6 Reasons Why Dahlia Leaves Are Turning Brown (And How To Fix It)

Dahlias, with their tuberous roots, retreat into dormancy during the winter only to eagerly reemerge in the spring. However, their journey to full bloom can encounter a few bumps along the way, impacting their overall health. Let’s dive into what might be causing these issues and how to nurse your dahlias back to pristine condition.

QUICK ANSWER: Brown leaves on dahlias could be a sign of too much sun, insufficient watering, or poor soil conditions. To fix this, first, make sure the soil’s pH is between 5.8 and 6.2. Next, address any pest or disease problems with a gentle application of horticultural oil. Lastly, find a spot for your dahlias where they can soak up around 6 hours of direct sunlight daily. By tackling these steps, you’ll be well on your way to reviving your dahlias’ vibrant health.

1. Overwatering

During the colder months of autumn and winter, dahlias shed their leaves and flowers, retreating back to just the tuber beneath the soil. In this dormant phase, as they begin to die back, there’s no need to water them. Overwatering at this stage can lead to waterlogged roots, causing damage.

This waterlogging can hinder the roots’ ability to absorb water and nutrients, a situation that’s not just limited to their dormant period. It can also happen in spring, as they start to sprout and grow once more. Overwatered dahlias often display yellow leaves, which may develop brown edges that are dry and crispy before the leaves finally brown and die off.

Solution:

1. Water your dahlias only when the top 2 inches of soil have dried out.
2. If there’s been a significant rainfall, skip watering.
3. Refrain from watering during the winter months to avoid issues.

2. Not Enough Water

Dahlias, more so than many other plants, require a substantial amount of water, especially if they’re planted in soil that doesn’t drain well, leading to runoff instead of proper absorption.

A lack of sufficient water will cause dahlia leaves to turn yellow, with the edges becoming brown, dry, and brittle. Over time, if this issue persists, the leaves will completely brown and eventually die.

Solution:

1. Water your dahlias once the top inch of soil dries out during hot weather. At other times, wait until the top 2 inches of soil have dried out before watering.
2. Apply a layer of organic mulch around your dahlias to help retain soil moisture and improve soil health.

3. Sunburn

Dahlias harness sunlight through chemical processes to generate the energy needed for their growth. Insufficient sunlight can impair their development, initially indicated by the yellowing of leaves, which then become brown, dry, crispy, and eventually fall off. Planting them in complete shade can be detrimental, leading to their demise.

While dahlias might survive temporarily on reflected light, the absence of direct sunlight will ultimately cause the leaves to wither and die. Conversely, too much sunlight, especially in areas with intensely hot climates, can scorch the leaves, resulting in brown edges.

Solution:

1. Position dahlias or move pots to a spot where they receive at least 6 hours of sunlight daily.
2. In hotter climates, choose locations where dahlias can enjoy partial shade during the peak heat of the day.

4. Nutrient-poor soil

The availability of nutrients to plants is significantly influenced by the soil’s pH level. For dahlias, if the pH is too high or too low, essential nutrients become inaccessible. Dahlias planted in nutrient-deficient soils, such as unenriched garden soil or clay-heavy soil, may exhibit browning leaves.

This issue may first manifest as yellowing leaves, which then turn brown and eventually die. Browning may also occur along the edges of the leaves. To determine the soil’s pH, you can use affordable pH testing kits available at garden supply stores or online.

Solution:

1. Aim for a soil pH between 5.8 and 6.2.
2. To increase the pH, add sulfur or organic matter.
3. To decrease the pH, incorporate some crushed limestone (lime) into the soil.

5. Pest Damage

Dahlias can fall prey to various insect pests, leading to browning leaves. Common culprits include mealybugs, scale insects, and aphids. Despite the variety of pests, they can all be managed with similar strategies. These insects feed on the leaves by extracting nutrients, and some may even attach themselves permanently to a spot for their entire life cycle.

Identifying these pests is straightforward. Scale insects, for example, appear as bumps on the stems and leave behind a black, crusty residue. Underneath the leaves, you might notice spider-web-like silk and tiny moving insects.

Solution:

1. In the evening, after the sun has set, thoroughly spray the affected areas with horticultural oil.
2. Apply the oil to new growth as well to prevent future infestations.

6. Diseases

Fungal spores, carried by the wind, can land on dahlias and start to colonize the leaves. These spores develop tiny roots that penetrate the leaves, extracting their nutrients. As the fungus grows, the leaves will eventually turn brown and dry out. However, if caught early, it’s possible to treat the fungus before it causes significant damage.

You’ll notice the fungus as a white or green layer on the leaf surface. A clear sign of an advanced infection is a small black dot surrounded by a brown, dead area on the leaf.

Solution:

1. Treat the leaves with a fungicide spray to kill the fungus.
2. Remove as many affected leaves as you can to prevent further spread.
3. Ensure good air circulation around your dahlias to reduce humidity and discourage fungal growth.

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