Updated: Jul 18, 2021
When I started flower farming, I saw my northern neighbors grow Dahlias by the bucket load. But when I began to research, what I read about growing Dahlias in the south was a little off-putting and somewhat disheartening. My first year attempting to grow them did not bolster my spirits. Almost all of them rotted. I thought pots were the answer (I read of a flower farmer who used pots for better drainage) but my potted ones rotted too. The few that survived looked sickly through our scorching summer and bug-ridden. Blooms in the fall were few (though pretty!!)
My data analyst mind took over and I began to look at every variable possible and set about improving each and every angle of the process. Though I've thrown in the towel on other crops (parrot tulips for instance) - I'm not moving on from Dahlias!!
1. Shade vs. sun: you'll traditionally read 'full sun' for Dahlias but I almost always question full sun recommendations. They were likely not made by someone as close to the equator as we are! :D We plant our Dahlias in partial shade and they thank us for it when we have months of triple digit dry scorching heat. Even the night time temperatures here are hot!
2. Soil - our soil is clay. This is a huge consideration and makes the approach to growing much different than loam or sandy soil. Dahlias grow from tubers and tubers are easily rotted when sitting in moisture. Clay loves to hold moisture for a long time. When is our clay holding moisture? Spring rains. When is our clay dry? Summer.
3. With the above in mind, I ditched the traditional wisdom of planting your tubers after the first frost and instead planted mine after the spring rains. Even though our last frost is typically March 15th, we have such a long growing season that putting off Dahlia planting for several months didn't hurt the time they'd have to mature. I planted mine mid-May in raised beds amended with lots of nice compost for drainage. No rot!
4. Here comes the southern twist - just when you've escaped the spring rains you're thrown into near drought conditions. So the dahlia's had to be watered. Alot. Ironic. :D [By the way, this is an excellent argument for rain water harvesting, just use your spring rains in the summer!]
5. Fertilize. When the tubers sprouted nicely, they needed nitrogen to grow. The soil was amended with lots of compost but we also fed them fish fertilizer once a month and compost tea almost as frequently.
6. Pinching. As July started heating up, we severely cut back our dahlias. By cutting them back from 12-18 inches to 6 inches, it promoted more growth, more branching, and cut out some bug issues that had started.
7. More water. Through-out August and into our unseasonably hot September, we watered our Dahlias for about an hour every other day. This was double last years watering amount and it made a humongous difference in the health of our plants. With water scarcity issues, we had to really consider how sustainable this crop was and determine if our rain water would be enough to make up for the thirsty plants. Considering the rest of our crops are pretty drought hardy, we decided the Dahlias could stay.
8. Though we have a long enough growing season, our bloom season is still pretty short. Dahlias like to bloom when the days are warm but the nights are cool. Also, once day-length falls below 12 hours, they go into tuber-production instead of flower production. We start cooling down here about the same time that day-length falls close to 12, so the harvest season isn't super long. In the middle of September, we switch from feeding them nitrogen to feeding them a bloom promoter - basically potash and which encourages them to set bloom.
9. The last, and perhaps most important consideration when growing Dahlias in the south is variety. This has been once of the most difficult things to find information about. The number of varieties are staggering and not all have been tested on a large scales in warm climates. So we're still learning. Hopefully soon, I'll be able to write a post about the best varieties!
10. The analyst in me had to get a round number! Support: we use tomato cages - at $2.50 per cage at Home Depot, they're affordable for us on a small scale. As we get a larger patch or try out bushier varieties we may experiment with other methods of support.