Dahlias are tuberous tender perennials that are not only some of the most beautiful flowers on the planet, they’re also a blast to grow once you get the hang of it. My guest this week, Kristine Albrecht, is a dahlia expert who is here to share dahlia basics as well as the advanced techniques for breeding new dahlia varieties at home.
Kristine is the founder of Santa Cruz Dahlias, an organic, no-till dahlia farm in California, and the author of “Dahlia Breeding for the Farmer-Florist and the Home Gardener: A Step by Step Guide to Hybridizing New Dahlia Varieties From Seed.” On just a quarter-acre in an urban area, Kristine grows more than 1,500 dahlia tubers annually. Her passion is hybridizing new dahlia varieties with complex colors and multiple forms.
Kristine is the president of the Monterey Bay Dahlia Society and travels with her blooms to local and national dahlia shows. She won the American Dahlia Society’s Derrill Hart Award and Lynn P. Dudley Award in 2014 for her cultivar KA’s Cloud and won the Derrill Hart Award again in 2018 for her cultivar KA’s Khaleesi.
Kristine’s first introduction to gardening was growing vegetables with her father. Then when she had kids, she and her son grew giant pumpkins — as big as 700 or 800 pounds. Once her son’s interest in giant pumpkins faded, her husband suggested she grow flowers instead to place in the rooms of their bed and breakfast. She started out with just a handful of dahlia tubers that her friend gave her, and the flowers were a hit. She grew 60 tubers in her second year, then hundreds — and kept expanding from there. There was no turning back.
Kristine joined the Monterey Bay Dahlia Society, which taught her how to grow dahlias really well. But she also credits her success to the giant pumpkin growers, who taught her the value of having great soil, watering well and pinching off. Giant pumpkins can grow 40 pounds a day, so they are really pumping in nutrients, she notes. Dahlias likewise benefit from fertile soil and pinching off.
Dahlias are super-prolific and like to be cut, Kristine says. When cut, they will give more blooms and will produce better tubers. Smaller varieties can give up to six blooms a week, and the giants can give a couple per week if you are lucky.
I grew dahlias a long time ago and recently got reacquainted with these beautiful flowers. I am making room for more flowers on my 5 acres in Georgia, and I am excited to add dahlias.
Dahlia History & Genetics
Dahlias are known for having beautiful colors and being double-flowered, but that’s not the case with all dahlias. In fact, the earliest dahlias came from Mexico, Central America and South America, and they were very basic in their looks. Then Europeans collected dahlias from the Americas, brought them home and began hybridizing them. Selective breeding gave us the dahlias that we know today.
Straight species dahlias have eight petals around a disc center. When breeders grew dahlias with 10 or 11 petals, they saved those plants and hybridized them further, for greater and greater petal count.
Most plants are diploids, which means they have two sets of chromosomes, while dahlias are octoploids, with eight sets of chromosomes, Kristine explains. “Because of the genetics they have a lot of diversity, and that’s unusual in the flower world,” she says. That’s why there are some dahlias that are under 2 inches and similar in appearance to cosmos and others that are 16 inches across, fully doubled, with petals going back to the stem.
Straight species dahlias still grow in the wild. Kristine recently joined a trip to Mexico for the American Dahlia Society Genome Project to collect wild dahlias for genetic sequencing. The project aims to discover if straight species dahlias have less DNA than the big, fully doubled dahlias on the market today.
The Best Place and Time to Grow Dahlias
Kristine finds that the best place to grow dahlias is where you have had success growing tomatoes.
Dahlias need good soil, six to eight hours of sun a day, a lot of mulch and cool roots. Because dahlias get quite tall — some varieties grow 6 to 7 feet tall — they require staking. Plants should be 16 to 24 inches apart.
Dahlias don’t like it super-hot, Kristine says. Speaking to that fact, she notes that species dahlias found in Mexico grow in the hills, where it’s cooler. Dahlia cultivars do well in coastal California and the Pacific Northwest, where temperatures don’t get that high. She says that if you were to grow dahlias in a hotter area, you’d likely need to install 30% shade cloth over them. The shade cloth will cut down on the heat while still allowing adequate light to get through.
Plant dahlia tubers after your area’s last frost date has passed. It’s also best to wait until the soil temperature has reached 50°.
Some growers in super-hot climates start their dahlia tubers super early. Then, during the hottest period of the summer, they cut the dahlias back to the ground to give them a break. The cut-back dahlias will sprout again in fall.
Cold is a problem for dahlias too. Because dahlias are frost sensitive, they are a challenge to grow in areas with a short growing season. The tubers can be pre-sprouted to give them a head start, or rooted cuttings can be used. The plants may still need to be protected with row cover early on to protect them from the frost. If the tubers are started too late, the first frosts of fall may kill the plants before they can flower.
There is no one-size-fits-all method for fertilizing dahlias. “Every soil’s so different and has different needs,” Kristine points out. She advises starting with a soil test and following the recommendations that come with the test results. Because she grows organically, she tells the soil testers that she only wants organic solutions for improving soil fertility.
Monitor plants, particularly mid-season, for signs of stress. That’s when Kristine tends to add compost, plus blood meal for nitrogen. Her soil is naturally high in potassium and phosphorus, so the fertilizer that works in her garden may not be what’s best for your garden.
Dahlia Pests & Diseases
In California, the top three dahlia pests that Kristine encounters are cucumber beetles, thrips and aphids. Another pest found in California but rarely seen elsewhere is lygus, the western tarnished plant bug. Outside of California, Japanese beetles are a common dahlia pest.
Because her dahlias can’t be damaged when she brings them to shows or sells them to florists, Kristine sprays her plants every 10 days with horticultural oil. The oil takes care of soft-bodied insects and powdery mildew. For chewing insects such as cucumber beetles, she sprays spinosad-based Captain Jack’s Deadbug Brew.
If you plan to pre-sprout dahlia tubers, the timing will depend on the variety. Determine how long the variety takes to sprout indoors and then count back from your last frost date. Cafe au Lait, a dinnerplate dahlia variety, will sprout in just 10 days. Other varieties will take as long as a month or six weeks.
The size of a dahlia tuber does not matter. A tuber will grow to produce flowers if it has an “eye.” Plant the tubers in slightly moist potting soil and keep them in a warm room. The tubers can start out without grow lights, but once they sprout they will require light. If they are not put under grow lights at this stage, they will become leggy. Kristine keeps the lights on for 14 hours a day.
In addition to putting up some green, the tubers will begin developing their root system as well, Kristine says. Not only does pre-sprouting give you a head start, but it also reveals which tubers are good and which are duds.
Pre-sprouted tubers can be watered right away once planted in the ground. That’s because they already have a root system to take up water. Unsprouted tubers shouldn’t be watered immediately because the tubers will just get wetter and wetter, which will make them vulnerable to rotting.
For bushier dahlias plants with more blooms, pinch off the tip of the stems before they get tall. Kristine pinches off her plants once they have three sets of leaves, which is when the plants are about 6 to 8 inches tall. At this stage, the plants are still in trays. Other growers like to wait to pinch off until the plants are in the ground and the stems are knee-high.
In warm areas like Santa Cruz where Kristine is, dahlia tubers can be overwintered in the ground. In areas where the ground freezes, the tubers must be deep mulched or dug up and stored. Otherwise, they won’t survive.
Even if freezes are not an issue in your area, the American Dahlia Society recommends digging up the tubers every fall. That’s because the tubers keep multiplying underground and form a big clump. It’s very crowded for the plants, and they will struggle for resources. The plants can’t put as much energy into producing big blooms. Crowding also makes dahlias more susceptible to powdery mildew. If you do overwinter tubers in the ground, the American Dahlia Society recommends breaking off all but two stems per clump to prevent crowding.
When storing dahlias tubers, the best temperature is 43° Fahrenheit. Too cold and the tubers won’t survive. Too warm and the tubers will think it’s spring and sprout prematurely.
Select Varieties That Perform Well Where You Are
Certain dahlias varieties grow better in certain areas than others. For example, Linda’s Baby, a variety of ball dahlia, does not grow well where Kristine is in California, but gardeners elsewhere in the country have great success with it. To find the varieties that are best suited to your area, Kristine says to ask someone who already grows dahlias where you are, or consult your local dahlia society, if there is one.
Dahlias reproduce through both seeds and tubers, and can even be grown from cuttings.
Tuber propagation is the most popular way to propagate dahlias. All it takes is digging up and dividing tubers each fall.
To propagate a cutting, you can use rooting hormone powder, but it is not necessary for success. However, the cutting must include a leaf axis (the place where leaves meet the stem) because that is where roots will form. Plant the cuttings in rooting cubes, washed play sand or seed-starting mix.
Tuber propagation and cutting propagation are both methods of cloning. That means the offspring will be identical to the parents.
Propagating dahlias from seeds is far less predictable than cloning. Each seed in a dahlia head needs to be pollinated individually. That means one dahlia seed head can have hundreds of different pollen parents. You can’t know what you’ll get when growing dahlias from seeds. “You could get yellows, reds, purples — all different colors,” Kristine says. For that reason, gardeners who want to have more control over their gardens should choose tubers.
Dahlia Breeding Challenges & Methods
When Kristine began hybridizing, she discovered that bees and other pollinators are more attracted to single-flowered dahlias with an open center. Single-flowered dahlias have much more pollen and are easier for bees to access, so bees visit them more often than double-flowered dahlias. Even though 90% of her dahlias were fully doubled, when she grew out the seed the following year, 99 percent of the hybridized plants were single-flowered.
Kristine learned that in order to hybridize fully doubled dahlias, she had to cull all single-flowered dahlias from her garden first. That way, the bees’ only choice is double-flowered dahlias.
Successfully breeding dahlias starts with setting your hybridizing goal. With an end result in mind, cull your garden of dahlias that don’t align with that goal. Add high-quality tubers of dahlias that do. If you want to breed a dahlia with a high petal count, then start with dahlias that have a high petal count. If you want orange dahlias, start with orange dahlias. If you want a lighter color, bring in white dahlias to mix with colored dahlias.
Even when you take steps to control the results, there is still a lot of luck involved, Kristine warns. When you get to your goal is hit or miss, especially when bees are choosing the pollen for you. Hand-pollination is an option but it is time consuming and sometimes not that successful because bees are so much better at pollination, she adds.
Kristine’s book includes helpful illustrations that show how to hand-pollinate dahlias. She puts an envelope below the flower to collect pollen and uses a paintbrush to apply the pollen to another flower. She writes down the parent plants on a tag and attaches it to the bloom. She covers each bloom with organza bags to prevent bees from further pollinating the flowers that she hand-pollinated. Six weeks later, the plant produces seeds.
Once the seeds have a little bit of a belly and are a nice dark color — brown to black — they are ready for harvest. Don’t delay because rot can set in if the seeds are left on the seedheads for too long. Dry the seeds out for 24 hours then place them in paper coin envelopes. Label the envelopes with the names of the parent plants and the number of seeds. Store the envelopes in a cool dry place.
Good tuber production and good storage capabilities are other considerations. If starting with a plant that only produces one or two tubers annually that store poorly, the resulting hybrid could be hard to replicate and sustain.
It’s super exciting to imagine what your hybridization project will result in, and that’s half the fun. Kristine describes it as a numbers game: If you plant 1,000 seedlings, you may get only 10 plants that you’ll want to keep.
Starting Dahlias Seeds
Kristine starts dahlia seeds using the “paper towel method,” which involves spreading seeds on a wet paper towel and keeping it moist. Dahlia seeds can take between two days and 22 days to germinate, though most sprout on day five.
When the seeds have sprouted, Kristine moves them into a 72-cell tray with Root Riot cubes that have been rolled in a mycorrhizal inoculant. You can use a smaller tray and any quality seed-starting mix. The inoculant is optional, though if you do apply it you may find that your plants grow to be bigger, healthier and more resilient. If you are seed starting in a cool room, a seedling heat mat will help to keep the seedlings adequately warm.
Kristine places the seedlings root-end down with half the seed out of the soil. Leaving some of the seed exposed reduces damping off, a fungus that can kill seedlings. She keeps the seedlings under grow lights so they don’t stretch out in search of light. “You want to keep your plants as kind of short and stocky and not leggy as possible,” she says. When the seedlings have grown a bit, she pots them up to 4-inch pots, taking care to label them accurately.
Dahlia seeds can be sprinkled on the ground in spring but you may have less success and it could be too late of a start for your growing season.
Using Sink Pots for Dahlias
Kristine does not remove her seedlings from their 4-inch pots before planting them. Rather, she digs a trench, lowers the pots into the ground, and covers them with soil. She calls these “sink pots.”
Sinking pots into the ground has the advantage of making it easy to cull the plants she does not want. (In agriculture, removing inferior or undesirable plants is called “roguing.”) Once a plant has put on a bloom or two, she inspects it and decides if she is interested in keeping it. If not, she can simply pop the pot out of the ground.
Without the pots, the undesirable plants’ roots would travel around and intertwine with the roots of desirable plants. And in fall, when it is time to get the tubers out of the ground, the pots can be pulled up easily.
I hope you enjoyed my conversation with Kristine Albrecht and feel more confident about growing dahlias. If you haven’t listened to our conversation yet, you can do so now by clicking the Play button on the green bar near the top of this post.
Have you enjoyed growing or breeding dahlias? Let us know in the comments below.
Links & Resources
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joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
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