Table of Contents
- Introduction
- Chewed Leaves and Petals: Identifying the Nighttime Visitors
- Sucking Pests: Why Your Foliage Looks Stippled or Yellow
- Deformed Flowers and Distorted Growth
- When It Isn't a Bug: Spotting Dahlia Diseases
- Cultural Prevention: Keeping Your Dahlias Strong
- Conclusion
- FAQ
Introduction
Watching a dahlia bud slowly unfurl its intricate petals is one of the most rewarding moments in the garden. These spectacular flowers are the stars of the summer and autumn landscape, offering a range of colors and shapes that few other plants can match in dahlia collections. At Longfield Gardens, we know that after all the effort of planting and tending your tubers, seeing holes in the leaves or chewed petals can be a bit disappointing.
The good news is that most dahlia visitors are easy to manage once you know what to look for. This guide is for home gardeners who want to identify exactly what is nibbling on their plants and find simple, effective ways to keep them healthy. We will cover the most common insects, how to spot their unique damage, and how to use gentle, practical methods to protect your blooms. If you’re ready to shop, start with our dahlias for sale.
By understanding the signs of common dahlia pests and diseases, you can take a few easy steps to ensure your garden remains a vibrant, beautiful space all season long. For a broader overview of planting, spacing, and support, see our How to Grow Dahlias at Home guide.
Chewed Leaves and Petals: Identifying the Nighttime Visitors
If you wake up to find jagged holes in your dahlia leaves or "Swiss cheese" petals, you are likely dealing with pests that prefer to do their work under the cover of darkness, whether you grow compact plants or dinnerplate dahlias. Identifying these requires a bit of detective work since they are rarely seen during the heat of the day.
Slugs and Snails
Slugs and snails are often the first pests to arrive, especially in the spring when dahlia shoots are tender and succulent. They leave behind a tell-tale sign: a shiny, silvery slime trail on the foliage or the soil nearby. Their damage appears as large, irregular holes with smooth edges. If you want a planting refresher before the season starts, our How to Plant Dahlias guide covers the basics.
To manage these slimy guests, focus on making the environment less inviting. Slugs love damp, dark hiding spots. You can discourage them by watering your plants in the morning rather than the evening, allowing the soil surface to dry out before nightfall. Removing low-hanging leaves that touch the ground also removes their "ladders" to the rest of the plant. If you need a more direct approach, organic slug pellets containing ferric phosphate are effective and safe to use around pets and birds when applied according to the label.
Earwigs
Earwigs are small, brown insects with a pair of pincers at the back. While they actually help the garden by eating aphids, they also have a taste for dahlia flower buds and petals, and the damage can stand out especially on ball dahlias.
A simple way to manage earwigs is to use their preference for tight, dark spaces against them. Place an upturned flower pot filled with damp straw or shredded newspaper on a stake among your dahlias. During the day, earwigs will crawl inside for shelter. You can then shake them out into a bucket of soapy water. Another trick is to apply a small amount of petroleum jelly to the main stem of the plant. This creates a slippery barrier that prevents them from climbing up to reach the flowers.
Caterpillars and Cutworms
Various caterpillars enjoy munching on dahlia foliage. You will usually see large chunks of leaves missing or small, dark green pellets (droppings) on the leaves. Cutworms are a specific type of caterpillar that stays near the soil surface and can "cut" through the stem of a young plant at night.
The easiest way to deal with caterpillars is to simply pick them off by hand when you see them. For cutworms, a "collar" made from a recycled plastic cup or a cardboard tube placed around the base of the stem at planting time provides a simple, physical barrier that keeps them away from your tender dahlia starts.
What to do next:
- Walk through your garden in the morning to check for slime trails or new holes.
- Remove any dead leaves or debris from around the base of the plants to eliminate hiding spots.
- Set up a simple "trap" if you suspect earwigs are the main culprit.
Sucking Pests: Why Your Foliage Looks Stippled or Yellow
Not all pests eat the leaves; some prefer to drink the plant's juices. These "sucking" pests can cause the leaves to look mottled, yellow, or distorted. Because these insects are often very small, you have to look closely to find them.
Aphids
Aphids are tiny, pear-shaped insects that cluster on the underside of leaves or around new flower buds. They come in various colors, including green, black, and yellow. As they feed, they excrete a sticky substance called "honeydew," which can attract ants and lead to the growth of a black, powdery fungus known as sooty mold.
You don't always need special products to handle aphids. A strong blast of water from your garden hose is often enough to knock them off the plant. Since they are soft-bodied and slow-moving, they usually can't find their way back up. Encouraging ladybugs and lacewings in your garden is another great long-term strategy, as these beneficial insects love to feast on aphids.
Spider Mites
If your dahlia leaves are starting to look bronzed, yellow, or "dusty," you might have spider mites. These are microscopic relatives of spiders that thrive in hot, dry weather. In severe cases, you may see very fine webbing between the leaves or stems.
Spider mites dislike humidity. Regularly misting your dahlia foliage during dry spells can help keep their populations low. If an infestation is already present, insecticidal soap or neem oil can be used, but be sure to spray the undersides of the leaves where the mites hide. For more growing tips and an overview of the plant, see All About Dahlias.
Thrips
Thrips are incredibly tiny, slender insects that are often hard to see with the naked eye. They feed by scraping the surface of leaves and petals, leaving behind silvery-gray streaks or flecks. They are especially fond of light-colored dahlias like white or pale pink varieties.
To check for thrips, gently tap a dahlia flower over a sheet of white paper. If you see tiny, dark specks that look like "moving dust," those are thrips. Keeping your plants well-watered helps them resist thrip damage. You can also use blue or yellow sticky traps hung near the plants to catch the adults before they can lay eggs.
Leafhoppers
Leafhoppers are small, wedge-shaped insects that jump or fly away quickly when the plant is disturbed. They suck juices from the leaves, causing a condition called "hopper burn," where the edges of the leaves turn yellow or brown and curl upward.
Because leafhoppers move fast, they can be tricky to catch. Keeping the area around your dahlias free of weeds can reduce their breeding grounds. If they become a significant problem, an application of insecticidal soap can help, but it must come into contact with the insect to work.
Deformed Flowers and Distorted Growth
Sometimes the damage isn't on the leaves at all, but on the developing flowers. If your dahlias are blooming with "lopsided" heads or if the buds are turning brown and falling off before they open, certain bugs are likely the cause.
Tarnished Plant Bugs
These are small, mottled brown insects that inject a toxin into the plant as they feed. This toxin causes the flower buds to develop unevenly. You might see a flower that only opens on one side or a bud that looks "blasted" (shriveled and brown).
Tarnished plant bugs overwinter in garden debris and weeds. One of the best ways to prevent them is to keep your garden tidy in the autumn and spring. If you see the insects on your plants, you can knock them into a jar of soapy water. For prized blooms, some gardeners use small mesh organza bags to cover the buds as they grow, providing a physical shield against these pests.
Cucumber Beetles
Cucumber beetles are bright yellow with either black spots or black stripes. While they love squash and cucumbers, they are also very attracted to dahlia petals. They chew holes in the flowers, often ruining the look of the bloom just as it reaches its peak.
If you have a vegetable garden, try to plant your dahlias as far away from your cucumbers and melons as possible. Hand-picking is effective if you only have a few plants. Like the tarnished plant bug, cucumber beetles can be kept off your best flowers using small mesh bags.
Capsid Bugs
Capsid bugs are similar to tarnished plant bugs. They feed on the growing tips of the dahlia, which can lead to "blind" shoots that don't produce flowers or leaves that emerge with many small, tattered holes. This damage often looks like someone poked the leaves with a needle while they were still folded up in the bud.
Because the damage happens early in the bud's development, you often won't see the insect by the time the leaves unfurl. Encouraging a diverse garden with plenty of predatory insects like spiders and birds is the best way to keep capsid bugs in check naturally.
Key Takeaway: Sucking pests and beetles often cause "indirect" damage like yellowing or deformed flowers. Keeping the garden clean, encouraging beneficial insects, and using physical barriers like mesh bags are the most effective ways to protect your dahlia's beauty.
When It Isn't a Bug: Spotting Dahlia Diseases
Sometimes the "eating" or damage you see isn't caused by an insect at all, but by a fungus, bacteria, or virus. Identifying these early is important for the long-term health of your dahlia collection.
Powdery Mildew
This is perhaps the most common dahlia issue, especially in late summer when the air is humid. It looks like a dusting of white flour on the leaves. While it rarely kills the plant, it can make the foliage look unattractive and weaken the plant over time.
To prevent powdery mildew, focus on air circulation. Give your dahlias plenty of space when you plant them so air can move freely between the stems. Avoid overhead watering; instead, aim the hose at the base of the plant to keep the leaves dry. If it appears every year in your climate, you can use a simple spray made of one part milk to nine parts water, which can help change the pH on the leaf surface and discourage the fungus.
Botrytis (Gray Mold)
In very wet, cool weather, you might see dahlia buds turn brown and become covered in a fuzzy gray mold. This is botrytis. It spreads quickly in damp conditions and can eventually travel down the stem.
The best fix for botrytis is "garden hygiene." As soon as you see a flower that is starting to fade or a bud that looks moldy, snip it off and put it in the trash (not the compost). Keeping the plant "deadheaded" (removing old flowers) prevents the mold from gaining a foothold.
Verticillium Wilt
If your dahlia suddenly wilts even though the soil is moist, or if the leaves turn yellow and brown starting from the bottom of the plant, it may be a soil-borne fungus like Verticillium wilt. This fungus clogs the plant's water-conducting tissues.
Unfortunately, there is no cure for wilt once it is in the plant. The best approach is to remove the affected plant to prevent it from spreading to others. To avoid this in the future, ensure your soil has excellent drainage. "Drainage" simply means that water moves through the soil easily and doesn't sit in puddles around the roots.
Dahlia Viruses
Viruses can cause stunted growth, yellow "mosaic" patterns on leaves, or weirdly twisted foliage. Unlike fungal issues, viruses are often spread from plant to plant by insects like aphids and leafhoppers.
If a dahlia plant looks significantly smaller than its neighbors and has distorted, mottled leaves, it is safest to remove the plant and the tuber at the end of the season. Do not save tubers from plants that show signs of virus, as the virus will live inside the tuber and reappear the following year.
Cultural Prevention: Keeping Your Dahlias Strong
The most important rule we follow is "Right plant, right place." A healthy dahlia is much better at defending itself against pests and diseases than a stressed one. By getting the basics right, you can avoid many of the problems listed above, and our Planning Guide for Dahlias makes it easy to compare forms and sizes.
Sunlight and Space
Dahlias are sun-lovers. They need at least six to eight hours of direct sunlight every day. Plants grown in too much shade will have "leggy" or weak stems and are more prone to powdery mildew. Space your dahlias about 18 to 24 inches apart. This might seem like a lot when you first plant the tubers, but they grow quickly and need that space for air to circulate.
Soil and Water
Dahlias need rich, well-draining soil. If your soil is heavy clay, adding some compost or organic matter can help loosen it up. When it comes to watering, "deep and infrequent" is the goal. Instead of a light sprinkle every day, give the plants a good soaking once or twice a week (depending on the weather). This encourages the roots to grow deep into the ground, making the plant more resilient.
Fertilizer Balance
While dahlias are hungry plants, be careful not to overdo the nitrogen. Nitrogen encourages lots of lush, green leaves, but too much of it can make the foliage "soft" and very attractive to aphids and slugs. Look for a balanced fertilizer or one with a slightly higher middle number (phosphorus) to support big, beautiful blooms.
End of Season Care
If you live in a cold climate and dig up your tubers for the winter, make sure they are clean and dry before storing them. This prevents rot and bacterial issues from overwintering. If you leave them in the ground (in warmer zones), a layer of mulch can help protect them, but remove it in the spring to prevent it from becoming a hiding spot for slugs and earwigs. For step-by-step help, see How to Lift and Store Dahlia Bulbs.
Simple Gardening Rules for Dahlias:
- Sun: Give them 6-8 hours of direct light.
- Air: Space them out to prevent mold and mildew.
- Water: Aim for the base of the plant to keep leaves dry.
- Hygiene: Remove old flowers and dead leaves promptly.
Conclusion
At Longfield Gardens, we believe that gardening should be a source of joy, not a cause for worry. While it can be a bit startling to find a bug on your favorite flower, remember that a few nibbled leaves are a natural part of a healthy, living garden. Most dahlia pests are easily managed with simple, hands-on techniques like evening patrols for slugs or using mesh bags for your show-stopping blooms. If you want to refresh your patch next season, browse our new dahlias.
By focusing on the basics—good sunlight, proper spacing, and consistent watering—you give your dahlias the best possible start. Healthy plants are naturally more resistant to visitors, and they have the energy to keep producing flowers all the way until the first frost. You can also explore our other dahlias for more options.
We encourage you to spend a little time each day in your garden, observing your plants and enjoying the progress. Often, catching a small issue early is all it takes to keep your dahlias looking their best. For order timing and delivery details, see our Shipping Information.
Next Step: Head out to your dahlia patch tomorrow morning with a cup of coffee and look at the undersides of a few leaves. If you see anything unusual, use the tips in this guide to help your plants thrive!
FAQ
Why are there small, round holes in my dahlia petals but I don't see any bugs?
This is usually the work of earwigs or small caterpillars that feed at night. Since they hide during the day, you may only see the damage they leave behind. Try setting a simple trap using an upturned flower pot with straw to see if you can catch the culprits.
Is it safe to use soapy water on my dahlias to kill aphids?
Yes, a mild solution of water and a few drops of dish soap can be very effective against soft-bodied insects like aphids. However, it is best to test it on a single leaf first and avoid spraying in the middle of a hot, sunny day, as the sun can sometimes "burn" the damp foliage.
Can I save dahlia tubers if the plant had powdery mildew?
Yes, powdery mildew is a surface fungal issue and does not usually affect the health of the tuber itself. Just make sure to cut back the foliage and dispose of it at the end of the season, then clean the tubers well before storing them for the winter.
Do I need to use chemicals to get rid of dahlia pests?
Not necessarily. Most home gardeners find that "cultural controls"—like keeping the garden tidy, hand-picking larger bugs, and encouraging birds and ladybugs—are enough to keep dahlia pests under control. Physical barriers like organza bags are also a fantastic chemical-free way to protect individual flowers. For more answers to common spring-planting questions, see our FAQs - Spring Planted Bulbs.