Helping Dahlias Through Hot Summers

Having originated in Mexican and Central American mountain plateaus, dahlias can suffer heat stress in late summer when daytime temperatures are over 90 degrees and nights stay warm. Here at OHG we cover our dahlia rows with weed mat and then straw to keep the ground cool and to help retain moisture. If you are seeing droopy leaves, this can be a sign of stress, not necessarily the need for water. Before we make the decision to water, we dig down 4" to see if the soil is dry. Often, it is still moist. Dahlias prefer deep watering less frequently as opposed to frequent watering which can lead to rot - even this late in the season. Overwatering is a much more common problem than having a dahlia fail due to being too dry. Some southern gardeners will mist foliage in extreme heat to cool the plants, and others find shade cloth that blocks 40% of sunlight to be helpful. And in a recent issue of the Bulletin of the American Dahlia Society, Tony Evangelista reports that spraying kaolin clay on dahlia foliage not only protected his plants from insect damage but decreased heat stress as well, since the white clay reflects some of the light before it can heat the leaves. (He notes that for exhibition varieties he stopped using it a month before showing them.) While we haven’t tried it ourselves, we’re intrigued, and we’d love to hear what has worked for you this year.