In the high mountain plateaus of Mexico where dahlias originated, no matter how hot the days get the nights are always cool — and many dahlias today still need cool night temperatures to grow and bloom well. Even here in Michigan, when nights stay in the 70s their growth often slows to a crawl, rebounding only with the approach of fall. That’s why we recommend most of our dahlias for zones 4-7b only — except on the West Coast where summer temperatures are cooler than in the corresponding zones elsewhere in the country, and there we recommend them through zone 9bWC only.
But some dahlias can handle warm nights better than most. We recommend these varieties — noted as “heat-tolerant” in their descriptions and “heat-ok” in our dahlia chart — through zone 8b in most of the country and 10b on the West Coast. (California, by the way, has six local dahlia societies in zones 9-11a.)
How you grow them makes a big difference, too. Dahlias are thriving for many of our customers in zones 8 and warmer, and here are some of their tips for growing and enjoying them where summer nights are never cool.
John Kreiner of the Dahlia Society of Georgia has been growing dahlias in the Deep South for over 30 years. He planted his first ones in zone-8b San Antonio way back when, and today he gardens in zone-8a Atlanta. Here’s his simple but important advice:
“There are a lot of things that we in the South must do to be successful in growing dahlias but the two most important in my opinion are:
“To keep dahlias alive through the heat of summer so we can enjoy the blooms that come when temperatures cool in the fall, mulching is a must. The sensitive feeder roots of dahlias grow just under the soil surface and can extend for up to two feet in all directions. To keep them cool, cover the soil with at least 2 inches of a good mulch that will let water through easily — and be sure to do it before July 1! Very light afternoon shade can be a help, too.”
“We learn this by growing them. The ones Old House Gardens offers that I know will grow well in the South are:
‘Kidd’s Climax’ — “This is probably OHG’s best large-sized dahlia for the South. It still wins every year on our show bench.”
‘Juanita’ — “This plant grows very well in the South and still wins. I have always loved it because its form as a cactus is the best.”
‘Thomas Edison’ — “You don’t see it in the shows very often, but it grows well here.”
‘Bishop of Llandaff’ — “This does well here, too.”
We asked John to trial several of our hard-to-find favorites, and some did so well he added them to his heat tolerant list:
‘Prince Noir’ — “This dahlia is the best of those you sent me. It was the second to bloom of all the dahlias in my garden, so it is an early bloomer. It had lovely, dark, maroon-red blooms that have been enjoyed by everyone who has seen them. It bloomed all summer and has grown very well. It put out plenty of laterals and lots of blooms and was a very nice addition to my garden. I highly recommend it as a heat-tolerant dahlia.”
‘Deuil du Roi Albert’ — “This lovely purple with white on the tips was absolutely outstanding in the HEAT this summer . It started blooming and never stopped until frost, with blooms coming continually during the month of August which had ten days of 100 degrees or higher. I highly recommend it for the Southern garden.”
‘Jersey’s Beauty’ — “As you told me, this plant does get very tall, taller than all of my modern dahlias except for ‘Spartacus’. The blooms after we started getting cool weather were absolutely beautiful, with a quality I really didn’t expect. I entered a bloom of it in our state fair dahlia show and got a blue ribbon. It also got a blue ribbon for an entry in the Tennessee Dahlia Show in Chattanooga by another grower, and I saw a very nice bloom of it in the Carolinas show last year in Asheville, NC. I highly recommend it as a heat-tolerant dahlia.”
‘Lavengro’ — “This plant grew real well in the heat but its buds didn’t open then. When the weather turned cool, though, it put on more buds and now in October it’s blooming with wonderful color. The centers haven’t stayed closed very long but at this time of the year we don’t expect them to. I recommend it as a heat-tolerant dahlia.”
‘Surprise’ — “This dahlia grew well and did fine in the heat, but its colors would benefit from shading in the South. I recommend it for the Southern garden.”
‘Bloodstone’ — “Grew well and bloomed late in the season.”
‘Princesse de Suede’ — “Grew well, bloomed late in the season.”
‘Winsome’ — “This was the second year of growing this variety and it has grown well both years and has put on buds. Last year a chipmunk go it and this year the heat got it. It was blooming very nicely and when the 100 plus degree days arrived it stopped blooming and never started again. I also think in a normal year it would continue to bloom without any problems.”
Some dahlias John trialed did NOT bloom well for him, though, so we DON’T RECOMMEND these for zones 8(10WC) and warmer: ‘Andries’ Orange’, ‘Kaiser Wilhelm’, ‘Nellie Broomhead’, ‘Old Gold’, and ‘White Aster’.
In zones 8-11 you can leave your dahlias in the ground all winter, but John says they will grow and bloom best if you dig and divide the tubers every 2-3 years anyway. Most growers cut the stalks down at some point, and John recommends capping each with a bit of tin foil and a rubber band so water doesn’t collect in them and rot the crown and next year’s sprouts. He also adds 4-6 inches of mulch directly over the tubers for their winter rest.
Even here in our zone-6 Michigan garden we’ve taken a tip from American Dahlia Society members in the Southeast who mist their dahlias on particularly hot, sunny afternoons. The evaporating mist helps to cool the plants and keep them thriving. Our misting techniques here are very low-tech. Basically we just spray them with our watering wand, but it seems to help.
You don’t have to be an expert to grow dahlias where summer nights are warm. Here’s what some of our customers who are ordinary, backyard gardeners in zones 8(10WC) and warmer told us about their successes. (If you’re growing dahlias in these zones, too, please email us your story. We’re always eager to learn!)
Though our friend Jonathan Lubar works at the Kanapaha Botanical Gardens in zone-8b Gainesville, Florida, he had never grown dahlias before trying a few of ours in 2008. He reported “great luck” with them, especially ‘Thomas Edison’ and the pompon ‘Yellow Gem’ . “I followed some recommendations from your Dahlias for the South page — heavy mulch, etc. They limped through the hot summer but took off in fall and are still blooming [Nov. 1]. ‘Tom Edison’ is an impressive monster!”
Miranda Hein in zone-8a North Augusta, SC, wrote us in April 2008: “I’m originally from Washington state and I LOVE my dahlias. I was advised not to grow them here, but couldn’t resist giving it a try. The first year I planted ‘Little Beeswings’ and ‘Clair de Lune’ on the hill in my backyard, but they were not happy there. So I dug them up and put them in the garage for the winter. Last year I put them on my back deck in whiskey barrel tubs, and they were beautiful! ‘Little Beeswings’ absolutely astounded me. Wow, does this flower rock! It bloomed for ages, tons of flowers, and, let me tell you, my back deck faces south and is absolutely incinerated by the sun from sun-up till sundown. We can’t even go outside from about 11 am till 6 pm or so because the boards are too hot to walk on. What an impressive little flower in every way! I left the bulbs in the tubs on the deck over the winter, and it just sprouted again yesterday!” In a follow-up email, Miranda told us she’s also having great success with the tough old pass-along Wisconsin Red.
Val Myers of zone-8b Georgetown, SC, 60 miles north of Charleston, wrote us in July 2006: “Last year I purchased ‘Claire de Lune’ and ‘Union Jack’. ‘Clair’ performed well its first year but ‘Jack’ was slow to start. This year, however, ‘Union Jack’ has not only returned but gone berserk. It has spread to about three feet wide and high and is fighting tooth and nail with gaura for space in my planter. It began blooming at least a month ago and is still trying even in the 90 degree heat with 90% humidity. It’s in a large planter filled with great soil and gets filtered shade most of the day with direct sun only late in the afternoon. It’s irrigated via a drip line and heavily mulched to prevent water loss.”
Kathryn Chauveaux of zone-9 Beaumont, Texas, on the Gulf Coast writes: “Dahlias thrive for me here. They seem to love the heat and humidity, and the more you cut them the more they bloom. I plant pompons just because I like their shape. They’re on the south side of my house in full sun, between my azaleas and wall of the house [OHG: This is a location we would never recommend, but . . . .] The soil is rich with organic matter and slightly acidic. I usually plant them in late May to avoid our sometimes heavy spring rains. I dig a hole 4-6 inches deep and sometimes add a little bulb food covered with a layer of sand. Then I put in a stake and fill up the hole. I add Super Thrive to the first watering and then wait for some green to show before I water again. When they bloom, I cut them for bouquets which stimulates more blooms. And that’s about it. Once planted, they are basically on their own.”
Della Smith is the proud grandmother of a thriving Houston dahlia. She writes: “One of your ‘Bishop of Landaffs’ is alive and well in zone-9 Houston, Texas! My daughter, who is a Master Gardener there, has had it return for three years now. She just leaves it in the ground over the winter and in the spring it pops back up. I was there last July, sweat box city, and it was gorgeous. I think that drainage is one of the keys for success there. It is planted in a raised bed.”
And Mary Peace Douglas who gardens in zone-8 Tucson, Arizona, writes: “June in Arizona is called ‘the death month.’ Hot, dry, windy, and waiting for the rains. Your single dahlias, though, are thriving. Pink ‘Bonne Esperance’ is in bloom and yellow ‘Clair de Lune’ will start next week. I know dahlias are native to the mountains of Mexico so should do well here but it is always a surprise to have a plant in such good form in June.”
Though the higher elevations of Sonoita and Tucson provide the cooler nights that dahlias love, still it’s clear: by choosing the right varieties and keeping them cool, gardeners in zones 8 and 9 of the South and Southwest can enjoy fabulous dahlias.