Learn How to Grow Beautiful Dahlias from Seed

By Steve K. Lloyd

Copyright © 2024 All Rights Reserved

Growing dahlias from seed is a fast, fun way to create beautiful blooms in your garden. When you start dahlia seeds indoors, you can have healthy plants ready to transplant outside as soon as the weather warms up. 

You don't have to be a seed-starting pro to give your dahlias a head start on the season. Learn a few easy tricks and you'll have a garden full of these colorful blooms and be on your way to dahlia-growing success!

Most of you know dahlias can be grown from tubers, underground fleshy roots that resemble bulbs. But dahlias also produce seeds, which can be harvested and planted just like other vegetables and flowers.

Because of this plant's unique genetics, seed-grown dahlias do not turn out like their parents. When you sow a dahlia seed, the plant it grows into can differ greatly from the parent plant. In fact, the new plant can show characteristics from several ancestors.

That's part of the thrill of growing dahlias from seed—it's always a wonderful surprise to see the array of colors and flower shapes the plant will produce.

One of the best things about starting your dahlias from seed is the sense of pride in your garden you will receive—knowing that one tiny seed produced all that beauty.

How early in the spring should you start indoor dahlia seeds?

Dahlias are sensitive to cold so you don’t want to transplant them outdoors too early. Your young plants should be sturdy enough to move to the garden about two months after seeding.

So the first step is to determine the last expected frost date in your area using a resource like the one provided by The National Gardening Association. 

That frost date is an average and can vary from year to year, but it gives you a general guideline. Next, count back six to eight weeks from the last frost date in your area and you’ll get the earliest date to start your dahlia seeds. 

I prefer to start them as early as possible so I have larger dahlia plants and earlier blooms. After all, dahlias grown from seeds will take 100-120 days before they bloom.

 Remember, you’ll need to move the seedlings to bigger pots, as they grow so be sure you’ll have a well-lighted area for all these plants.

Packaged dahlia seeds can be purchased from most retail outlets that sell other garden seeds. These dahlias typically grow into "bedding dahlias," knee high, with open-centered daisies or collarette flowers in solid colors.

You can get dahlia seeds for taller varieties from specialty dahlia growers. A few mail-order nurseries sell them, and many small dahlia farms collect and sell seeds from the dahlias they grow for cut flowers or tuber production.

If you grow dahlias, you can easily save seeds from your plants. At the end of this article there are links to more articles about saving dahlia seeds.

Fill your seed starting pots or cell trays with a moist seed starting mix. I plant my seeds in 2 ½ inch pots to avoid moving them to large pots—called potting up, so soon.

You can also start the flower seeds in plug trays to save space initially, but you will need to pot your dahlias up into a larger container at least once before transplanting them into the garden. 

Sow dahlia seeds an ⅛ inch deep. Tap the soil with your finger to ensure soil contact around the flower seed. The ideal temperature for germination is 70-75⁰F, and you should see your seedlings emerge in 7-14 days. 

Like all seedlings, light is vital for photosynthesis. Your seedling tray needs to be close to your light source when growing dahlias indoors. As soon as the seedling emerges, the plant needs bright light to grow sturdy so the light should be only a couple of inches above the soil.

As the dahlia plants grow taller, you can slowly raise the light source, always keeping it within two or three inches of the leaves.

Here’s the best way to water your dahlia seedlings.

I prefer bottom watering because it allows the soil to act as a wick and take up the water without disturbing the seeds. My four-inch pots are placed in 1020 size heavy duty trays underneath my pots and cell trays when starting seeds. I fill the tray about 1 inch or 1 ½ inches with water and let the soil soak up the water. After about an hour, I drain out any excess water so the soil doesn’t remain too soggy.

Bottom watering saves time and helps prevent damping off of your tender seedlings.

Before germination, be sure the soil stays moist. Dry soil prevents sprouting and can stunt or kill a tiny seedling. If the top layer of your soil is dry, use a fine mist spray bottle to dampen the very top layer of soil. 

When should you pot up your dahlia seedlings?

Dahlia seedlings must be potted up if they can't be planted in the garden yet. They are not cold-hardy, so you must wait to plant them until all danger of frost is gone. I like to pot up my dahlias when they have filled out the pots nicely with roots. 

It's essential to keep them growing. If the seedlings sit in the pots or trays long enough to become root-bound, they will stunt their growth. Potting them up will keep your plants growing strong and produce flowers earlier. Once a week, lift each pot to see if roots are poking out the bottom. If so, move them up to a larger container. 

Potting up your dahlias will give them plenty of growing room to stay in the growth stage without the risk of them becoming pot-bound in small containers.

How to harden off your dahlia seedlings:

Once your temperatures are warmer and no longer freezing, it's the perfect time to let your seedlings get a taste of the great outdoors. If you've been nurturing your dahlia seedlings indoors or in a greenhouse, it's crucial to ease them gradually into their new environment. 

This critical process is called "hardening off," it takes about a week to do it right—don't rush it!

Think about it: your dahlia seedlings have been living in a cozy, controlled space with stable temperatures day and night. They haven't felt the sun's warmth, which is significantly stronger than indoor grow lights. These young plants are delicate and can easily get sunburned or stressed by wind, rain, and temperature swings.

So, take it slow. Gradually introduce your growing dahlias to the outside world, starting on calm days. Increase their outdoor time each day until they've also spent a few nights outdoors. Be cautious—too much wind or rain during this process can harm your tender plants.

This careful approach will help your dahlias thrive in their new surroundings.

Prepare the garden for your new dahlia plants.

Dahlias need full sun. Choose a location with well-draining soil and six or more hours of sun daily—morning sun with late afternoon shade is best.

Before planting, enrich the soil with compost or well-rotted manure. Adding sand, peat moss, or organic compost to heavy will help loosen the dirt and improve drainage. 

Dahlias grown from seed can be planted about 12-18 inches apart for the shorter varieties, and taller varieties will need 18-24 inch spacing. Remember, those tall-growing dahlias grown from seed will need stakes or other support, just like those produced from tubers.

Growing dahlias from seeds is an incredible experience that brings great joy to your garden and a cost-effective way to fill your flower bed with beauty.

Imagine the thrill of watching and waiting for that first bloom, not knowing what color or texture your flowers will have. And the best part? You're growing a special flower that's unique to you!

If you haven't tried growing dahlias from seed yet, give it a go this season! It's a fantastic way to add your personal touch to your garden.