Mildew on dahlias

Question

Every year my dahlias develop mildew as we come into autumn. It then spreads around the garden and limits the plants’ ability to thrive. Is there anything I can do to prevent mildew?

Answer

As dahlias die down naturally in autumn it’s very common for mildew to appear. To reduce the incidence and spread of mildew, remove up to fifty percent of the leaves on your dahlia plants. Leave the rest to die down to allow your next season's dahlias to have enough nutrients.

Dahlias grown in an open sunny position in soil that is well-drained tend to exhibit less mildew in autumn. You may want to consider shifting your dahlias, once they have died down, to a sunnier position with well-drained soil. If some individual dahlia plants exhibit a lot of mildew each season, you may wish to cull these.

Garden hygiene is very important in any garden. At the end of the growing season, once the remaining leaves have died down (brown), remove these and do a general tidy up around plants so you start the next season with a clean slate. Fortunately, the existence of mildew in autumn does not appear to harm dahlia growth in spring.

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Dahlia with mildew iS181061967S.jpg