1. Home
  2. Expertise
  3. Diseases & Pests Advice
  4. Dahlia


Series: Fresco™, Harlequin™


1. Soil and pH Needs

Dahlias grow best in a deep, fertile, well-drained soil; they are not tolerant of water-logged soils. Heavy clay soils will retard root growth and starve the plant of oxygen, which can lead to root diseases. Soil should be amended with fully composted organic matter prior to planting and in the case of clay soils the addition of sand equal to at least 25% of the soil volume will aid in aeration. If adding organic matter, be aware that organic matter that has not been fully composted will tie up nitrogen and sulfur leading to nutrient deficiencies and poor growth.  Soil pH should be 6.0 – 7.0 for optimum growth; and should be tested prior to planting but after adding any amendments. This can be done quite easily with a portable pH tester. Mix one part soil with two parts distilled water, stir well and wait thirty minutes, then follow the instructions for using the pH meter. If pH is too low, the addition of lime is warranted. However, the rate will vary depending on how much you need to raise the pH and the type of soil you are dealing with. Clay soils or those with a lot of organic material have a high buffer capacity and require more lime than sandy soils, which have a low buffering capacity. A good soil testing lab can determine the lime requirement index (LRI) of your particular soil and can recommend types and rates of lime to use. The frequency of watering will depend on the type of soil, weather conditions and the amount of mulch. Mulch will not only reduce soil water evaporation but will also reduce splashing of water onto the lower leaves, moderate soil temperatures and reduce weed competition. 


2. Fertility

Dahlias require low nitrogen fertilizers, “Do not overfeed your dahlias”, doing so will promote lots of foliage, but not a lot of bloom.  An application of a balanced controlled-release fertilizer (CRF) such as 5-10-10 with a 3-4 month formulation applied to the bed at planting will keep the plants well fed, depending on the amount of irrigation required and the average daily temperature. In areas of high irrigation and high temperatures it may be better to use a formulation with a slower release rate since higher temperatures will cause the fertilizer to be released quickly. Fertilizers that are not CRF are not recommended since the fertilizer will be leached out of the root zone long before the crop needs the nutrients. In high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce chlorosis (yellowing) of the foliage.  Yellow foliage on Dahlias may also be caused by a nitrogen deficiency within the plant. Both can be corrected with the proper supplemental fertilizers. 


3. Disease

Water correctly to prevent most of the fungal diseases that can cause problems for Dahlias such as: botrytis and powdery mildew. Under wet conditions, Botrytis can be identified when masses of gray spores form later turning brown on the buds, leaves and stems. Powdery mildew is a dry, white mealy fungal growth that occurs on the upper surface of the foliage. Dahlias are also susceptible to Tomato Spotted Wilt Virus (TSWV).  Symptoms of an infected plant include the leaves being mottled with yellow spotting, dead flecks or line/ring patterns. Plants may also be distorted, show flower color breaking or may be stunted. Infected plants should be immediately destroyed since there is no cure. Implementing a good thrips control program will keep TSWVV in check since the virus is vectored through by thrips. The best preventative practice is to use drip irrigation. At the very least, direct water from the hose onto the soil or mulch, and not on the plants themselves. 



Botrytis affecting flower buds


Botrytis symptoms on the foliage


Powdery mildew attack on Dahlias


Infected Dahlia leaves with TSWV


More infected Dahlia leaves with TSWV


4. Insects

Dahlias are susceptible to aphids, spider mites, thrips and leaf miner. Aphid nymphs and adults feed on the plant sap by piercing the tissue. This slows growth and can cause leaf curling. Aphids also excrete honeydew. The honeydew will lead to sooty mold formation which will reduce growth and production. Thrips cause damage to the plant by piercing and sucking cells on the leaf surface. This causes silver-grey spots to form on the foliage. At high infection rates, the leaves may wither. Thrips are also a primary vector for virus transmission. Spider mites feed on the underside of the leaves. This feeding causes yellow spots to form which results in decreased plant growth and production. Leaf miner damage is caused by the larvae mining below the leaf surface after the female has laid her eggs. The leaf tissue dries and turns brown.  Young Dahlia plants may also be susceptible to slugs and snails. Remember that healthy, vigorous plants are less susceptible to pest damage than unhealthy plants.  



Adult aphids


Foliage with spider mite damage


Stippling of foliage caused by thrips damage


Leaf miner damage to the foliage


5. Tips for Success

  • Dahlias prefer a sunny area with a minimum of 6 hours exposure.
  • Dahlias require low nitrogen fertilizers in the landscape, do not over-feed. 
  • If you want your Dahlia to provide a continuous, extended flower show, you will need to remove the spent buds promptly.
  • Remove weeds from the plantings that compete with the Dahlia for moisture, nutrients and light.


Information provided by Syngenta Flowers