When and How to Pot Up Your Dahlia Tubers

It's hard to contain our excitement for dahlia season! If you want to know when and how you can get an early start, read on. Gardening expert Melissa Strauss has the lowdown on jumpstarting your tubers.

Close-up of potted up dahlia tubers covered in fresh soil, set on a green lawn in the garden.

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It’s been several years since I began growing dahlias, and much like everyone else, my enchantment was instant. As soon as that first Cafe au Lait flower bloomed, I knew that my love for these plants would be intense and lifelong. 

I live and garden right on the cusp of Zones 8 and 9, where dahlias can stay in the ground all winter. Anywhere from here to Zone 11, the same is true, but north of Zone 8, you’ll have to dig them up in the fall and store them during the cold months. There’s also a question of how long your season lasts and how to get the earliest possible start. 

Naturally, an early start will mean better tuber development, as well as a longer flowering season. If you live in a cooler climate, you can get your tubers started in pots. This way you can transplant them as soon as the weather warms up, and you’ve got a head start on the competition. Let’s talk about when and how to start your tubers in containers

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Step 1: Start Early

Bright orange dahlias with layered, slightly curled petals bloom in a terracotta pot, surrounded by deep green, serrated leaves on sturdy stems.
Give plants six to eight weeks for healthy development.

Dahlias can tolerate some cold weather before they sprout, but the tender foliage will die in a late freeze. You don’t want to risk them sprouting during a warm spell and then dying in a frost. Starting them in pots eliminates this risk. When you get started will depend on your growing zone and your last expected frost date.

 A good rule for planting ahead is to give your tubers six to eight weeks to get a head start. This leaves you with enough time to get them growing and branching. You’ll have strong, sturdy stems by the time your last expected frost day rolls around. 

Identify your zone and last expected frost date, and then plan accordingly. Count backward eight weeks from that date. This is when you can begin. If that date has passed, it’s ok, go ahead and get going now. You’ll still get a head start.

Step 2: Select the Right Potting Mix

Close-up of a gardener's hands in white gloves mixing fresh potting soil in a large black plastic pot.
Mix compost for better soil structure and plant health.

There are three important factors in selecting a potting mix for your tubers. The first is acidity. Dahlias prefer a soil pH of between 6.2 and 7. That is just slightly acidic, so you want a mixture that contains some materials that will get to that level. Start with a good quality, standard potting mix, and work from there.

The second important factor is nutrients. These plants respond well to an initial supply of organic compost to kick things into gear. Adding organic compost to your mixture will not only enrich the soil with nutrients it will also lower the pH a bit and improve the soil structure. This can come in the form of leaf compost or some other material like worm castings

The last issue of importance is drainage. Most tubers and bulbs are sensitive to overwatering. It’s easy to end up with fungal rot, which will drastically decrease your success rate. Since compost can be dense and heavy, it’s a good idea to lighten up the soil with some larger particles to improve drainage. Perlite or vermiculite are good examples of materials that will improve drainage. 

Step 3: Select the Right Container

Numerous large black and brown plastic pots filled with rich soil hold freshly planted tubers, arranged neatly in a garden setting.
Bigger containers help tubers grow strong and healthy.

While you can grow dahlias from seeds, tubers offer some advantages. Growing from seeds will save money and typically result in more plants. However, if you want a specific variety and a clone of that one plant, tubers are a sure thing. Seeds can produce hybrid variations with characteristics of either parent.

Given that we are discussing tubers, container is a major factor. Tubers are larger, and they set down roots faster, so they need more space to grow initially. Choose your container based on the size of the tubers.  A good size is about 12″ deep and wide. If you intend to leave them in containers for the season, you may go even larger and opt for a wider pot. 

Also important is drainage. Because they can rot easily, your container should have adequate drainage holes. This way, any excess water passes through rather than pooling in the bottom. You can label your containers as well to help keep track of varieties. 

Step 4: Separate Tubers if Necessary

A female gardener in a bright yellow knitted cardigan uses blue pruning shears to divide tubers in the garden.
Make sure each section has at least one eye.

If you already separated them in the fall before storing them, you’re a step ahead. If you decide to wait for spring, do this before planting. It’s not a complicated process, but there are important factors in success. 

Make sure, first, that you use a clean, sharp tool to separate them. A knife is best, as you can make a cleaner cut than you would with hand shears. If you use it for gardening, wipe it with alcohol before you cut it to avoid contaminating your dahlias with bacteria or fungus. 

Identify the eyes on your tubers. These are growth points, and you can identify them as buds at the top of the cluster. Cut your bulbs apart, leaving at least one eye on each section. Leaving two eyes will usually result in two central stems. 

Step 5: Situate Your Tubers Correctly

A female hand plants tubers in a clay pot on a wooden table, surrounded by a shovel full of soil and several divided tubers.
Position roots with eyes facing upward for healthy growth.

Placing your roots in the right position is important, as they sprout from the eyes. Fill your container about halfway with soil; less for larger clusters. If you desire to, you can add in some starter fertilizer to increase the nutrients available to your young plants.

Set your tuber or cluster on top of the soil with the eyes or sprouts pointed upwards. Fill in around and on top with more potting mix. Don’t compact the soil. It’s good to leave it loose and fluffy. Lightly water them in, leaving the soil moist but not wet. They will initially only take in a small amount of moisture, but they’ll need more after they sprout.

Step 6: Choose the Right Spot

Several identical black pots with tubers planted in soil, topped with a layer of pebbles, arranged in a small greenhouse in a sunny garden.
Aim for temperatures between 60-70°F (16-21°C) for healthy growth.

Dahlias like warm weather. They do their best growing in soil between 60-70°F (16-21°C). If you want to get them off to the fastest start, place them in a warm, brightly lit space. Direct sun can be harsh for young plants, but they prefer plenty of light, so indirect light is not a necessity. 

If you have a greenhouse, that’s a great place to get these beauties growing. If not, a warm spot in the house next to a sunny window is great, too.

Step 7: Pinch the First Shoots

Tiny green shoots emerge from dark soil in a pot, surrounded by a few remaining pieces of dried tuber.
Pinch early shoots to promote branching and abundant flowers.

Like many plants, these benefit from pinching when they are young. Pinching is the practice of cutting young shoots just above a leaf node. It encourages branching, which leads to more flowers. Some varieties have a tendency to grow tall and narrow. Pinching these will create shorter, bushier plants with strong stems and lots of blooms. 

You can use your hands to do this on the first go-round. The stems will be tender and easy to pinch off. This first round should happen when there are at least two sets of leaves. Then, you can repeat this in about a month when the initial stem branches and there are two sets of leaves on each branch. As the stems get tougher, a pair of hand shears will give you a cleaner cut that will heal faster.

Step 8: Thin the Resulting Shoots

Small seedlings with bright green, serrated leaves sprout from dark soil in multiple pots with yellow sticks.
Thin out stems to prevent overcrowding and promote health.

Depending on how many stems you end up with, you may want to thin them out. You want the plant to be able to support all the branches, and you want them to be sturdy. Too many stems will make for a heavy plant that requires a lot of water and nutrients to support it. 

Remove the weaker stems, pinching them off at the juncture of the main stem. Leave the strongest and thickest stems. More stems will overcrowd things and also inhibit the sun from reaching the interior. It can also lead to powdery mildew, as crowded stems reduce air circulation. 

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Step 9: Transplant at the Right Time

A female gardener wearing yellow and orange gloves carefully transplants young green seedlings from pots into rich soil in a garden flower bed.
Choose a sunny, well-drained spot for healthy growth.

Transplanting time will again depend on your zone and your last frost date. Putting them in the ground before this time passes can end in disaster. A frost will kill off or severely damage foliage, especially young, tender stems. In cold climates, this can mean waiting until June to transplant, but usually, it’s more like April and May.

You want to wait until the soil is warmed to the 60°F (16°C) mark. You don’t want to deal with transplant shock, and at cooler temperatures, your dahlias won’t grow as quickly. You could see growth halt altogether 

Choose a spot with plenty of sun and good drainage. Dahlias perform well in raised beds, but any loose, well-draining, nutrient-rich soil will do. Dig your hole slightly larger than the container so you don’t have to disturb the roots. 

If the roots become crowded in the container, loosen them at the outside and spread them out loosely in the hole. Water the hole, and add soil to cover the roots and tubers. Water them in again and cover them with mulch to help retain moisture in the soil. While they don’t like to have wet feet, these plants have high water needs, especially when they grow large. 

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Several rounded seeds with papery brown outer layers are nestled in moist, dark soil, surrounded by vivid pink and green plants, with a small hand trowel resting nearby.

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