What is eating ragged holes in my dahlia leaves and flowers at night?

Most likely earwigs or slugs and snails, both of which feed after dark and hide by day. Earwigs leave irregular holes in buds and petals; slugs leave holes plus slimy trails. Trap earwigs with rolled newspaper or oil traps, and patrol for slugs at night or use an iron-phosphate bait.

What is the best organic control for dahlia aphids?

Start with a forceful spray of water to knock aphids off, repeated as needed. For more, insecticidal soap or horticultural oil applied to cover the insects works well, and lady beetles and lacewings help if you let them. Go easy on nitrogen, which pushes the soft growth aphids prefer.

How do I stop powdery mildew on dahlias?

Prevent it. Plant in full sun with good spacing and air movement, since mildew favors shade and crowding at moderate temperatures. Sulfur-based fungicides used before symptoms appear act as protectants. Once a leaf is coated, no spray makes it clean again.

Why do my dahlia tubers keep rotting?

Almost always too much water and poor drainage. Plant into well-drained soil or a raised bed, do not water tubers before they have roots, and keep water from pooling at the crown. The same rule governs storage: barely moist, never soggy, with airflow.

Dahlia Pests and Problems: A Field Guide

Updated 2026-06-13

Dahlias are tougher than they look, but a few pests find them reliably. The good news is that most of the trouble is visible and fixable. Catch it early, know what you are looking at, and you rarely lose a plant.

This is the field guide. For each common pest and problem, here is what it looks like, what it does, and how to control it, both the gentle organic way and the stronger conventional one. The single best prevention runs through all of it: a healthy plant in full sun with good air movement and clean soil shrugs off far more than a stressed one.

We name no farm here. Pests are a fact of growing this flower, not a mark against anyone who grows it.

Earwigs

Earwigs are the slim reddish-brown insects, about three-quarters of an inch long, with the pincers at the tail end. The pincers look alarming but are harmless to you. The damage is the issue. Earwigs feed at night and hide by day in dark, moist places, including down inside the flowers themselves, and they chew ragged, irregular holes in leaves, buds, and petals.

Because they hide by day, you often see the damage before the bug. The fix is trapping. Set rolled newspaper, short lengths of hose, or bamboo tubes near the plants at dusk, then in the morning shake the earwigs that crawled in for shelter into soapy water. A low-sided can sunk to soil level and filled with vegetable oil plus a little fish oil or bacon grease also draws and drowns them. UC IPM advises trapping every day until you stop catching them.

Clear away mulch, weeds, and debris near the plants, since that is where earwigs shelter and breed. Daily trapping usually brings them down to a level you can live with, so insecticides are rarely needed. When one is warranted, products containing spinosad are the more selective choice.

Slugs and snails

Slugs and snails do their worst early in the season, when young dahlia foliage is soft and tender, and they can strip a new shoot overnight. The signs are ragged holes with slimy trails nearby, and feeding mostly at night or after rain.

The cheapest control is a nightly patrol. Go out after dark or early morning with a flashlight and collect them by hand. Keep the soil surface dry and clear of damp hiding places, since slugs need moisture. Beer traps, shallow dishes sunk to soil level, draw and drown some of them.

For more pressure, iron-phosphate slug baits, sold under names like Sluggo, are a pet-safer option than the older metaldehyde baits. Scatter them lightly per the label, not in piles. A copper barrier ring around prized plants also deters them.

Aphids and thrips

Aphids are tiny soft-bodied insects, green, black, or other colors, with a pair of little tubes, called cornicles, sticking out the back end. They cluster on new growth and flower buds, curl and distort the leaves, and leave behind sticky honeydew that turns black with sooty mold. They also move viruses from plant to plant, which matters a great deal in dahlias.

For aphids, start with a forceful spray of water to knock them off, repeated as needed. Insecticidal soap or horticultural oil, applied to cover the insects, handles heavier infestations, and natural enemies like lady beetles and lacewings do real work if you let them. Avoid heavy nitrogen feeding, which pushes the soft new growth aphids love.

Thrips are slivers almost too small to see. They rasp at petals and leaves and leave discoloration and distortion on both blooms and foliage. Keeping plants watered and unstressed helps, and insecticidal soaps are the common control. Because both aphids and thrips can spread virus, controlling them early protects more than just the one plant.

Spider mites

Spider mites are not insects but tiny relatives of spiders, and they thrive in hot, dry, dusty conditions. They are easy to miss until the damage shows: foliage mottled with fine yellow stippling, leaves that wilt and drop, and, in a bad case, fine webbing strung between the leaves and stems.

Heat and drought stress are the open door, so the first defense is keeping plants well watered and not baking. A strong spray of water on the undersides of leaves dislodges many of them and raises the humidity they dislike.

For more, insecticidal soap or horticultural oil sprays work, with thorough coverage of the leaf undersides where mites live. Avoid broad-spectrum insecticides for mites, since those often kill the predatory mites that would otherwise keep the pest in check, and a flare-up follows.

Powdery mildew

Powdery mildew is the white, dusty coating that spreads across dahlia leaves, usually in the second half of the season. Unlike most fungal diseases, it does not need wet leaves to take hold. UC IPM notes it favors moderate temperatures, roughly 60 to 80 degrees Fahrenheit, along with shade and crowding, and it can grow even on dry foliage in humid air.

Prevention beats cure here. Site plants in full sun, space them for good air movement, and avoid crowding, since still, shaded, packed plantings are where mildew thrives. Overhead watering in the morning can actually wash spores off and let leaves dry by midday, though many growers prefer to water at the base for other reasons.

If you spray, treat it as protection, not rescue. Sulfur-based fungicides used before the disease appears act as protectants, and horticultural oil can knock back an existing infection, but do not apply oil in high heat or close behind a sulfur spray. Once mildew has coated a leaf, no spray makes that leaf clean again.

Tuber and stem rot

Rot is less a pest than a consequence, almost always of too much water and not enough drainage. A tuber sitting in cold, soggy soil rots before it sprouts. A stem can rot at the base if water pools around the crown, and a hollow cut stalk left open in the rain funnels water straight down into the plant.

The defenses are all about water and air. Plant into well-drained soil, or onto a mound or raised bed if your ground stays wet. Do not water tubers at planting before they have roots. Keep mulch pulled back from the stems, and when you cut stalks, do it so water does not run down the hollow center.

Rot also haunts winter storage, where the same rule holds: barely moist, never soggy, with airflow. A tuber that has gone soft and brown through the crown is finished, so cull it before it spreads to its neighbors. Most rot is preventable, and almost all of it traces back to water that had nowhere to drain.

Common questions

What is eating ragged holes in my dahlia leaves and flowers at night?: Most likely earwigs or slugs and snails, both of which feed after dark and hide by day. Earwigs leave irregular holes in buds and petals; slugs leave holes plus slimy trails. Trap earwigs with rolled newspaper or oil traps, and patrol for slugs at night or use an iron-phosphate bait.
What is the best organic control for dahlia aphids?: Start with a forceful spray of water to knock aphids off, repeated as needed. For more, insecticidal soap or horticultural oil applied to cover the insects works well, and lady beetles and lacewings help if you let them. Go easy on nitrogen, which pushes the soft growth aphids prefer.
How do I stop powdery mildew on dahlias?: Prevent it. Plant in full sun with good spacing and air movement, since mildew favors shade and crowding at moderate temperatures. Sulfur-based fungicides used before symptoms appear act as protectants. Once a leaf is coated, no spray makes it clean again.
Why do my dahlia tubers keep rotting?: Almost always too much water and poor drainage. Plant into well-drained soil or a raised bed, do not water tubers before they have roots, and keep water from pooling at the crown. The same rule governs storage: barely moist, never soggy, with airflow.