How to Propagate Dahlia Cuttings

Most gardeners begin their dahlia growing journey by planting tubers—swollen roots similar to daffodil, lily, and tulip bulbs. All dahlia tubers contain the genetic coding along with the stored nutrients needed to produce an exact duplicate of their parent plant.

When dahlia tubers are planted into warm soil, they produce roots that will deliver moisture and soil nutrients to the plant. They also sprout a growing shoot from one or more eyes, specialized plant cells, on the crown of the tuber. Without an eye, the plant cannot sprout.

For the beginner, dahlia propagation using cuttings starts with a tuber. Although more advanced techniques involve making cuttings from other parts of a growing dahlia plant, we'll start with the basics. 

Every dahlia variety is suitable for making cuttings, but not every tuber performs equally. It is best to select a reasonably large tuber since small tubers can quickly deplete stored energy to grow the shoots. The tuber should be plump and firm—wrinkled skin and a shriveled appearance suggest that the tuber has dried out and may not perform well.

Bacterial diseases are a different story! The two most troubling are leafy gall (Rhodococcus fasciens) and crown gall (Agrobacterium tumefaciens).We won't get into the details here, but know that these diseases are bad news for dahlias! Once a plant is infected, the disease is carried by its tubers and will be transferred to any plant propagated from cuttings of the infected tuber.

Cuttings are typically grown in late winter or early spring, depending upon climate zone and the last frost date. The goal is to produce small, actively growing plants ready to go into the ground about the time you would direct-plant dahlia tubers in your garden. 

Consider taking dahlia cuttings about 4 to 8 weeks before you expect to plant them outside.

Here’s a list of suggested supplies and tools:

• Dahlia tubers, either individual or undivided clumps;

• Potting soil, compost, or other growing medium;  

• Small pots, trays, or larger containers;  

• A very sharp knife, scissors, or scalpel;  

• Rooting cubes or small segmented seed-starting trays;

• A humidity dome or other clear top to cover the baby cuttings;  

• Bright lights to position over the cuttings, preferably with an automatic timer;  

• A warm room that maintains an all-day temperature of at least 65F (18C);  

• Plant labels (such as small plastic stakes); and 

• Garden marker (not a Sharpie).  

• Recommended: A seed-starting heat mat with automatic temperature control  

• Optional, but recommended: Rooting hormone powder or gel.

Before I get into the step-by-step process, I'd like to summarize how this all works so you'll have a clearer picture when I get into the specifics.

As a tuber warms up and breaks dormancy—often called waking up—it triggers specialized parts of the tuber to grow. The eye swells and prepares to make a growth shoot, which will become the stem of the dahlia plant. Next, small roots form at the other end of the tuber. 

I’m talking about stem cutting, carefully removing the growing shoot from the tuber with some leaves attached. This small section of stem is placed in a growing medium where it is kept moist and warm.

When exposed to bright light for at least 14 hours per day, the cutting is triggered to form roots and as it grows taller, it forms new leaves.

Meanwhile, the tuber senses the growing shoot is gone. In nature, maybe something ate it, or an animal stepped on it and broke it off. So the tuber directs energy to the eye, forming one or more fresh shoots to replace the one we removed. This regeneration process allows us to take multiple cuttings from a single tuber in a single season.

About 14 days after being removed from the tuber, the new dahlia cutting has a small root system and can be transplanted into a larger pot. It will continue to grow under bright light until the weather is warm enough to begin its transition outdoors.

As it produces shoots the tuber draws from the food reserves in the tuber. Even once the tuber forms roots, it only draws a little nutrition from the surrounding soil at this early growth stage. 

That's why bigger tubers make better cutting parents—like a fat bear, they have more energy to produce a series of new shoots for us to take as cuttings.

So that's the big-picture version. Now, let's talk about how to make it happen!

Set up your cutting bench or table in an area where it can remain undisturbed for at least a few weeks. I’ve converted part of my garage into a dahlia-cutting and seed-starting room. If you have a greenhouse, that's even better. Others have used their basement, a spare bedroom, or any room with electricity that you can maintain at a suitable temperature.

Dahlia cuttings need a precise amount of light each day. Dahlias are sensitive to what they perceive as day length, which we manipulate by placing the lights on an automatic timer. The magic number for dahlia cuttings is 14 hours of intense, bright light followed by 10 hours of darkness or dim, diffused window light. My timer turns the lights on at 6 AM and off at 8 PM. Although faint natural light enters the room between 8 PM and 6 AM, when the lights are off, it's dark enough that the dahlias sense it is night.

Tubers for cuttings may be planted individually in pots, or multiple tubers can be placed in a larger container such as a tray, plastic storage bin, or any other sturdy container that is about 6 inches (15 cm) deep or more.

Fill the pots or containers with good-quality potting soil. The exact formulation isn’t critical, but you want to select a product that retains moisture and offers a bit of nutrition to the early roots formed by the tubers.

In the garden, we plant dahlia tubers horizontally and cover them with 4 to 6 inches (10 to 15 cm) of soil. However, when waking up tubers to take cuttings, we need to access the crown of the tuber, where the fresh growth happens. So place the tuber on its end with the crown up. Gently firm the potting soil around each tuber, adding a bit more material as needed so the slender necks of the tuber is covered and only the crown shows. 

Potting soil may be slightly moist right from the bag. If so, you don't need to add any water. If the mix you're using feels very dry, moisten it slightly. We don’t want to over-water the tubers when first placing them in potting soil! The cutting doesn’t have roots, and can rot in wet soil.

Keep track of the dahlia variety names as you plant the tubers. I use small plastic plant stakes, which I label with a garden marker and insert next to each tuber. You can use a Sharpie or other felt-tip marker, but remember that the ink will fade and become unreadable. I use this type of  garden marker  that remains legible even after years in the garden.

Place your newly planted tubers on a flat surface. Although it is possible to use an electric heat mat to maintain the 60-70F (15.5-21 C) temperature they require, bottom heat also contributes to tuber rot. It's better to keep the room temperature within the desired range and let the air warm the tubers.

The tubers don’t need light while the eyes wake up and prepare to sprout. But as soon as the first tuber shoot appears, treat it like a growing plant and use your lights and automatic timer to deliver that steady 14 hours of light daily.

Now, the waiting begins. It takes dahlia tubers anywhere from one week to 2 months to wake up and start growing though most only take a week or two. Each eye can produce new growth, so the number of shoots will correspond to how many eyes each tuber has. 

Wait for the developing shoot to reach a length of about 3-4 inches (7-10 cm), when it should have either two or three pairs of leaves. This is the perfect length to take cuttings.

Prepare a new home for the cuttings as you watch your tubers grow. Although there are many DIY options, the best and most effective method is to use a 10x20-inch (25x50 cm)  seed-starting tray  with a clear plastic cover called a humidity dome. These are available in many garden stores, home centers, greenhouse supply stores, and online. 

Choose a tray without drainage holes—you will keep your new cuttings very moist, so keep the water from escaping the tray.

Inside the tray, you can use small pots, such as the 3.5" (9 cm) square pots that are sized to fit inside the seed tray. You can fit around 15-18 pots per tray, depending on the exact size you choose.

Many growers—including me—use a smaller insert that  provides 72 cells  in a standard tray rather than 15-18. This permits growing a lot of cuttings at once, which can be crucial. There are also soil blocks, such as the popular  Root Riot cubes , that fit conveniently inside standard trays.

Whether you use pots or 72-cell inserts, fill them with high-quality potting soil. This soil will provide the nutrients and water your cuttings need during their first weeks of life. If you're using soil blocks, everything the cutting will need is already there.

Place the soil-filled pots or inserts in the tray and water them thoroughly. Allow the moisture to soak in, then water them again. You want the growing medium to become almost completely saturated but not glistening or dripping wet. Think of a damp kitchen sponge allowed to drip for a few minutes without being wrung dry—that's what you're after.

The day before you take your first cutting, place the tray with the dampened potting soil on a heat mat set to 65-70F (18-21C). While it isn’t a necessity to use bottom heat, cuttings root more readily and grow faster if they receive warmth from the soil and the surrounding air.

Place the clear humidity dome on the tray to keep the growing medium from drying out too quickly. As the water in the soil evaporates, moisture will remain suspended in the air inside the humidity dome, which will help keep the new cuttings hydrated. This is critical! Because dahlia cuttings do not have roots at first, they can only receive moisture by absorbing it through their tiny leaves and stems.

Get ready to take your first dahlia cutting! Choose a tuber with a shoot that is 3-4 inches (7-10 cm) in length. It’s best to have at least two pairs of leaves (not counting the tip), but 3 or 4 pairs of leaves are even better.

Here is a photo of dahlia ‘Emory Paul’–the very same cutting illustrated in this post–in full bloom. The photo was taken on September 23rd, almost seven months after this tiny shoot was taken as a cutting on March 3rd. This cutting of ‘Emory Paul’ grew to a magnificent dahlia, and you can duplicate these results in your own garden!

Sanitation is essential, and you need to clean your knife, scalpel, or other cutting tool between each plant. This helps reduce the chance of transmitting disease cells from a sick tuber to the cutting of a healthy plant.

I keep a small open jar of 10% bleach solution (1 part liquid household bleach mixed with 9 parts tap water) on the bench beside my tools. After each cut, I dip and swirl the blade in the bleach solution to sanitize it. One grower I know labels her scalpels and uses the same tool for every cutting she takes from that tuber. Both methods can work well.

Repeat the steps above if you are taking additional cuttings that same day. Work on a single tuber and cutting at a time, and label everything as you go. To help avoid confusion, I write the new tag before I pick up my scalpel. I leave the label on the cutting bench, name side up, where I can grab it, and stick it next to the cutting as soon as I've settled the soil around the stem.

Once all your cuttings have been taken for the day, place the humidity dome on the tray and return it to its table or shelf under the lights. 

If using a heat mat, ensure the temperature probe has been correctly placed. Since I use 72-cell inserts, I purposely leave one cell unplanted, but filled with potting soil. I put the probe into this cell and it monitors the temperature of all the cells in the tray.

Remember that a consistent 14 hours of intense light daily is essential. Keep the lights positioned just above the clear plastic tray covers. The setup I use has two 4-tube LED shop lights, each 4 feet long, placed over a 24x48 inch (61 x 122 cm) shelf. One shelf holds four 1020 seed trays holding 71 cuttings each (remember, I reserve one of the 72 cells for the temperature probe). This setup provides room for 284 cuttings on a single shelf.

Since the cuttings take about 14 days each to root, I can cycle 3 or 4 batches of cuttings across a single shelf each season by staggering my cuttings. That's around 800-1,000 plants!

Once the cuttings have been placed in their trays beneath humidity domes, the growing medium must always be kept moist. This, besides warmth, encourages the cuttings to form roots. Because the cuttings don’t have roots, their primary means of taking in water is through their stems and leaves. I recommend keeping a spray bottle or pressurized mister nearby and thoroughly spritzing all the cuttings from above at least once per day (twice is better). You will see condensation clinging to the inside of the humidity domes—this proves the system is working!

After 2 weeks, you can check the most robust-looking cutting to see whether it has developed roots. If you use rooting cubes, gently lift the cube from the tray. You should be able to see a small network of thin white roots along the cube's edges. If you don't, return the cutting to the tray and check it every few days.

If you're using tray inserts with potting soil, take the blade of a butter knife and gently lift one of the soil blocks. Again, you should see a miniature network of roots, which signifies that the cutting is ready to be potted up.

Note: If you have used 4-inch pots to root your cuttings, it is unnecessary to transplant them yet. Dahlias will live happily in this size pot for a long time.

However, many growers don't use 4-inch pots to start their cuttings because they take up a lot more room than, say, a 72-cell tray insert. With limited space under the lights and on top of the heat mats, we couldn't grow as many cuttings as possible using the method I'm describing.

When your cuttings are ready to be potted up, you can choose from several methods.

I favor 3.5" square plastic pots, which come in sheets of 18 pots (3 rows of 6) and fit exactly into a standard 1020 seed tray. Place the pots in the tray and fill them halfway with high-quality potting soil. Nearby, I keep a tray with extra dirt I can easily reach. I also place an empty seed tray on the bench where the newly potted cuttings will be placed.

Take one half-filled pot on the table and extract a rooted cutting from its insert. Use a butter knife or similar tool to ease it out without the soil crumbling away from the roots. Work slowly and handle the cuttings gently while you get the hang of it.

Place the cutting with its small cube of soil in the center of the 3.5" pot so it rests on the soil you placed inside. Support it with one hand and use the other to sprinkle more soil around the miniature root ball. Use your fingers to press the soil lightly around the roots. As before, the soil will compress to create more room in the pot. Repeat this process once or twice until the soil is at or just below the pot's rim.

It's okay if the cutting is slightly deeper in the pot than before. Some growers strip off the lowest pair of leaves and position the cutting so those leaf nodes are also buried. This can help the growing dahlia establish a more robust root system and may encourage more active tuber growth later in the season.

Note that in these photos, I am moving a dahlia that was rooted in a 3.5” pot into a 5.5” pot. Although not everyone does it, I believe strongly in stripping lower leaves and planting dahlias more deeply every time they are repotted into a larger container, then again when they are moved to their final home in the garden.

Move the plant tag from the cutting into the new pot!

Repeat these steps until you’ve potted all your rooted cuttings. You will be pleased to see how quickly an 18-pot tray fills up with small, healthy-looking baby dahlias. When the tray is complete, or when you're done transplanting for that day, water all the newly potted dahlias liberally. The soil should be damp but not nearly as wet as you kept it the first two weeks.

Return the transplanted dahlias to their shelf or table with the lights a couple of inches above the plants. You may keep the humidity domes in place for another week or two but leave them slightly ajar to allow some air circulation. Your new dahlias have a developing root system and can get moisture from the soil, not just the air. If the foliage is kept too wet, mildew can quickly become a problem.

As the dahlia grows, monitor the soil and water lightly as needed. Like any growing plant, they need adequate water but won’t do well in soaking wet soil. 

With 14 hours of daily light, you will see they grow quickly and soon resemble the magnificent flowering plants they will become. 

But don't rush right out and plant them yet! They have been raised under pampered, protected conditions and need to be introduced to their new home gradually. This process is called hardening off and is simple but essential.

When the daytime air temperature is above 50F (10C), carry the trays containing your potted cuttings outside and find a place in the SHADE to protect them from wind and the possibility of some marauding critters snacking on them.

Do not put them in the sun—like a pale tourist on a tropical beach, they will fry!

The idea is to introduce your plants to the great outdoors. Insects, breezes, raindrops—all these natural wonders will be brand-new experiences for your new dahlias. Every day, they’re becoming outdoor plants, and the more gradually you can expose them to direct sunlight and weather—the better.

If you're an experienced gardener, particularly if you have started plants indoors from seed and later planted them outdoors, you know exactly how to do this.

The trick is to give your dahlias a little more sun each day. Unlike dahlias grown from tubers in the garden, a dahlia from a cutting must be introduced to outdoor living a little at a time. After that, it will be just as robust as its tuber-grown cousin.

Propagating dahlias from rooted cuttings is a unique way to experience the science behind the plants we love to grow. We will never have a 100% success rate, but practicing these techniques will open new pathways to multiply your dahlias.

Have fun, and happy growing!