Dahlia Problems

Dahlias are rewarding plants to grow; they give you cut flowers for months through summer and autumn. President of the Dahlia Society of NSW & ACT, Graeme Davis shares common problems with solutions to fix them. Once you know these four common problems, your dahlias will be happy and healthy.

Why are plants not growing?

The tuber will not sprout. Dahlias are often grown from tubers, like potatoes. However, unlike potatoes, dahlias only grow from the small piece of old stem attached to the tuber. If there is no stem, there is no growth. If the connection between the stem and the tuber is broken there will be no sprouting. Check your dahlia tuber for a stem at planting time and discard if there is none.



Tuber rot. This can be an issue at planting time when your soil is cold and wet. Wait until the soil warms. Before planting. Good drainage is essential. If your soil is heavy, it’s good to add grit or sand to lighten the texture and facilitate better drainage. Note, however, that very dry soilis not conducive to plant growth. Soil should be moist and warm for the tuber to ‘shoot’.

Why are there so few blooms?

There are three main causes for limited flowering: not enough sun, not enough food, and not enough water. Dahlias need seven hours of sunshine to produce good flowers.



Water is essential for dahlias. A constant supply of moisture is required for them to perform well. However, excess water in a poorly draining soil can cause problems. Seven-to-twelve centimetres of mulch will keep the soil cool and damp and promote healthy growth and flowering.







 

Why are young leaves and flowers distorted?

There are two main culprits: broad mites and broadleaf herbicide damage.

Broad mites attack the growing tips of the plant. You will not see the mites, but you will see their damage. These can be controlled by predatory mites or if the problem is well established, by a commercial spray such as eco-neem, every three-to-four days.



Broadleaf herbicide damage occurs when the soil dahlias are grown in soil that has been contaminated either by earlier spraying or by using mulch, compost, or manures that have been contaminated with herbicide. Dahlias are highly sensitive to such contamination. Avoid using products that have any chance of having been contaminated by broadleaf herbicides. This includes animal manures, where animals have been fed products from crops that have been sprayed; the residue persists into the manure. If you unknowingly introduce broadleaf herbicide, the addition of Biochar may help clear the contamination.

Why are leaves yellowing?

Yellowing leaves can be caused by a range ofthings, including nutrient deficiency, virus,  spider mite and other sap-sucking insects and old age. Dahlia leaves age as the season progresses and tend to yellow at the base of the plant. Just remove those leaves to improve airflow around the plants.



Nutrient deficiency: dahlias require minerals and nutrients. Regular addition of seaweed or fish fertiliser will help, particularly if this is applied as a foliar feed.



Sap-sucking pests: Yellowing can be caused by sucking insects such as whitefly or, more problematically, spider mites. Predatory mites can be used to control these mites, particularly if introduced early enough before the outbreak becomes well-established. Commercial sprays are available as well.



Viruses: yellowing leaves can be a sign of a virus. Viruses are present in many dahlias in Australia but not all plants show symptoms of infection. If your dahlia is not growing well you should discard it into green bin.

 

Learn More

If you want to know more, join your local dahlia society and meet people with a similar passion for these gorgeous flowers. You will meet people with lots of experience and those who have only recently caught the bug. All are happy to share what they have learnt. For more information on the Dahlia Society of NSW & ACT, visit their website: www.dahliasocietynswact.org.au