Delphiniums are perennial flowering plants of the family Ranunculaceae. These plants are native to tropical Africa and the Northern Hemisphere and bear flowers from late spring to late summer. They comprise about 300 species and are popular for their spikes of colorful flowers that come in shades of pink, blue, white and purple. Unfortunately, they are also prone to diseases and conditions that cause yellowing leaves.
Delphinium leaves turning yellow: What are the reasons?
Overwatering
Delphinium leaves may turn yellow because you are overwatering them. The leaves may not become droopy, but the roots are drowning in moisture which could lead to root rot and plant death.
In this case, refrain from watering the plants and evaluate the conditions. Observe the plants over the next few days for signs of improvement or worsening.
Insufficient fertilization
Delphiniums are considered heavy feeders, so they need regular fertilization at certain stages of their plant and flower development. If they are underfed with nutrients, their leaves will tend to turn yellow. You need to know how frequently your plants need feeding and the last time they were fertilized before you feed delphiniums with yellowed leaves. Plants in pots or planters generally need more feeding than those planted in the ground.
Too much or too little sunlight
Delphinium leaves may turn yellow as an indication of too much or too little sunlight. Delphiniums thrive best in sunny spots and prefer moist and fertile soil. Make sure the plants are exposed to some sun for a few hours, but be mindful not to overdo it. Too much sunlight could lead to yellowing leaves. Place the plants in an area with a cool shade if the sun gets too hot.
Nutrient deficiencies
Delphiniums in a highly alkaline and nutrient-deficient soil may have yellowing leaves. Plants growing in alkaline soil tend to be iron-deficient, since they absorb iron best when the soil pH is between 5 and 6.5. Nutrient deficiencies such as manganese and zinc can also cause chlorosis, or yellowing of leaves.
By observing which leaves are affected first, you can determine which nutrient deficiency is causing the chlorosis. The younger leaves turn yellow first in case of iron deficiency, while older leaves are affected first if it is a manganese or zinc deficiency.
Diseases
Delphinium leaves may also turn yellow due to diseases such as bacterial bud rot, crown rot or stem rot. These are fungal infections characterized by stunted growth and blackened stem bases around cracks and wounds. The pathogens causing these diseases are Erwinia carotovora and Erwinia chrysanthemi. Destroy all infected plants at once and avoid overhead irrigation.
Powdery mildew may also be the culprit; symptoms include dry, white and mealy fungal growth on the upper areas of leaves. The leaves may turn yellow and die from the base of the stem upward. Be sure to apply fungicides and destroy plant debris at the end of the season.
Aside from the reasons mentioned above, other factors that may play a role could be soil conditions or pests. Check the soil using a soil test kit to ascertain any issues that need addressing. The presence of pests in the plants could also trigger the yellowing of leaves; pests suck the sap of plants causing them to become weak and susceptible to diseases.
The leaf yellowing may also happen as plants prepare for dormancy or when they complete their flowering period. There is nothing to worry about in this case, as it is part of a plant’s natural life cycle. What you can do is remove affected flowers and foliage to breathe new life into the plants.
Drying winds and humidity may also cause yellowing leaves, as can transplant shock. In case of the latter, the plants should be able to adjust after a few weeks.
Conclusion
Delphinium leaves turning yellow could be due to a number of reasons, including overwatering, too much or too little sunlight, or nutrient deficiencies. It could also be due to pest infestation, lack of fertilization, or diseases such as crown rot or bacterial bud rot.
Image: istockphoto.com / Suprabhat Dutta