Dahlia leaves turn brown due to overwatering, verticillium wilt, necrotic spot virus, frost damage or poor draining soil. Under-watered dahlias can also develop brown leaves as they dry out and die back. Dahlia leaves will naturally turn brown and die back when the weather turns cold in the fall and winter.
If you are not sure why your dahlia leaves are turning brown, check out this article to find out what is happening with your plants.
8 Reasons why dahlia leaves turn brown
Check out the top 8 reasons why dahlia leaves turn brown and what you can do to solve each one.
1. Normal winter die-back
The top reason why dahlia leave turn brown is the natural die back process that happens over the cooler months. After the dahlia has finished flowering in summer and the weather cools in fall the leaves will die off.
The leaves will wither and the whole stem will turn brown and die back. The tuber underground will re-absorb the nutrients from the leaves and stem and store it ready to grow again in spring the next year.
For a healthy dahlia tuber and the best flowers the next year allow the dahlia to die back completely before trimming off any remaining stems during winter. When spring arrives the tuber will sprout new stems will flower again.
2. Overwatering
Dahlia leaves will turn brown if the plant and tuber has been overwatered. The tuber is very good at absorbing water and adding too much will cause the tuber and roots to rot.
Overwatering dahlias will remove the oxygen out of the soil and can stop the roots from absorbing the nutrients they need. Dahlia leaves will often droop, turn yellow and then brown over time if they are overwatered.
To avoid overwatering dahlias make sure they are planted in well draining soil or good quality potting soil. Make sure you avoid watering your dahlias if there has been rain and aim for 2 deep waterings per week in summer.
Stop watering your dahlias when fall arrives and the weather cools. The dahlia will die back drawing the nutrients into the tuber and storing to to grow back the next year.
3. Necrotic spot virus
Necrotic spot virus can cause wilting, yellowing and browning of dahlia leaves. The disease will look like spots on the leaves almost like chickenpox.
Plants are infected by thrips, small sap sucking insects that carry the virus. Thrips can be treated with Neem oil. You will need to apply this at least 2-3 times until all of the thrips have been treated.
4. Verticillium wilt
Verticillium wilt is a soil born fungus that can infect dahlias and cause the leaves to become yellow lower down and then brown. If you cut a piece of the stem you can often see brown or black streaks through the center.
The best way to try to save your plant is to give them a nitrogen based fertilizer and water regularly. You could add a high nitrogen manure based fertilizer like pelleted chicken manure or a synthetic fertlizer.
Water the fertilizer in well and this can help the plant to recover and grow new stems and leaves.
Avoid moving the soil to other areas of your garden or re-using the potting soil. Thsi will stop the disease from spreading to other plants in your garden.
5. Under-watering
Under-watering dahlias can cause their leaves to become brown. Dahlias like regular water in well draining soil to avoid rotting their tubers.
If you live in a dry climate, check the soil twice per week and deep water to avoid the plant drying out. If hot weather is predicted, water your dahlias in the morning to pre-soak the soil and stop them from drying out.
6. Frost damage
Dahlias are sensitive to frost so early spring frosts can damage new leaves and cause them to become yellow or brown.
Protect your dahlias from early spring frosts by covering them overnight with a light hessian or cotton cover. Remove this cover after the frost has passed in the morning.
7. Poor drainage
Soil that is poor, hard and compact that drains poorly can cause dahlia leaves to become brown. This happens if the soil is high in clay or has not been turned over or loosened before planting dahlia tubers.
This essentially staves the roots of air and nutrients and will stunt the growth of your dahlias.
Before planting dahlia tubers improve your soil with aged cow manure and compost. This will help the soil to drain and will stop the tuber from rotting and will keep your dahlia leaves green.
For more tips on planting dahlias in the perfect soil, check out this video.
8. Not enough sun
Dahlia leaves will turn brown if they are not getting enough sun. Dahlia grow best when they get at least 6 hours of sunlight so their leaves can photosynthesize and their flowers can bloom.
Dahlias that don’t get enough sun will start to develop yellow leaves that will eventually turn brown. They will become leggy and the stems will not be able to support their heavy flowers.
Choose a position for your dahlias that gets morning light and at least 6 hours of sun throughout the day.
Why are my dahlia leaves turning brown? | Summary
Dahlias are beautiful showy flowers that are worth the effort to grow for their amazing spring color. If the leaves are starting to turn brown, check for pests, good soil drainage and make sure they are getting plenty of sun. If fall has arrives it could be that your dahlias are just going through their natural winter die back and will be ready to grow again next year.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.