Climate change is creating many new challenges for the plants in our gardens. These include frequent and often dramatic swings in temperature and rainfall. While some plants can take these weather fluctuations in stride, others really struggle.
Dahlias rank high on the list of plants that dislike weather extremes. They prefer temperatures that vary no more than about 20 degrees: 70-80°F during the day and 60-70°F at night. When temperatures rise above 85°F or nights don't cool down, dahlias may begin exhibiting signs of stress. These include stunted growth, wilting leaves, reduced flower production, misshapen flowers, and increased susceptibility to disease and pests.
But here's the good news. Gardeners grow dahlias in almost every state in the country. In this article, you'll find hot-weather dahlia growing tips from gardeners in Georgia and Texas, as well as a list of the heat-tolerant dahlia varieties they recommend.
Dahlia Cornel
Dahlias Like it Warm, Not Hot
Like most plants, dahlias evolved in a specific location and unique ecosystem. The ancestors of today's dahlias are native to the mountains of southern Mexico and Central America, where summer days are pleasantly warm (80°F) and nights are cool (65°F). In the US, similar growing conditions can be found on the Pacific Northwest coast, which is why gardeners who live there can grow such fabulous dahlias.
Full Sun Isn’t Always Best
Dahlias are sun lovers and more sun usually means more blossoms. But if the sun is too hot and temperatures are too high, the plants may stop flowering. If excess heat is a concern, choose a planting location with half day rather than full day sun. Ideally, your plants would get full morning sun and filtered light in the afternoon. You can also try planting your dahlias beneath a shade tree. Overhead branches will protect the foliage from hot sun and also help reduce the air temperature.
No Shade? Consider a Shade Structure
If you live in a hot climate and want to grow a lot of dahlias, consider putting up a shade structure. It's an ideal solution for yards without any natural shade. Polyethylene shade cloth lasts for many years and depending on the weave, you can filter out as little as 20% of the light or as much as 80%. For growing dahlias, 40% shade is usually a good option. Shade fabric will also shield the flowers from damaging rain and hail.
Mulch to Cool the Soil
Dahlias have shallow feeder roots that are only about 1” below the soil surface and they can extend as much as 18” in all directions. Covering the soil with 2-3” of mulch will insulate these roots and help protect them from extreme heat. Make sure the mulch is no more than 2 to 3” thick, and that it lets water flow through into the soil. The best time to mulch is early July -- once your plants are growing strong and before it gets too hot.
Dahlia Rip City
Provide Good Drainage and Lots of Organic Matter
Dahlias thrive in rich, humusy soil. At the beginning or end of each growing season, enrich the planting area with a generous amount of compost, shredded leaves or rotted mature. Loose soil that contains lots of organic matter encourages strong root growth and also absorbs moisture, so it’s available to the plants as needed.
Dahlias must be planted in well drained soil. This is especially important in areas that get heavy summer rains. Soggy soil can suffocate the roots, stress the plants, and cause tubers to rot. If the soil in your yard is naturally heavy, consider growing your dahlias in raised beds. This way, you can fill the beds with the best possible soil and ensure there will always be excellent drainage. You can also consider growing dahlias in containers filled with a high-quality growing mix that's been amended with compost.
Water Wisely and Consider Using Drip Irrigation
Dahlias don't like wet soil, but drought can cause almost as many problems. If natural rainfall is inadequate, it's best to water your dahlias deeply once per week rather than watering them lightly several times per week. Drip irrigation is great for dahlias. The foliage stays dry, and water can percolate down to the roots, slowly and evenly with no runoff. If you are hand watering, direct the water right at soil level and avoid wetting the foliage as this can encourage powdery mildew. You'll find more tips here: How to Water Your Plants.
Dahlia Lavender Perfection
Mist On Hot Days to Keep Dahlias Cool
Dahlia growers in hot climates such as Australia, use water to help their plants survive the heat. On hot days, a misting system (on a timer) surrounds the plants in a cloud of cool, moist air. This brings down the air temperature and also ensures the plants stay well hydrated. If installing a misting system sounds daunting, use a watering wand to spray the foliage several times during the hottest parts of the day.
In warm climates, it's important to get your dahlias established before it gets really hot. During the hottest part of the summer the plants may not flower at all. But that's OK because once the weather cools down, they will start setting buds and produce flowers right through October.
Feed Your Plants to Keep Them Strong
It's difficult for hungry plants to stay strong and resilient, so do your part to make sure your dahlias have all the nutrients they need. In addition to enriching the soil with compost, it's helpful to incorporate a 5-5-5 fertilizer at planting time.
Once your dahlias start to bloom, fertilize the plants every two weeks during July and August. Use a water soluble fertilizer formulated for roses or tomatoes, or a low or no-nitrogen organic fertilizer such as Alaska MorBloom. Alternatively, you could use a time release fertilizer such as Osmocote. Just remember to reapply it in mid-summer.
Dahlia Arabian Night
Heat-Tolerant Dahlia Varieties
Through trial and error, warm climate gardeners have discovered that some dahlia varieties are naturally more heat tolerant than others. The following list was gleaned from a number of different dahlia organizations in the southeast.
Akita, Babylon Red, Ben Huston, Belle of Barmera, Bishop of Llandaff, Blackberry Ripple, Cornel, Ferncliff Copper, Hamari Gold, Kelvin Floodlight, Kidd’s Climax, Lady Darlene, Lavender Perfection, Mingus Alex, Otto’s Thrill, Penhill Dark Monarch, Penhill Watermelon, Pooh, Rip City, Senior Ball, Show N Tell, Spartacus, Tartan, Thomas Edison, White Perfection, Winkie Colonel, Vasio Meggos, Zorro.
Be sure to ask around for recommendations from local gardeners. This may even yield some pass-along varieties that have proven to be particularly good for your region. An alphabetical list of heat-tolerant dahlias, recommended by members of the Dahlia Society of Georgia can be found HERE. We hope some of these garden-tested techniques will help you be successful growing dahlias in hot weather.
