My dahlias are struggling this summer for some reason. The flowers aren’t opening properly and the petals are going brown at the tips. The plants are growing in good, rich soil in a nice sunny spot in the garden where they’ve always done well before now. Any advice?
T Dunphy, Cork
Judging by your description, it sounds like your dahlias are suffering from thrip damage. These tiny, sap-sucking insects, which are hard to spot unless you look very closely, can sometimes infest dahlia plants at this time of year, causing deformation of both the flowers and leaves as well as discolouration. As is true of many kinds of insects that feed on plant tissue, they also spread plant diseases.
A hot, dry environment suits them to a tee, which is why thrips are much more common in warm summers or on plants grown under cover in a polytunnel or glasshouse. To check if your plants are indeed affected, examine the flowers carefully and look for multiple tiny dark insects, each one no bigger than a poppy seed, moving rapidly among the petals.
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The best long-term solution to keeping your dahlia plants free of this destructive little pest is to keep them well-watered, especially during any prolonged hot, dry spells. Always make sure to water around the base of the plants, gently soaking the soil so that it penetrates well below the surface. Although wetting foliage and flowers is generally not recommended when watering plants, gently misting all parts of your dahlias with a fine water spray will also help to rid them of thrips, which dislike a moist atmosphere.
As the insects can overwinter as eggs with the potential to cause problems for your plants again next summer, I’d also suggest mulching the ground with a generous layer of well-rotted organic matter in spring. This will help the soil retain moisture as well as boosting plant health.
In the short term, you could also use the biodegradable, organically-acceptable product known as Uncle Tom’s Natural Soft Soap (from fruithillfarm.com). These kinds of insecticidal soaps have been used for centuries to treat a range of common plant pests including aphids, red spider mite and thrips. They only work when they come into direct contact, so need to be diluted in water and then applied liberally to all parts of the plants as a fine spray. The best time of the day to do this is in late evening when beneficial insects are much less active and temperatures are lower, reducing the rate of evaporation.
