It can be scary when the leaves on a beloved plant start to turn red. The good news is that most of the time, red leaves are the plant’s stress response to something in its environment. If you identify what’s stressing your plant, you can take steps to solve the problem.
Hydrangea leaves turn red because of stress. Most of the time, this is caused by extreme temperatures, either hot or cold. When exposed to these extremes, the plant produces anthocyanin pigments that protect it in several ways and help to extend the life of the leaf.
I’m going to explain the most common reasons for Hydrangea leaves turning red and give you some ideas about how to return them to their lush, green glory.
Contents
Causes of Hydrangea Leaves Turning Red and Dying
Anthocyanin Pigments
In most cases, when hydrangea leaves turn red, it’s due to the production of anthocyanin pigments in the leaf cells. Plants produce these pigments in response to many different sources of stress, including extremes of temperature, too much or too little light, and coming under attack from pests.
There are over 600 kinds of naturally occurring anthocyanins, and scientists are still working on understanding exactly how they protect plants.
Studies show that they act as a kind of sunscreen against harmful UV rays, increase the effectiveness of photosynthesis and act as antioxidants – protecting against damage from free radicals.
Solution
The only way to stop plants from producing anthocyanins is to ensure that they don’t experience any stress. For hydrangeas, this means planting them in rich, well-draining soil, keeping them well watered but not wet, and making sure they catch the sun in the morning but get plenty of shade during the hottest part of the day.
If grown in the ground, they don’t need feeding but will appreciate a compost mulch once a year. In pots, a slow-release fertilizer developed especially for hydrangeas applied once a year is a good option.
Cold Weather
When the weather gets cold, Hydrangea leaves usually turn red before dropping off the plant. This is a natural part of the life cycle and is usually nothing to worry about.
Hydrangeas are deciduous. This means that in the fall their leaves die off before regrowing in the spring. During winter, a deciduous plant is dormant – meaning it doesn’t grow or require much water or nutrition.
Before they drop off the plant, the leaves of deciduous plants make less chlorophyll (the chemical responsible for the green color in plants) and more anthocyanins.
So as the chlorophyll fades, we see more of the orange and red colors coming through. This enables the leaves near the end of their lives to continue to be productive for as long as possible.
Solution
There is no way to stop your deciduous hydrangeas from losing their leaves in winter, but they’ll be fine outside with no protection unless temperatures drop below 0 °F (-17.7 °C). Come spring, they’ll produce new growth that should be green, not red.
A few types of Hydrangea are evergreen, meaning they won’t turn red and lose their leaves in winter. If you’d prefer a plant with glossy green foliage all year round, you could give the climbing Hydrangea integrifolia or Hydrangea seemanii a try.
Growing In Full Sun
Hydrangeas thrive best when they get plenty of sunshine in the morning when temperatures are cooler and shade during the afternoon.
If your plant is in full sun throughout the day, it will protect itself by producing anthocyanins – turning the leaves red.
These pigments protect the cells of the leaves from damage and are not harmful, although they are a sign that the plant isn’t in its preferred situation.
Solution
Make sure to plant your hydrangea where it gets direct sunlight in the morning but not in the afternoon. Most types like around 4 – 6 hours of sunlight per day, with some types needing more or less.
If you can’t move the plant, consider ways to provide it with some shade during the hottest part of the day. You could plant other trees or shrubs or use some kind of structure like a fence to provide shelter.
Lack of Light
Hydrangeas growing in a spot that is too shady might also produce red leaves as anthocyanins can increase the ability of cells to photosynthesize. So if your plant gets less and 4 hours of sunlight a day and is turning red, this could be the issue.
Lack of light more usually causes pale growth that leans toward the nearest light source, or in some cases, a combination of pale and red growth.
Solution
Make sure that your plant is getting enough sunlight. If it is in a position with full shade, you should move it to a brighter situation while still ensuring that it is not in full sun.
If you can’t move the plant, consider ways that you could increase the amount of light it gets in the morning, for example, by cutting back other trees and shrubs nearby.
Phosphorus Deficiency
Hydrangeas are not heavy feeders, and too much fertilizer can reduce the number of flowers on the plant or even cause damage to the root system. However, if you see that the lower leaves of your plant are turning purple from the tips, it’s a sign of phosphorous deficiency.
One thing to note is that when soil temperatures drop below 55°F (13°C), Hydrangeas struggle to take up phosphorous.
This problem is made worse by wet soil, which also makes it difficult for the plant to take in nutrients. It’s therefore vital to keep your plant in a well-draining situation.
Remember that in winter, most types of Hydrangea become dormant and don’t need to take in nutrients, so you don’t need to worry about the soil temperature preventing phosphorous uptake in fall and winter.
Solution
Use plenty of high-quality compost when you plant your Hydrangea, and apply a compost mulch once or twice a year. Compost contains all the nutrients that the plant needs in an easily accessible form.
Unless your soil is very deficient in nutrients, you probably won’t need to use fertilizer. However, if you do want to use extra fertilizer, use one developed for hydrangeas and only apply it once or twice a year, according to the instructions.
Underwatering
The word ‘Hydrangea’ comes from the Greek words for water (hydros) and jar (angos). This should give you some idea of the plant’s water requirements!
Although hydrangeas don’t like wet soil, they do need a lot of moisture. This can be a tricky balancing act for even the most experienced growers!
If you notice your plant’s leaves drying out, curling, and turning red, it could be a sign that it needs more water.
Be careful, though – before you increase the amount you water, make sure that the plant isn’t suffering from root rot as this can also cause symptoms of dehydration.
Solution
Once you’ve determined that the plant doesn’t have root rot, it’s easy to solve this problem. Just give your plant more water!
During summer, you’ll probably need to water two or three times a week. Consider using hardwood mulch to help the soil retain water.
The soil must be well-draining, as hydrangeas don’t like having wet feet and are prone to root rot. In winter, reduce watering and only water if the soil is completely dry.
Xanthomonas and Cercospora Leaf Spot
If you’ve noticed red or purple spots gradually spreading from the lower leaves of your Hydrangea, it could be Xanthomonas or Cercospora leaf spot.
Xanthomonas is a bacterial disease, and Cercospora is a fungal disease, but they both have similar symptoms. They spread from infected organic matter and can be quite difficult to treat once they have taken hold.
Usually, the bacteria or spores move from infected matter to the plant through splashing water, for example, rain or when the plants are watered from above, but it is also possible to pass on these infections through equipment or even on your hands. (Source: Michigan State University Extension)
Solution
Aim to prevent both these infections by removing fallen leaves and other decaying organic matter from around the plant. This will ensure that there are few places for the bacteria or fungal spores to hide, reducing the likelihood of infection.
Once established, a Xanthomonas infection is hard to treat. There are no effective bactericides available, so the best thing you can do is remove infected parts of the plant and hope for the best.
Some plants survive with the infection; others don’t. If the infection is very severe, it’s best to destroy the plant.
Cercospora leaf spot can be successfully treated with fungicide if you catch the infection in time. If less than 20% of the plant is infected, the chances of survival are good.
For heavily infested plants, it’s probably better to destroy them and try again. Make sure not to reuse soil and pots from infected plants without sterilizing them first.
Phytophthora Root Rot
The Phytophthora fungus that causes root rot thrives in wet soil and is easily passed through splashing water or infected soil.
It’s a killer for all plants and can be a particular problem when growing Hydrangeas because of their water requirements. If your plant is in waterlogged soil, it is likely to develop root rot.
Root rot usually causes symptoms of dehydration as it prevents the plant from taking in water through its roots. If your plant has drooping, dry, reddening leaves but is getting plenty of water, root rot could be the issue.
Solution
Always make your that your Hydrangea is in well-draining soil. Never let it sit in waterlogged soil for any length of time. Consider relocating the plant or improving soil drainage if the situation becomes waterlogged.
Once it has taken hold, root rot is very hard to treat. You can try by removing any infected roots, which will look brown and slimy, replanting in well-draining soil, and holding off watering for a while.
The plant may recover. However, root rot is a death sentence for many plants, and it might be better to dispose of the plant and try again.
Should You Worry About Hydrangea Leaves Turning Red?
As red leaves are a symptom of stress, they are often a sign that the plant isn’t happy in its situation. While not necessarily a reason to worry, you should try to work out what’s causing the issue – then you can decide what to do about it.
For example, red leaves in the fall are normal and don’t need any treatment. Red leaves caused by too much sun, though, are a sign that your plant will be much happier elsewhere. And red leaves caused by root rot means that the plant requires immediate attention.