Why Dahlias And Sunflowers Are Not A Good Match In The Garden

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Why Dahlias And Sunflowers Are Not A Good Match In The Garden

It is not uncommon to want to plant your favorite flowers together in the garden, but sometimes certain plant pairings don’t seem to work. One plant may thrive while the other withers, growing so well that it crowds out its neighbor entirely. This situation can be frustrating, but there is a scientific explanation for it. It’s called allelopathy, which is the process by which certain plants protect themselves by producing biochemicals that can inhibit or encourage other plants’ growth.

Sunflowers and dahlias are two flower garden favorites that, unfortunately, will never grow well next to each other, despite looking beautiful in a traditional cut bouquet. This is due to the sunflower’s natural allelopathic tendencies. Moreover, these two flowers are in the same plant family, which means they attract a lot of the same pests and diseases. These are the two fundamental reasons why you may want to reconsider planting sunflowers and dahlias together.

Even though sunflowers and dahlias are both striking blooms and thrive in full sun and similar soil environments, they are not natural companion plants. If you want to include both sunflowers and dahlias in your garden, you should plan to plant them far apart.

The suffocating sunflower

sunflower field

Naphtalina/Getty Images

Most serious gardeners know the best practices surrounding alternating plant families in your soil year to year. What this means is that plants with similar characteristics — plants classified within the same family (like dahlias and sunflowers) — can’t be planted in the same plot in successive years because they attract the same pests and spread the same diseases, having a negative impact on the soil over time. However, not as many people are as familiar with the concept of allelopathy, particularly because allelopathic properties aren’t consistent across plant families. That is, not all allelopathic plants are in the same family, not all plants in a single family are allelopathic, and allelopathy may affect some plants in a family but not others.

In the case of sunflowers, they are extremely allelopathic, with every part of the plant producing biochemicals capable of suffocating or inhibiting the growth of neighboring plants. Not everything is susceptible to these biochemicals – impatiens and marigolds are excellent companion plants to sunflowers — but unfortunately, dahlias are. The allelopathic chemicals of sunflowers act as almost a dahlia-specific herbicide, preventing the growth of the normally abundant and brilliantly diverse dahlia flower. Try mixing your dahlias with zinnias or lilies instead.

Shared pests and diseases

curled petals of infected dahlia

bkel/Shutterstock

Dahlias and sunflowers are both members of the Asteraceae plant family, and therefore often attract the same garden pests. Spider mites and aphids are some of the most common pests of Asteraceae flowers, destroying foliage, sucking away vital sap, and transmitting diseases. Both are tiny as well but multiply at terrifying rates, and often the signs of their presence — like curled or yellowed leaves — are only clear when an infestation is well underway. Since these pests are attracted to both dahlias and sunflowers, having them next to each other in the garden may appear like an all-you-can-eat buffet, creating a much bigger and more devastating outbreak.

Likewise, sunflowers and dahlias also are susceptible to the same types of diseases. Fungal growths such as Botrytis Blight, leaf spots, rusts, and powdery mildew are all common on the leaves and petals of these flowers and can quickly spread among susceptible species, destroying entire crops in their wake. Even if the disease is identified before it veers out of control, already infected stems will likely have to be removed entirely to keep it from spreading further. A preventative measure many gardeners use against the spread of such harmful afflictions, however, is simply not planting similarly affected plants next to one another in their flower beds, which means sunflowers and dahlias are poor bedfellows.

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Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.