Dahlias are beloved for their vibrant colors and oversized blooms, but they can be tricky to grow. If you’re scratching your head over a mystery ailment or wondering what went wrong with your dahlia garden, don’t fret. In this article, we’ll explore the most common problems gardeners encounter when growing dahlias and provide solutions to help you turn things around.
No Blooms at All
By far, the biggest issue reported by dahlia growers is the lack of blooms. There are a few reasons why this might happen. First, dahlias require at least 6 hours of direct sunlight to bloom. If they are grown in shade or part shade conditions, they will not produce flowers. Make sure your dahlias are receiving enough sunlight.
Another factor to consider is soil temperature. Dahlias need soil temperatures of at least 60 degrees to grow and produce blooms. If you planted them too early when the ground was still cold, their bud growth might have been stunted. Wait for the soil to warm up before planting your dahlias.
Dry and crumbly soil can also affect bud production. If the soil surrounding your dahlias is dry, they might not be receiving adequate nutrients. Make sure the soil is moist but not soggy, as this will allow the dahlias to absorb the necessary nutrients for bud growth.
Lastly, be cautious with fertilizer. Using a fertilizer with more nitrogen than phosphorus or potassium can encourage foliage growth rather than blooms. Use a fertilizer with an NPK ratio of 5-10-5 and only fertilize once or twice during the season.
Not Enough Blooms
If your dahlias have few blooms, the issue might be related to maintenance. Most dahlia varieties will naturally produce a couple of large, showy blooms instead of multiple smaller ones. To encourage more abundant blossoms, pinch your dahlias early in the season to promote side shoots and multiple bloom points.
When the plants reach around a foot in height and have 3-4 sets of leaves, cut the top shoots off just above the third or fourth set of leaves. Repeat this process in a few weeks, and you’ll have bushier and more abundant dahlia blossoms.
Another reason for reduced flower production is not deadheading your dahlias. If you leave faded and spent blooms on the plant, it signals to the roots that reproduction has occurred, diverting energy towards root and foliage growth instead of producing new flowers. Regular deadheading by snipping off spent blooms just above a set of leaves or an intersecting branch will significantly boost flower production.
Stressed dahlias, whether due to disease or insect infestation, can also result in reduced blooming. Keep an eye out for additional symptoms that might indicate an issue.
Blooms Are Distorted
If your dahlia blooms are oddly shaped or don’t match the pictures on the nursery tag, they might be affected by an insect or disease. Aster yellows, a bacterial disease transmitted by sap-sucking leafhopper insects, can cause malformed buds. Look for distorted leaves, stunted stems, and disrupted hormone flow as signs of aster yellows. Unfortunately, there is no cure for this disease, and affected plants should be removed to prevent spreading.
Tarnished plant bugs, attracted to dahlia flower buds for their sweet nectar, can also cause distorted blooms. Their toxic saliva distorts growth. To discourage plant bugs, maintain weed control, apply horticultural oil, and manage their habitat. The same applies to borers and thrips, which also deform dahlia buds.
Buds Won’t Open
If your dahlias have plenty of buds but they never open or blossom into regular flowers, watering practices and insect damage might be the culprits. Waterlogged roots can prevent proper nutrient absorption and shut down bud production. Ensure your dahlias are planted in well-drained soil that doesn’t puddle or retain standing water.
Conversely, if the buds appear dry and brown, they might be stunted due to a lack of moisture. Water them deeply but less often to prevent root drying.
Examine the buds for small holes, as thrips or earwigs might be causing damage. Punctured or chewed buds are unlikely to open properly. Manage thrips with neem oil or a heavy hose spray, and deter earwigs with newspaper piles or oil traps.
Leaves Have White Fuzz
If you notice a white fuzz or powdery substance on your dahlia leaves, it’s likely powdery mildew. This fungal disease is prevalent during periods of high daytime humidity and low nighttime temperatures. While it doesn’t kill the plants, it can be unsightly and spread. Promptly remove and dispose of affected leaves and stems, and cut stems off at the tuber when storing for winter.
Leaves Have Gray Fuzz
If the fuzz on your dahlia leaves has a grayish tone, it might be gray mold or Botrytis. This disease tends to occur after rainy or cloudy periods with high humidity. Although not fatal, it should be controlled through diligent pruning and removal. Cut dahlias down completely at the end of the season, clean tubers thoroughly before storage, and ensure they are completely dry.
Entirely Yellow Leaves
Yellow leaves can be caused by improper irrigation or nutrient deficiency. Soft and droopy yellow leaves indicate excessive watering, while yellow, crunchy leaves that curl under suggest insufficient moisture. Adjust your watering practices accordingly, ensuring the soil is moist but not waterlogged.
Pale yellow leaves with green midribs and veins indicate a nutrient deficiency. Perform a soil test to determine the pH level and adjust it with lime or peat moss to achieve a slightly acidic pH of 6.5. Older yellow leaves signify magnesium deficiency, while newer leaves indicate iron deficiency.
Partially Yellow Leaves
Yellow spots, streaks, or splotches on leaves are typically caused by insect damage. Aphids can cause stippled, yellow appearance and leave behind a sticky black substance. Control aphids by spraying their leaf undersides with water or introducing natural predators. Mottled yellow leaves that curl up and have a sticky web presence are likely due to spider mites. Keep the leaves moist and shake off infested leaves to reduce mite populations.
Brown Leaves
Brown leaves can result from excessive sunlight, wilt disease, or dormancy. If the leaves are drying and crunchy from too much sun, provide some shade and regular watering. Wilt disease can cause stems to ooze when cut and requires affected plants to be removed. Brown leaves may also indicate dormancy as the season ends. Dig up the dahlias and store them in a cool, dark place with temperatures between 32 and 50 degrees.
White Spots on Leaves
White spots on leaves can be caused by powdery mildew, smut dahlia, mosaic virus, or thrips. Powdery mildew starts as scattered white spots that spread over time. Remove affected leaves, as it’s easier to control when caught early. Smut dahlia is characterized by pale yellow to white spots with black centers, often caused by extended periods of rain and humidity. Prune off affected leaves. Mosaic virus causes a stained glass appearance on leaves and is transmitted by aphids. Dispose of infected plants. Thrips can also cause white spots with their piercing damage, so control their populations with neem oil or a strong hose spray.
Chew Marks on Leaves
Irregular holes or shredded margins on leaves are signs of damage by slugs, snails, caterpillars, deer, or rabbits. Keep the leaves dry to deter slugs and snails, maintain a tidy garden, and use deterrents like chives, mint, or garlic. Remove caterpillars manually or encourage natural predators. Use fencing and eliminate hiding places to deter deer and rabbits.
Tall and Leggy Stems
If your dahlias have tall and leggy stems with few flowers, they might not be receiving enough sunlight or have received improper fertilization. Ensure they receive at least 6 hours of natural light or consider moving them to a sunnier location. Prune the tops of young plants to encourage bushier growth. Avoid excessive nitrogen in fertilizers, as it promotes stem and leaf growth over blooms.
Collapsing Stems
Black and mushy stems indicate diseases such as sclerotinia, verticillium, or fusarium wilt. Remove affected stems and consider applying fungicide. Prevent future outbreaks by removing fallen petals and monitoring wet periods. Look for wilt disease if stems are brown or black and falling over, and remove affected plants. Plant dahlias in a new bed and remove the soil from the disease-infested area.
Growing dahlias can be a rewarding experience, but it’s essential to understand and address the common issues that can arise. By following the solutions provided and staying vigilant in your garden care, you can ensure healthy and vibrant dahlia blooms.