13 Common Problems With Dahlia Flowers

Dahlia gardening is a true act of love and optimism. In temperate climates, dahlia devotees spend the long winter months checking on dormant tubers in the cellar, ordering new varieties from grower websites, and eagerly anticipating the next season’s mid-summer bloom.

We’re addicted to their oversized, technicolor flowers and do everything we can to help them thrive in our backyards. Given them full sun and well-drained soil, carefully staked their fragile stems, and made sure they aren’t overcrowded. Pinched, deadheaded, and watered only at the base.

But dahlias can be a little tricky, and things can go wrong in any number of ways. If you find yourself puzzled by a mystery ailment or wondering what went wrong, don’t worry. Here are some of the most common dahlia problems you might face, along with a few suggestions to help get things back on track.

No Blooms at All

By far, the most common dahlia problem I hear is that the plants look healthy, but they never bloom. There are a few reasons why this might happen.

First, dahlias need at least six full hours of direct sunlight per day to flower. These hours don’t have to be consecutive, but they must add up to six. In shady or partially shaded areas, most dahlias simply won’t bloom.

Second, dahlias require soil temperatures of at least 60°F (16°C) to grow and produce flowers. If you planted them too early, when the ground was still cold, you may have unintentionally stunted their buds.

Third, if the soil around your dahlias feels dry and crumbly, the cause of this dahlia problem may not lack of nutrients. Aim to keep the soil consistently moist (but not soggy) to support healthy bud development.

Lastly, using a fertilizer with too much nitrogen can encourage leafy growth at the expense of flowers. Choose one with a balanced NPK ratio like 5-10-5, and apply it just once or twice during the growing season to promote blooming.

Not Enough Blooms

Another common dahlia problem is sparse blooming. This can stem from the same causes as non-blooming plants (see above), but it may also be tied to a few maintenance-related factors:

Left to their own devices, most dahlia varieties will produce a few large, showy blooms rather than clusters of smaller ones. If you’re aiming for quantity over size, pinch your dahlias early in the season to encourage side shoots and multiple blooming points.

When new plants reach about a foot in height and have 3 to 4 sets of leaves, use a small pruning tool to snip off the top shoot just above the third or fourth leaf set. Repeat the process a few weeks later, and you’ll be rewarded with bushier plants and more abundant blooms.

If you haven’t been removing faded blooms promptly, you may notice a decline in flower production. Spent flowers signal to the plant that reproduction has occurred, prompting it to shift energy toward root and foliage growth instead.

To deadhead properly, use your fingers or a small pruner to remove spent blooms just above a set of leaves or an intersecting branch. On dahlias, spent blooms are pointed, while new buds are rounded. Keep this in mind when trimming. Regular deadheading gives your plant a significant boost in flower production.

If your dahlias still aren’t blooming as they should, they may be stressed by disease or insect activity. Check below for more symptoms to help diagnose the dahlia problem.

Blooms Are Distorted

Commonly transmitted by sap-sucking leafhoppers, aster yellows is a bacterial disease often identified by the presence of malformed buds. This dahlia problem disrupts photosynthesis and hormone flow, leading to abnormal bloom development.

Additional symptoms include distorted leaves and stunted stems. Unfortunately, there’s no cure for aster yellows. Infected plants must be removed from the garden, and leafhopper populations should be managed to prevent the spread to nearby plants.

If your dahlia’s blooms appear misshapen or fail to open properly, tarnished plant bugs may be the culprit. These pests use piercing-sucking mouthparts and leave behind toxic saliva that distorts growth.

Attracted to the sweet nectar of flower buds, tarnished plant bugs can be discouraged through careful weed control (especially flowering weeds), horticultural oil applications, and general habitat management. Since they overwinter in leaf litter, keeping beds clean is essential.

Similar to plant bugs, borers and thrips also attack and deform dahlia buds by sucking or tunneling through them. Damage to the leaves often accompanies bud deformation, helping to identify the presence of these pests. Manage them as you would plant bugs: debris removal, weed control, and the use of insecticidal soaps.

Buds Won’t Open

If your dahlias have plenty of buds but they never open or develop into full blooms, take a closer look at your watering habits and check for insect damage to identify the cause of this dahlia problem.

Waterlogged roots can’t transport nutrients effectively, and as a result, bud development may stall. Excess moisture also makes roots more vulnerable to rot and fungal infections, which can damage or kill young buds. To support healthy bud and flower production, plant dahlias in well-drained soil that doesn’t puddle or retain standing water.

On the other hand, if the buds appear dry and brown, your dahlias may not be getting enough moisture. Deep, infrequent watering is best. It allows the roots to access adequate hydration without becoming waterlogged.

If your plant’s stems and leaves look healthy but the buds refuse to open, inspect the buds closely for tiny holes or other signs of insect activity. Thrips and earwigs can cause significant damage by puncturing or chewing through developing buds. Buds that are repeatedly pierced may become infertile and fail to open at all.

If earwigs are the dahlia problem, try luring them away with loosely rolled newspaper traps or shallow containers filled with oil. You can also hand-pick them at night. If you suspect thrips, treat the plant with neem oil or use a strong spray from the hose daily to dislodge them.

Leaves Have White Fuzz

If you suddenly notice a white fuzz or powdery coating on your dahlias’ leaves, they’re likely dealing with powdery mildew. This common dahlia problem typically appears as white fuzz on leaves and sometimes on stems.

Watch for it during periods of high daytime humidity paired with cool nighttime temperatures. It’s most prevalent in early spring or late fall.

Although powdery mildew is primarily an aesthetic issue and won’t kill your plants, it can be unsightly and may spread if left untreated. Promptly remove and dispose of affected leaves and stems. When preparing dahlias for winter storage, cut stems off at the point where they meet the tuber to help prevent the disease from carrying over.

Leaves Have Gray Fuzz

If the fuzz on your dahlias’ leaves has a slightly gray tone and also appears on stems and blooms, gray mold (also known as botrytis) may be the culprit. This disease often follows rainy, overcast weather with high humidity.

Like powdery mildew, gray mold won’t kill your plants outright, but it should be managed carefully. Keep it in check through regular pruning and the prompt removal of affected plant parts.

To prevent gray mold from overwintering, cut your dahlias down completely at the end of the season. When digging up tubers for storage, clean them thoroughly and allow them to dry completely before storing.

Entirely Yellow Leaves

There are several reasons why a dahlia’s leaves might turn completely yellow, most of which stem from improper irrigation or nutrient deficiencies.

If the yellow leaves are soft and droopy, the plant may be receiving too much water. Excess irrigation can waterlog the tuberous roots, causing the leaves to lose their ability to photosynthesize.

Make sure you’re watering slowly and deeply to maintain moist (but not muddy) soil conditions. Working some organic matter into the soil can also help improve drainage.

On the other hand, leaves that are yellow, dry, and curling under may indicate the plant isn’t getting enough moisture. This is especially common during droughts or the peak heat of summer, when roots conserve groundwater, leaving the foliage to fend for itself.

Dry, yellow leaves should be addressed promptly with supplemental watering to keep soil evenly moist at all times. Adding a layer of mulch can help insulate the soil and reduce moisture loss.

If your dahlias have pale yellow leaves with green midribs and veins, they’re likely experiencing a nutrient deficiency. This disruption in the photosynthetic process affects the plant’s ability to function properly.

Yellowing older leaves often signal a magnesium deficiency, while yellowing in newer leaves typically points to an iron deficiency. Conduct a soil test to determine your soil’s pH. Dahlias prefer slightly acidic conditions, with an ideal pH around 6.5. To adjust the pH, add lime to raise it or peat moss to lower it, depending on your test results.

Partially Yellow Leaves

Leaves with yellow spots, streaks, or splotches are often suffering from insect damage. If the leaves appear stippled with yellow and have a sticky black substance that’s attracting ants, aphids are likely the cause. These rapidly multiplying, pear-shaped insects can cause significant damage in a short time and should be dealt with promptly.

A natural way to remove aphids is to regularly spray the undersides of affected leaves with a strong stream of water. For more severe infestations, you can introduce beneficial insects like parasitic wasps or lady beetles to control the population. Horticultural oils applied to the undersides of leaves can also be effective.

If your dahlia leaves are mottled yellow, curled, and accompanied by fine, sticky webs, spider mites may be to blame. Mites usually work from the ground up, first targeting stems and lower foliage near the soil line.

Because spider mites thrive in dry, dusty conditions, keeping your dahlia leaves slightly moist can help discourage them. Regular pruning and gently shaking off heavily infested leaves can also help reduce their numbers.

Brown Leaves

If your dahlia’s leaves have turned brown, first check that they’re receiving adequate water and the right amount of sunlight. Then, evaluate them for signs of disease to identify the dahlia problem.

Too much sun can cause leaves to become dry and crunchy. If your dahlias are getting more than 8 hours of direct sunlight per day, moisture may be evaporating faster than the plant can replace it. Try offering some protection from the late afternoon sun and water them regularly. They may bounce back.

If leaves are dark, wilted during the day, but perk up at night, your dahlias could be infected with wilt disease. A telltale sign is stem ooze. If you cut into the stem with a sharp tool and notice any discharge, you’re likely dealing with a bacterial infection. In this case, remove the affected plants along with the surrounding soil and discard them.

If it’s late in the season, browning leaves may simply indicate that your dahlias are ready for dormancy. In that case, dig up the tubers and prepare them for winter storage in a cool, dark space with temperatures between 32 and 50°F (0-10°C).

White Spots on Leaves

There are several reasons your dahlia’s leaves may develop white spots, most often due to a fungal issue or insect damage.

Before powdery mildew spreads into a white fuzz, it begins as a scattering of white spots that gradually grow and merge. Catching this fungal disease in its early stages makes it easier to manage. Prune off affected leaves and remove them from the garden to prevent further spread.

If you notice pale yellow to white spots with dark centers, your dahlias may be showing signs of smut, a fungal condition that typically appears after prolonged rain, high humidity, and overcast skies. Though unsightly, smut is not usually fatal. Simply prune off and dispose of the affected leaves.

A viral dahlia problem like mosaic virus presents differently. In the early stages, infected leaves develop a stained glass appearance, with spots that may be white, light green, or yellow, scattered irregularly across the surface.

Watch for this condition if your garden has a persistent aphid population, as aphids are common carriers of the virus. Use insecticidal soap or introduce beneficial predators to reduce aphid numbers. Unfortunately, mosaic virus is eventually fatal and spreads easily. Remove and dispose of any affected plants immediately.

Thrip damage can also result in white, papery-looking scars on dahlia leaves. Thrips are especially active during dry weather, so consider this a likely cause if conditions have been hot and arid. Keep your dahlias evenly watered, spray the leaves with a strong blast from the hose to dislodge thrips, and use neem oil to manage heavier infestations.

Chew Marks on Leaves

If your dahlia leaves have irregular holes or shredded edges, check for a slimy mucus trail. This usually indicates the presence of slugs or snails. Take a flashlight into the garden at night and inspect the undersides of leaves, when these pests are most active.

To deter slugs and snails, keep foliage as dry as possible by watering only at the base of the plant. Maintain a tidy garden, free of leaf litter and other potential hiding spots. Planting deterrents like chives, mint, or garlic can also help prevent these dahlia problems. Use a beer or soda trap to kill them off.

If the leaves appear to have small, punched-out holes, young caterpillars are likely the cause. Rolled leaves held together by webs are another clear sign of caterpillar activity. These pests can cause significant damage in a short period of time.

The most effective way to manage caterpillars is to remove them by hand. Encouraging natural predators (such as wasps, spiders, and birds) can also help keep populations in check.

Non-discriminating feeders like deer and rabbits are drawn to herbaceous plants for their carbohydrates, minerals, and moisture, dahlias included. Ragged, uneven chew marks between 1 and 4 feet off the ground often point to deer, while clean, angled bites and snapped stems usually indicate rabbits.

To prevent these mammals from feasting on your flowers, install fencing and eliminate nearby hiding places. Keep the area around your garden free of clutter and potential nesting spots. Long grass and strappy perennials are also appealing to these animals as shelter, so consider removing them if wildlife pressure is a concern.

Tall and Leggy Stems

Dahlias that don’t receive at least 6 hours of natural sunlight will stretch toward the light at the expense of bloom production. The result is often a bed of awkward, leggy stems and leaves with few or no flowers. If you can’t provide 6 hours of sun, it’s best to plant them in another location, or consider skipping them altogether.

If you catch leggy growth early in the season (when sprouts are around a foot tall), you can snip off the tops to encourage side branching and create a fuller, bushier plant.

Otherwise, you’ll likely end up with a tall, spindly dahlia later in the season. If that’s the look you’re after, just stake it and enjoy. But if you prefer a more compact shape with lower growth, go ahead and pinch early.

Overfertilizing, or using a fertilizer with the wrong N-P-K ratio, can also lead to excessive leaf and stem growth rather than blooms. Avoid using lawn fertilizers near your dahlias, as they often contain high levels of nitrogen. Instead, choose a fertilizer with more phosphorus than nitrogen or potassium to support flower development.

Collapsing Stems

If your dahlia stems are black and mushy, they are likely suffering from a disease. Consider the following potential dahlia problems to determine whether the condition is manageable or fatal.

If the stems appear brown or black and are collapsing, take a sharp knife and make a lateral slice along one of the stems. If you find a string of tiny black spores inside, it’s a clear sign of sclerotinia disease.

Sclerotinia doesn’t have to be fatal if caught early. Remove the affected stems immediately and consider applying a fungicide at the end of the season to prevent the pathogen from overwintering. To reduce the risk of future outbreaks, promptly remove fallen dahlia petals from garden beds and monitor plants closely during wet weather.

If you notice darkened stem tissue near the soil line and streaky brown or black roots, wilt disease is the likely cause. This fungal condition thrives in fluctuating temperatures (especially during sudden shifts from cool to warm) so keep an eye out during those transitions.

Unfortunately, wilt attacks the plant’s entire feeding system, and infected dahlias will not survive. Remove and destroy affected plants, and avoid planting dahlias in the same bed next year. The soil in which the diseased plants were grown should also be removed or replaced to prevent further contamination.