilize my cosmos plant?

It is recommended to fertilize your cosmos plant every four to six weeks during the growing season with a balanced fertilizer.

What is the best location for planting cosmos?

Cosmos plants prefer full sun and well-draining soil. Plant them in a location that receives at least six hours of direct sunlight per day and avoid areas with standing water or poor drainage.

In conclusion, yellowing of cosmos leaves can be due to a variety of reasons. By identifying the underlying cause and taking appropriate measures, you can save your plant and enjoy its beautiful flowers all season long.