13 Common Problems With Dahlia Flowers
Do your dahlias have problems growing this season, but you aren't quite sure what's happening to them? Dahlias can fall victim to many different issues, depending on your climate. In this article, certified master gardener Liz Jaros walks through the most common problems with Dahlias, and how to fix them.
Dahlia gardening is a true act of love and optimism. In temperate climates, dahlia devotees pass the long winter days checking on dormant tubers in the cellar, ordering new varieties from grower websites, and anxiously anticipating next season’s mid-summer dahlia display.
We are addicted to their oversized, technicolor blooms and try to do everything right to ensure they thrive in our backyards. We’ve given them full sun and well-drained soil. Then we carefully staked their fragile stems and made sure they weren’t crowded. And we’ve pinched and deadheaded and watered just the roots.
But dahlias can be a little tricky to grow, and things can go wrong in a lot of different ways. If you find yourself scratching your head over a mystery ailment, or wondering where you went wrong, don’t fret. Here are some of the most common issues you’ll encounter in the dahlia garden, and some suggestions for turning things around.
No Blooms at All
By far the biggest dahlia problem I hear about is that their leaves look just fine but they never had any flowers, and there are a few reasons why this might happen.
Dahlias require 6 full hours of direct sunlight in order to bloom. The hours do not need to be consecutive, but they must add up to 6. In shade and part shade conditions, most dahlias will not produce flowers.
Dahlias need soil temperatures of 60 degrees in order to grow and produce blooms. If you jumped the gun and planted them in the cold ground, you may have stunted their buds.
If the dirt surrounding your dahlias is dry and crumbly, they might not be drawing adequate nutrients from the soil. Make sure the soil in which they’re planted is moist but not soggy and they will be better able to generate buds.
Treating dahlias with a fertilizer that has more nitrogen than phosphorus or potassium will encourage foliage growth rather than blooms. Make sure you use an NPK ratio of 5-10-5, and fertilize just once or twice during the season.
Not Enough Blooms
Another common dahlia problem is that blooms are few and far between. This can be due to the same factors as non-blooming dahlias (see above) but it also might be related to one of the following maintenance issues:
If left to their own devices, most dahlia varieties will send up a couple of large, showy blooms rather than a cluster of slightly smaller blooms. If you’re going for quantity over quality, you’ll want to pinch your dahlias early in the season to encourage side shoots and multiple bloom points.
When new plants reach a foot in height and possess 3-4 sets of leaves, use a small pruning tool to cut the top shoots off just above the third or fourth set of leaves. Repeat in a few weeks and you’ll have bushier, more abundant dahlia blossoms.
If you haven’t been lopping off your dahlia’s blooms as soon as they are faded and spent, you will likely find yourself with diminished flower production. Dead flowers are a signal to your dahlia’s roots that reproduction has occurred, and the plant will then direct its energy toward roots and foliage.
To deadhead properly, use your fingers or a small pruning tool to snip off spent blooms just above a set of leaves or an intersecting branch. Remember that spent blooms are pointed and new buds are rounded on a dahlia plant. Regular deadheading will give dahlias a major boost in flower production.
Dahlias that are not blooming properly and/or prolifically may be stressed by disease or insect infestation. See below for additional symptoms that will help you zero in on what might be afflicting them.
Blooms Are Distorted
Commonly transmitted by sap-sucking, leafhopper insects, aster yellows is a bacterial disease that’s often diagnosed once malformed buds are observed. Blockage in the photosynthesis process leads to a disrupted hormone flow which leads to abnormal bloom production.
Look for other symptoms such as distorted leaves and stunted stems to help identify this disease. Unfortunately, there’s no cure for aster yellows and plants will need to be removed from the garden. Leafhopper populations should also be managed to discourage the spread of nearby plants.
If your dahlia’s blooms are oddly shaped or don’t open properly, tarnished plant bugs may be responsible. These pests have piercing-sucking mouthparts and leave toxic saliva behind that distorts growth.
Attracted to flower buds for their sweet nectar, plant bugs can be discouraged with careful weed control (particularly weeds that flower), horticultural oil application, and habitat management. They overwinter in leaf debris, so keep your beds tidy.
Like tarnished plant bugs, borers and thrips will attack and deform dahlia buds by sucking and tunneling through them. Look for considerable damage to leaves as well to indicate the presence of these insects. Manage them in the same way you manage plant bugs, with debris removal and insecticidal soaps.
Buds Won’t Open
If your dahlias appear to have plenty of buds, but they never open or blossom into regular flowers, evaluate your watering practices and look for insect damage.
Dahlia roots that are waterlogged will not transpire nutrients properly and bud production might be shut down. Soggy roots are also susceptible to root rot and fungal conditions that can cripple young buds. To facilitate healthy bud growth and flower production, dahlias should be planted in well-drained soil that never puddles or holds standing water.
Conversely, failed dahlia buds that appear dry and brown may be stunted due to a lack of adequate moisture. Water deeply and less often to prevent roots from drying out without waterlogging them.
If the stems and leaves of a plant appear normal but buds are not opening, examine them for small holes to determine if thrips or earwigs might be doing puncture or chewing damage. Buds that have been pierced or feasted on repeatedly might be damaged to the point of infertility and would therefore not open properly.
If earwigs are the culprit, consider luring them away with nearby newspaper piles or sunken oil traps, or pick them off by hand at night. If thrips are suspected, use neem oil or a heavy hose spray daily to physically remove them from your plants.
Leaves Have White Fuzz
If you suddenly notice a white fuzz or powdery substance on your dahlias’ leaves, they are likely suffering from powdery mildew. A common dahlia foe, powdery mildew presents with white fuzz on leaves and/or stems.
Look for it during periods of high daytime humidity with low nighttime temperatures. It is most prevalent in early spring or late fall.
While the disease is mostly aesthetic and will not kill the plants, powdery mildew can be unsightly and it can spread. Remove and dispose of affected leaves and stems promptly, and cut stems off where they meet the tuber when storing for winter.
Leaves Have Gray Fuzz
If the fuzz on your dahlias’ leaves is slightly more gray in tone and can also be found on their stems and blooms, gray mold (or Botrytis) might be the culprit. Look for this disease after rainy, cloudy periods when humidity is high.
Similar to powdery mildew, gray mold will not kill the plant but the disease should be kept in check with diligent pruning and aggressive removal.
Prevent gray mold from overwintering by cutting dahlias down completely at the end of the season. When tubers are dug up for storage, clean them thoroughly and dry them completely before storing.
Entirely Yellow Leaves
There are a number of reasons why a dahlia’s leaves might turn completely yellow. Most of them are the result of improper irrigation or nutrient deficiency.
If yellow leaves are soft and droopy, it’s possible they are getting too much water. Tuberous roots can be waterlogged by excessive irrigation and leaves will lose the ability to photosynthesize.
Make sure you are watering slowly and deeply to encourage moist, but not muddy, soil conditions. Consider working some organic material into the ground to facilitate good drainage.
Leaves that are yellow, crunchy, and curling under are not getting enough moisture. This is particularly true in times of drought or during summer’s peak heat months when roots will conserve groundwater, leaving plant parts above ground to fend for themselves.
Dry yellow leaves should be corrected promptly with supplemental irrigation to promote even, moist soil at all times. Consider adding a layer of mulch to insulate dahlia beds.
Dahlias that have pale yellow leaves with green midribs and veins are likely suffering from some kind of nutrient deficiency. This means a disruption has occurred in the photosynthetic process, and leaves are not transpiring properly.
If older leaves are turning yellow, this is a sign of magnesium deficiency. If newer leaves are turning yellow, this is a sign of iron deficiency. Perform a soil test to determine your pH level. As dahlias prefer slightly acidic soil with a pH sweet spot of 6.5, work some lime into your beds to bring acidity down or work some peat moss into the dirt to bring it up.
Partially Yellow Leaves
Leaves that have yellow spots, streaks, or splotches are typically suffering from insect damage. If leaves have a stippled, yellow appearance and there is a sticky black substance on them that’s attracting ants, aphids are probably to blame. These rapidly multiplying, pear-shaped insects can do a lot of damage quickly and should be addressed promptly.
A natural approach to aphid removal involves diligent spraying of their leaf undersides with the hose. For more severe infestations, parasitic wasps or lady beetles can be introduced to control populations. Horticultural oils applied to leaf undersides can also be effective.
Mottled yellow leaves that curl up and have a sticky web presence are typically indicative of spider mites. Since mites typically work from the ground up, they will first appear on stems and foliage near the soil.
Since mites thrive in dry, dusty conditions, keeping your dahlia leaves moist will help discourage them from taking over. Aggressively pruning and hand-shaking leaves that are overrun with mites can also help to reduce populations.
If your dahlia’s leaves have turned brown, confirm that they’re getting enough water and appropriate amounts of sunlight, then evaluate them for disease.
Too much sun can make the leaves dry and crunchy. If their daytime total exceeds 8 hours, moisture might be evaporating from their leaves faster than it can be replenished. Give them some protection from the late afternoon sun and water them regularly. They may bounce back.
If you notice that leaves have turned dark and are wilting during the day, but seem to perk up at night, your dahlias might be infected with wilt disease. If stems ooze when you cut through them with a sharp tool, you can be pretty sure they have something bacterial. Remove the affected plants, along with the dirt in which they were planted, and throw them away.
If it’s late in the season, your dahlias may be telling you they’re ready for a long winter’s nap. Brown leaves can be an indication that the plant has entered dormancy. Dig them up and prep them for storage in a cool, dark room with a temperature between 32 and 50 degrees.
White Spots on Leaves
There are quite a few reasons your dahlia’s leaves may have white spots. The condition is likely caused by a fungus or insect problem.
Before white fuzz covers a leaf, powdery mildew begins with a smattering of white spots that slowly spread and grow together. If you catch this fungal disease in its early stages, it’s easier to control. Prune off affected leaves and remove them from the garden.
Pale yellow to white spots with black centers point toward a dahlia smut diagnosis. Also a fungal condition, smut will often present after extended periods of rain, humidity, and cloudy skies. While it may be unsightly, smut is not typically a fatal disease. Prune off affected leaves and dispose of them.
When this viral condition first attacks a dahlia, leaves will have a stained glass appearance. Spots may be white, light green, or yellow and will be randomly distributed on a leaf’s surface.
Look for this condition if your garden has an aggressive aphid population, as they are common carriers of the disease. Take steps to deter them with insecticidal soaps or predatory insects. Dispose of any plants you suspect have mosaic virus, as the disease is eventually fatal and will spread.
The scars left by thrip piercings can appear white and papery. Thrips thrive in times of drought so consider this a possible culprit if the weather has been dry. Keep your dahlias evenly watered, and hit their leaves with a strong hose spray to physically knock thrips from their surface. Neem oil can be used to control severe infestations.
Chew Marks on Leaves
If leaves have irregular-shaped holes in them and/or shredded margins, look for a slimy mucus trail to suggest the presence of slugs or snails. Take a flashlight out to the garden at night and examine the undersides of your dahlia’s leaves to look for these creatures when they are most active.
To deter snails and slugs, keep leaves as dry as possible by watering just the soil and roots. Maintain a tidy garden that’s free of leaves and other potential habitats. Deterrents such as chives, mint, and garlic can also help discourage these slimy, chewing pests.
Leaves that look like holes have been punched in them are most likely from young caterpillars. Rolled leaves held together by webs are also tell-tale signs of caterpillar habitats. Caterpillars can do major dahlia damage in a short period of time.
The most effective way to address unwanted caterpillar feeding is by removing them manually. Allowing and encouraging wasps, spiders, and birds to feed in the area can also help control populations.
Non-discriminating feeders like deer and rabbits get carbohydrates, minerals, and moisture from herbaceous plants, and dahlias can deliver all of those things in spades. Look for ragged, uneven leaf chew marks between 1 and 4 feet off the ground to indicate deer. Look for clean, 45-degree angle stem breakage and leaf bites to indicate rabbits.
The best way to prevent these mammals from chomping on your flowers is with fencing and habitat elimination. Keep the surrounding area free from junk and hiding places to discourage nesting.
Long grass and other strappy perennials are also attractive sleeping spots for these critters, so consider eliminating them from the landscape if this is a concern.
Tall and Leggy Stems
Dahlias that are not receiving at least 6 hours of natural light will stretch toward the sun at the expense of bloom production. The result will be a bed full of awkward-looking stems and leaves with minimal to no flowers. If you can’t guarantee 6 hours of sun, plant in another location or skip them altogether.
If leggy dahlias are noticed early in the growing season (when sprouts are about 1 foot tall), you can snip off their tops to encourage more side branching and a fuller, shrubbier habit.
Otherwise, you will likely find yourself with a gangly, singular dahlia later in the season. If that’s what you’re going for, stake it and enjoy. But if you’d rather have some growth near the ground, pinch away.
When fertilized too frequently or with an improper N-P-K ratio, dahlias will grow stems and leaves rather than buds and flowers. Make sure dahlias planted near lawns are not the unintended recipient of grass fertilizers and choose a fertilizer that’s higher in phosphorus than nitrogen or potassium.
If your dahlia stems are black and mushy, they are likely diseased. Consider the following potential infectors to determine if the condition is fatal or can be managed.
If your dahlia stems are brown or black and falling over, take a knife and make a lateral slice up one of their stems. If a string of tiny black spores is present, sclerotinia disease can be confirmed.
Sclerotinia does not have to be fatal if caught early. Remove affected stems and consider applying a fungicide at the end of the season to prevent overwintering. Prevent future outbreaks by removing fallen dahlia petals promptly from your garden beds and monitoring carefully during wet periods.
Look for dark stem tissue near the soil base and streaky brown or black roots to indicate wilt disease. Another fungal condition, wilt thrives when temperatures change suddenly from cool to warm so look for it during these conditions.
Since wilt attacks your dahlia’s entire feeding system, plants will not survive and should be destroyed. Dahlias should be planted in a new bed next year, and the dirt in which the disease plants were grown should be removed.
New gardeners should expect a learning curve and should not be deterred by a season or two of less-than-perfect specimens. With a little trial and error, and some guidance from a seasoned expert, you’ll get better at winnowing down the possible reasons for your dahlias’ struggles. With this new know-how and a little determination, you’ll be better able to help them thrive and grow to their fullest potential.