Yellowing lilac leaves are indicative of an environmental factor causing the plant stress. The key to fixing the problem is correctly identifying the cause of the discoloration.
The most common causes of yellowing lilac leaves are overwatering, underwatering, iron deficiency, diseases and pests.
In this article, we will discuss the different causes of yellowing lilac leaves and how to remedy each one.
Why are my lilac leaves turning yellow?
Overwatering
The most common cause of yellowing lilac leaves is overwatering. When you overwater your lilac bush, the leaves will turn yellow, brown and then fall off until the whole plant eventually dies.
Overwatering damages your plant by suffocating and drowning its roots with all the excess water. Plant roots need to dry out between watering in order to take in oxygen. Air needs to be able to pass through the soil and between the roots for the plant to breathe properly. If the soil around your plant’s roots is too compact and retains water too well, the roots will stand for too long in the soggy soil. They will drown, die, and your plant will suffer root rot. Root rot happens when the dead parts of the root become susceptible to bacterial and fungal diseases. The rot climbs up the stem and into the leaves, which is what causes them to turn yellow.
Check whether your plant is being overwatered by removing it from the soil and looking for brown or black roots. These roots are dead and probably have rot. Cut them off using a sterile knife or scissors. Then, lay the plant on a tray lined with dry tissue and let it air dry. After the plant’s roots have dried, replant it in a new pot with drainage holes, using well-draining soil.
You can avoid overwatering by making sure you use soil that is well-draining, and a pot that has drainage holes at the bottom. This allows any excess water to pass through easily instead of staying in the pot.
You should also know when to water your plant. There is no set number of days between watering, because this is dependent on the weather and the season. You might need to water your plants more frequently during the summer and less frequently during the winter.
Check the soil around the plant by pressing your finger into the top two inches of soil. If it feels dry, you can water the plant. If the soil still feels moist, wait one or two days and check the soil again.
Underwatering
Inadequate watering can also cause the leaves on your lilac bush to turn yellow. Lack of water can dry out your plant, turning its leaves yellow and brown until they fall off. Plants need water to transport nutrients and minerals from the soil into the plant. So, not only is the plant lacking moisture, but it also cannot get the nutrients it needs to grow properly.
Make sure you water your lilac plant regularly. Check whether the soil is dry and, if so, flush it with water until the excess water flows through the drainage holes at the bottom of the pot. In the hotter months, you may need to water the plant more frequently, while in the colder months, you may need to water it less.
Iron deficiency
Chlorosis is caused by a lack of iron in the soil around the plant, or alkaline soil that keeps the plant from taking up any iron. Plants need their soil to be slightly acidic so that iron can be transported from the soil into the plant. If there is no iron uptake, eventually the leaves will become yellow with green veins.
You can treat chlorosis by topdressing the soil around the plant with iron sulfate to help lower the soil pH until it is around 7.0. Water the soil well after applying the recommended dose. Remember that this solution is temporary and you will need to reapply the iron sulfate every couple of years.
Disease
There are many diseases that can infect your lilac plant. Some fungal diseases can cause yellowing or browning of the leaves and even leaf drop. These diseases may be difficult to catch in the early stages, especially in mature, more established plants.
Botrytis blight, also called gray mold, causes yellowing of lilac leaves with a distinctive gray spore on them.
Another disease that affects lilacs is verticillium wilt. This is a fungal disease that attacks the plant’s vascular system and can also cause the leaves to turn yellow and brown. Another sign that your plant has verticillium wilt is if the wood under the bark is discolored.
You can avoid these diseases by cleaning up any fallen leaves and other debris from around the plant and disposing of them properly. You should also prune off infected leaves and branches. Try not to water the plant from above, because the splashing of water can cause the spores to reach other plants, thus spreading the infection.
Pests
The two most common pests that attack lilac plants are nematodes and scale insects. These pests cause the foliage of the lilac to turn yellow.
Nematodes live in the soil around the plant and feed on its roots. Because the roots are compromised, the plant is unable to take up water and nutrients effectively. They may not do enough damage to kill the plant, but they can definitely impact the growth and vigor of the plant. The best way to treat nematodes is to provide the plant with correct and adequate care.
The oystershell scale insect is a type of armored scale insect usually found in the plant’s bark and leaves. They feed on the plant using their straw-like mouth. A small infestation is manageable for the lilac plant, but the bigger it gets, the more symptoms appear.
You can eradicate scale insects using natural enemies or pesticides. You can also use horticultural oil to kill them.
Conclusion
The leaves on your lilac plant will turn yellow if there is an environmental factor causing it stress. You will need to identify the exact cause of the discoloration in order to fix it quickly.
The most common causes of yellowing lilac leaves are overwatering, underwatering, iron deficiency, diseases and pests.
The closer you are able to simulate the plant’s natural habitat, the better the plant’s growth will be.
Image: istockphoto.com / Ilona5555