Dahlias are a genus of flowers in the daisy family native to Central and South America. With eight sets of chromosomes they vary in color, shape and size far more than most flowers. So, whether you want bold drama or whimsy, classic formality or a romantic cottage-garden feel, you can find the perfect dahlia for your garden that will bloom from mid to late summer until frost.
If you’re thinking about growing dahlias for the first time, it may be because you’ve seen a photograph of a spectacular flower - an enormous Cafe au Lait, a sculptural ball-form like cornel bronze, or an otherworldly cactus-form dahlia - and you want one for your own garden. In that case, you’ll want to purchase tubers - large underground roots like flower bulbs that form and multiply underground as a dahlia plant grows. Growing dahlias from tubers is the only way to get an exact clone of a specific variety you may want.
However, if you’re less particular about growing specific varieties, it is also possible to grow dahlias from seed that will produce tubers by the end of the growing season.
Why Grow Dahlias from Seed (VS Tubers)
The most important thing to know when deciding how you will grow your dahlias is that dahlia tubers will produce a clone of the original plant; one with exactly the same characteristics. Whereas dahlia seeds will not produce exact clones of their mother plant and may even look quite different. In other words, unlike tubers, dahlia seeds are not true to type.
At the same time, seed-grown dahlias aren’t completely random. They are likely to share at least some characteristics of the original plant. So it’s possible to buy seed mixtures that will produce a range of dahlias within certain parameters. For instance, Sarah Raven’s popular Bishop’s children mix will produce dahlias in a range of colors, sizes and petal shapes that all have dark purple to black foliage. Other mixes allow you to select for characteristics like height, double blooms, or a collarette or cactus shape.
On the other hand, if a random mix of dahlias is what you are looking for, some Dahlia farms, like the popular “bee’s choice” mix from Floret, offer random mixes of seeds from their farm’s selection. You may also be able to find a similar mix at a small dahlia farm that’s local to you. Keep in mind, though, that these sorts of mixes will tend to produce mostly single open-centre types of dahlias.
You may want to grow dahlias from tubers if:
- You want a specific variety in your garden.
- You prefer elaborate/non-single varieties of flowers.
- You don’t have the time to start and care for a lot of seedlings, stake the plants and dig the tubers in winter
You may want to grow dahlias from seed if:
- You want to save money in your garden - even if it means spending more time - by growing more plants for the cost of a few packets of seed.
- You want to learn to grow dahlias before investing in tubers. There is a lot of room for error when growing dahlias, from planting to staking and tuber storage. The stakes won’t be as high while you learn if your dahlias were grown from seed.
- You like surprises in the garden. With dahlia seeds you can never be sure exactly what flowers you will get, which can be part of the fun of gardening.
- You’re interested in creating your own Dahlia varieties
- You’re in it for the bees and pollinators. Seed grown dahlias tend to have open centers full of pollen that the bees absolutely love.
- You want to grow dahlias as annuals. Dahlias are not a low-maintenance flower, but you can eliminate a lot of the time required if you grow them from seed as annuals.
- Your garden is new and you’d like to fill in space until you can plant trees, shrubs and perennials, or until existing plants grow to their full size. Dahlias are perfect fillers as they can grow huge in one season.
Looking for more ways to create an exuberant garden without spending a fortune? Try propagating boxwood from cuttings.
How to Start Dahlia Seeds
Unless you live in a very warm climate with a long growing season, or don’t mind waiting till late summer for blooms, you will need to start your dahlia seeds indoors. In general, the best time to start is about 6-8 weeks before your last frost date, around the time you start your tomatoes. The exact right time for you will depend on a number of variables like your climate, how early you’d like to get blooms and the amount of effort you’re willing to put into caring for seedlings.
If you’d like to get dahlia blooms earlier in the season, you can start your dahlia seeds even earlier, anywhere from 8-12 weeks before your last frost, though you will need to pot them up into larger containers one or two times before it will be safe to transplant them outdoors. This approach has the added benefit of giving you enough time to start more seeds if the germination rate of your initial seeds is low.
Notes on Dahlia Seed Germination
Dahlia seeds tend to have a relatively low germination rate that can be anywhere from 30% - 75%. This means you shouldn’t expect all of the seeds you sow to germinate and that is perfectly normal.
To maximize your germination rate:
- Start more seeds than you think you will want or need.
- Keep your dahlia seeds warm as you wait for them to germinate. Dahlia seeds like warm temperatures between 18F and 21C (64-70F). Placing your seed trays on a heat mat will help maintain that temperature. If you don’t have a heat mat you can try placing them in a relatively warm place in your house like a sunny window or on top of your refrigerator or dryer.
- Keep the growing medium moist but not soggy. If you have it, sprinkle the top of your seed trays with a thin layer of fine vermiculite and/or keep them covered with a plastic dome or plastic wrap until the seeds germinate so the top of the soil does not dry out.
Starting seeds with low germination rates can waste growing medium and space under your grow lights as cells remain empty with seeds that don’t germinate. This can be especially frustrating if you’re short on growing space or are trying to fit your seeds on a heat mat. There are a few germination strategies you can try to save space when working with tricky-to germinate seeds like dahlias:
- Try over-sowing seeds closely together in shallow single-cell trays rather than sowing one or two seeds in individual cells of standard cell trays. Plastic takeout containers and aluminum grilling trays work well for this. Once germination begins, prick the tiny seedlings out of the tray and transfer them to individual cell trays.
- Try the paper towel method of germination. Place your seeds on a damp paper towel, fold the towel over and place inside a plastic bag. Check regularly to see if the seeds have begun to germinate and transfer any that do to cell trays.
- Try starting your dahlia seeds in individual small ¾” soil blocks. Among its other advantages, soil-blocking is a great space-saving solution for when you are starting a lot of seeds, especially flowers and herbs with tiny seeds and slow growth or anything with low germination rates, like dahlias.
What you will Need to Grow Dahlias from Seed:
- Dahlia seeds
- Seed starting mix or soilless potting mix
- Seed trays or pots
- Heat mat (optional but recommended)
- Indoor grow lights
How to Start Dahlia Seeds: Step-by-Step
- Pre-moisten your growing medium. Ideally a seed-starting mix or a soilless potting mix. Avoid using soil from your garden.
- Fill your seed trays or pots with the growing medium, tamping it down to ensure the trays are full.
- Plant your dahlia seeds by placing them on the top of the growing medium and pressing them a few millimeters into the soil. If you are using cell packs, use 1-2 seeds per cell. If you are using a shallow tray with the intention of pricking seedlings out after they germinate, broadcast the seeds closely together over the top of the tray.
- Cover the tops of the seeds with the growing medium and top with a thin layer of fine vermiculite, if you have it.
- Mist the tops of the trays lightly with water and use either a plastic dome or a piece of plastic wrap to trap moisture around the seeds.
- Label your trays with the variety names.
- Place your seed trays or pots in a warm location, preferably on top of a heat mat.
- Once the seeds begin to germinate, move the trays beneath grow lights hung 1-2 inches above the trays.
For more details on how to start seeds, check out my guide to the basics of starting vegetable seeds indoors!
Temperature, Moisture, light & Feeding
- Dahlia seedlings do best in slightly cooler temperatures than those required for germination. Remove them from the heat mat after germination.
- Maintain good ventilation around your seedlings. Don’t crowd them together too tightly if you can and use a small oscillating fan to mimic a slight breeze.
- Check the growing medium regularly and maintain a consistent level of moisture. You want the growing medium to be moist but not soggy. Ideally, water from the bottom.
- Dahlias need a lot of light. Keep them under grow lights for 14-16 hours a day, turning them off at night. If they get too large to fit under your grow lights you can transfer them to a bright south-facing window. As temperatures rise above 10C (50F) in the daytime, you can also begin to harden them off outside during the day, bringing them back inside at night.
- Once your dahlia seedlings have their first set of true leaves, fertilize them regularly. There are many approaches to doing this. Fertilize every 2 weeks or so and use whatever balanced water-soluble fertilizer you can find, dilute it with water to between ¼ and ½ of the recommended application and bottom-water your seedlings with it.
Pinching out the tops of your dahlia plants is a pruning technique that encourages some plants, like dahlias, to produce more branching, sturdier stems and more blooms. Like all pruning, pinching is a way of working with your plants, not against them, to produce the best possible plant. It can be difficult to do after taking so much care to grow your seedlings in the first place, but it’s for the best. I like to think of pinching out like killing your darlings - removing a sometimes beloved piece to make the whole better. Getting used to doing this will strengthen your plants and make you a better gardener.
To pinch your dahlias, wait for the seedlings to reach 6-12 inches tall and use a pair of sharp clean pruners to snip off the top three to four inches from the main growing stem, cutting just above a leaf node. Dahlias are resilient plants, often growing back even when they are snapped off at ground level by the wind. They will quickly produce new growth after being pinched and by the end of the season you will have sturdier plants with far more blooms than if you had left the main stem to grow.
Depending on how early you start your dahlia seeds, you may need to pot them up once, or even twice, into larger containers before transplanting them into the garden. As your seedlings grow, periodically check their root growth by looking at the bottom of the tray to see if any roots are emerging from the drainage holes or popping them out of their pots to take a look at the roots. If the roots are filling up the growing medium and beginning to spiral, if the seedlings are getting very top-heavy for their pots or if they start drying out very quickly, it’s probably time to transfer them to a larger pot.
Transplanting Dahlia Seedlings Outside
Timing & Hardening Off
- Dahlias are very frost sensitive so you need to wait until any chance of frost has passed before transplanting your seedlings outside into the ground.
- Dahlias prefer night time temperatures of at least 15C (60F) (similar to warm-season vegetables like peppers and eggplants) So, depending on how quickly your climate warms up in the spring, you may need to wait a few weeks longer than your last frost date to plant them outside.
- A few weeks before you plant to transplant your seedlings, begin to harden off your dahlia seedlings by gradually introducing them to outdoor conditions. Start them off with short periods outside in a protected shady location and slowly move them to sunny areas for longer periods of time. Be sure to bring them back inside at night.
Location & Spacing
- When ready to transplant, choose a warm location that gets full-sun and has well-draining soil. Dahlias don’t love soil that’s too dry but they also don’t like heavy and overly wet soil. If your soil is too wet or heavy, add compost or grit at planting time. Dahlias work well in their own dedicated bed, especially if you plan to use them as cut flowers, or tucked in amongst other plants in herbaceous borders. You can also grow dahlias in pots, though they will require more maintenance. Use the largest pots you can and choose shorter or dwarf varieties.
- Dahlias can grow quite large, so plan to give each dahlia two or three feet if planting in mixed borders (though I like a closer spacing at about 12-18” between each plant). If planting in a dedicated dahlia bed, plant them closer together with 12” between each plant in rows about three feet apart.
Support & Staking
- Dahlia plants have delicate hollow stems so they will usually require some sort of staking or support. Stake your dahlias as early as possible, preferably at planting time.
- There are many ways to support your dahlias. Some people use 1-3 stakes per dahlia plant. I like to group 3 dahlias together around a sturdy foraged branch. If your dahlias are spaced closely together in dedicated dahlia beds, you may want to build a framework or grid around them using stakes and twine or netting. Foraged branches, bamboo canes and metal poles all make good staking materials.
- If you’d rather not bother with staking, choose shorter dahlia varieties like dwarf or bedding types, be sure to pinch the plants back to encourage sturdier bushier plants, and plant them in areas where they will be protected from strong winds. You risk losing plants if you don’t support them, though the plants will often grow back.
Care Over the Growing Season
- Dahlias are heavy feeders. I like to top with compost after planting, which I use as mulch, and sprinkle some all purpose slow release fertilizer - like a granular chicken manure - or bone meal over the soil. As the season progresses, fertilize with a water-soluble fertilizer every few weeks but avoid fertilizers that are high in nitrogen.
- Like most plants, dahlias need about 1 inch of water per week, sometimes 2 in the hot summer, but this will vary based on where you live, how much rain and humidity you get and how windy your garden is.
- Mulch your dahlias to conserve moisture and water deeply about once per week, 2-3 times a week during the hottest months of summer, unless it has rained a lot. Avoid watering over the top of your plants, aiming the water instead towards the base of the plants.
- Deadhead dahlias regularly to ensure a long flowering season.