How to Grow Dahlias

Dahlias may look impressive, but they are surprisingly easy to grow. Give them sunshine, well-drained soil, consistent moisture once they begin growing, and sturdy support and they will reward you with armfuls of flowers from midsummer until frost.

This guide is intended for growing dahlias from tubers.

Before Planting Your Dahlia Tuber

Dahlia tubers naturally vary in size, shape, and appearance. A small or unusually shaped tuber can produce a plant just as beautiful as a large one. 

The most important part of the tuber is the crown, where the tuber was attached to the original plant. The eye, or future sprout, develops from the crown, not from the body of the tuber. Every viable tuber must have a portion of the crown with at least one eye.

Your tuber may already have a visible eye or sprout when it arrives. Others remain dormant until they are exposed to warmth. Both are normal.

Keep your tubers in a cool, dark, frost-free place until you are ready to plant them. Do not allow them to freeze, become wet, or sit in a sealed container where condensation may develop.

When to Plant Dahlias

Wait to plant until:

• The danger of frost has passed.
• The soil has begun to warm.
• The ground is not cold, soggy, or waterlogged.

A simple guideline is to plant dahlias around the same time you would plant tomatoes outdoors. Here in central Pennsylvania, that is generally around the middle of May, although spring weather varies from year to year.

Dahlias planted into cold, wet soil are more likely to rot. It is better to wait an extra week than to plant too early.

Choose the Right Location

Dahlias grow best in a location with:

• At least six hours of sunlight each day.
• Loose, fertile, well-drained soil.
• Good airflow around the plants.
• Protection from strong winds when possible.

Morning and early-afternoon sun are ideal. Dahlias can tolerate some afternoon shade, particularly during very hot summers.

Avoid locations where water collects after heavy rain. If you have heavy clay soil, improve the drainage with compost and organic matter or consider growing your dahlias in raised beds or 7-gallon grow bags. 

How to Plant a Dahlia Tuber

1. Install the support first

Most dahlias grow into tall, heavily branched plants and need support to prevent their stems from bending or breaking. Install your support system before or at the time of planting, since adding stakes later can accidentally puncture the tuber or damage its roots.

At Thief of Weeds, I use T-posts and the corral method, wrapping rows of sturdy twine around the posts as the plants grow. This creates a supportive framework that keeps the dahlias upright while still allowing them room to branch

2. Dig the planting hole

Dig a hole approximately 4 to 6 inches deep.

3. Position the tuber

Lay the tuber horizontally on its side with the eye or sprout facing upward. Position the crown close to the support stake so the growing stem will be easy to secure later.

Do not worry about making the tuber perfectly level. The new growth will naturally find its way toward the surface.

4. Cover the tuber

Gently cover the tuber with soil. Do not press or compact the soil heavily.

5. Allow room to grow

Space most full-size dahlias approximately 18 to 24 inches apart. Generous spacing improves airflow and gives the mature plants room to branch. Very large varieties may need additional space.

6. Label your plant

Place a weather-resistant variety label near each plant. Once dahlias begin growing, many varieties look nearly identical until they bloom, and labels can sometimes be moved, damaged, or lost during the season.

To help prevent mix-ups, I use several methods to track my dahlias. I write the variety name directly on each tuber with a permanent marker, label every grow bag, and record each plant’s location in a spreadsheet. Keeping multiple records makes it much easier to maintain accurate variety names throughout the growing season and when the tubers are dug and divided in the fall.

Watering Newly Planted Tubers

One of the most common mistakes new dahlia growers make is watering too much before the plant emerges.

After planting, water the tuber in once to help settle the soil around it. After that initial watering, allow the tuber to begin growing before watering again. Dahlia tubers contain the moisture and stored energy they need to sprout, and cold, constantly wet soil can cause them to rot.

Wait until you see growth emerging from the soil or the first leaves beginning to develop. Once the plant is actively growing, begin watering more consistently, adjusting for rainfall, temperature, and how quickly your soil drains.

Do not become concerned if your dahlia does not emerge right away. Depending on the variety, soil temperature, planting depth, and weather, a tuber may take several weeks to sprout.

Watering Established Dahlias

Once dahlias are actively growing, they need consistent moisture.

Water deeply at the base of the plant rather than giving the surface a light sprinkling. As a general guideline, established dahlias need approximately one inch of water each week from rain or irrigation. They may need more during extreme heat, dry weather, or when grown in containers.

Allow the soil to drain between waterings. It should remain evenly moist but never constantly soggy.

Watering at soil level also helps keep the foliage dry. If overhead watering is necessary, water early in the day so the leaves have time to dry before evening.

Supporting Your Dahlias

Dahlia stems can break under the weight of their blooms, especially during heavy rain or wind.

Secure plants to their stakes as they grow, adding ties approximately every 12 to 18 inches. Use soft ties that will not cut into the stems. Tomato cages, horizontal flower netting, or a corral-style support system can also work well.

Check the plants regularly. It is much easier to support a branch before it bends or breaks.

Pinching for More Flowers

Pinching encourages a young dahlia to branch, creating a fuller plant with more flowering stems.

When the plant is approximately 8-12 inches tall and has three to four sets of leaves:

1. Locate the main growing tip at the top of the plant.
2. Use clean snips or your fingers to remove the center tip.
3. Make the cut just above a full set of leaves.

I will admit that pinching was extremely difficult for me when I first began growing dahlias. After watching a healthy plant grow, cutting off the top felt completely wrong. Since I usually need to see the results for myself, during my first serious year of growing dahlias, I pinched some plants and left others unpinched.

The difference convinced me. The pinched plants developed more branches, produced more usable flower stems, and had a fuller, more manageable shape. The unpinched plants tended to grow one thick central stem with fewer side branches.

Pinching is optional, and an unpinched dahlia will still grow and bloom. However, if your goal is to produce more flowers and longer stems for cutting, I definitely recommend pinching.

Feeding Your Dahlias

I cannot emphasize this enough: healthy soil is the best place to begin! Fertilizer can supplement your soil, but it cannot replace good soil structure, drainage, and balanced nutrients.

A soil test will tell you whether your garden needs additional nutrients and help prevent unnecessary fertilizer use. Always refer to your soil test results before adding fertilizer, as the recommendations will tell you exactly what your soil needs.

Adding finished compost before planting can improve soil structure and provide valuable organic matter. When I plant my dahlia tubers, I also apply a timed-release fertilizer to provide steady nutrition as the plants become established. Always follow the application rate listed on the fertilizer label. Using too much fertilizer can damage the tubers or encourage excessive leafy growth.

Once the plants are approximately 12 inches tall, I begin spraying the foliage with diluted fish fertilizer approximately every two weeks. I apply it according to the product directions and spray early in the morning. Never apply foliar fertilizer during extreme heat or intense, direct sunlight.

Avoid fertilizers that are excessively high in nitrogen. Too much nitrogen can produce large, leafy plants with fewer flowers.

Improving Airflow

As your dahlias grow, remove leaves from the bottom 8 to 10 inches of the plant. This opens the base of the plant, improves airflow, and makes it easier to water at soil level.

Also remove damaged, heavily spotted, or yellowing leaves throughout the season. Place questionable foliage in the trash rather than leaving it beneath the plant.

Use clean tools and sanitize them when moving between plants, particularly when cutting stems or removing suspicious growth.

Disbudding for Larger Flowers

Dahlia stems often form a cluster of three flower buds: one central bud with two smaller side buds.

For a larger flower on a longer, stronger stem, gently remove the two side buds and leave the center bud. The plant will direct more energy into the remaining bloom.

Disbudding is completely optional. There is no need to do it when you prefer a natural-looking garden plant with as many flowers as possible. It is most useful when growing dahlias for cutting, arranging, or larger individual blooms.

Deadheading and Cutting Flowers

The more dahlias you cut, the more flowers the plant will continue to produce.

Harvest flowers during the cooler part of the day, preferably in the morning or evening. Use clean, sharp snips and place the stems directly into clean water.

When cutting, follow the stem down into the plant and make a longer cut rather than simply removing the flower head. This encourages the plant to produce useful new stems from lower leaf nodes.

Remove faded flowers regularly if you are not cutting them for bouquets. Deadheading prevents the plant from directing energy into seed production and keeps it producing new buds.

Common Dahlia Pests

Young dahlia shoots are especially attractive to slugs and snails. Begin watching for damage as soon as the plants emerge.

Other possible pests include:

• Japanese beetles
• Earwigs
• Aphids
• Thrips
• Spider mites

Inspect the undersides of leaves and developing flower buds regularly. Catching a problem early is much easier than treating a large infestation.

Japanese beetles can be knocked into a cup or container of soapy water. During beetle season, I walk through my garden several times a day and remove them by hand. Catching them early and checking frequently helps prevent them from causing extensive damage to the flowers and foliage.

I then feed the collected beetles to my chickens as a snack. However, never feed insects to chickens if they may have been exposed to an insecticide or other pest-control product.

Remove heavily damaged buds and flowers so the plant can direct its energy toward healthy new growth. When using any pest-control product, always follow the label directions and make sure it is approved for the intended use.

Growing Dahlias in Containers

Dahlias can grow successfully in containers, but they need plenty of room and excellent drainage. I grow my dahlias in 7-gallon grow bags, which provide enough space for the roots and tubers to develop while allowing excess water to drain easily.

Full-size varieties need larger containers and strong support, while compact or border dahlias are easier to manage in smaller spaces.

Container-grown dahlias usually require:

• More frequent watering, especially during hot or windy weather.
• Regular feeding because nutrients wash out of containers more quickly.
• Strong support to prevent tall plants and heavy blooms from bending or breaking.

Grow bags can dry out much faster than dahlias planted in the ground, so check the soil regularly. Water deeply when needed, but never allow a container to sit in standing water or remain constantly soggy.

What to Do After Frost

Dahlias are tender perennials. Their foliage will be killed by frost, and their tubers will not reliably survive a freezing Pennsylvania winter in the ground.

You may treat dahlias as annuals and replace them the following spring, or dig and store the tubers over winter.

To save your tubers:

1. Clearly label each plant before digging.
2. After frost damages the foliage, cut the plant back, leaving several inches of stem.
3. Begin digging approximately 10 to 12 inches away from the stalk to avoid damaging the tuber clump.
4. Loosen the soil on every side before carefully lifting the entire clump.
5. Handle the tubers gently. The narrow necks are especially fragile immediately after digging.
6. Allow the clump to dry in a protected, well-ventilated location. I let mine dry for approximately 24 hours. Avoid leaving them exposed for an extended period in warm or very dry conditions, as the tubers may begin to dehydrate.
7. Store the tubers in a cool, dark location that remains safely above freezing.

Growers use several different storage methods. Tubers may be packed in vermiculite, wood shavings, peat moss, or coco coir, while some growers wrap individual tubers in plastic.

A storage temperature of approximately 40 to 50°F works well. I store mine in the garage, but only because it remains above freezing. Check the tubers periodically throughout the winter. Remove any that develop rot, and watch for excessive shriveling, which may indicate that the storage conditions are too dry.

A Few Final Reminders

• Be patient. Some varieties sprout much faster than others.
• Do not overwater a dormant tuber.
• Install support before the plant becomes tall.
• Pinch young plants for additional branching.
• Keep cutting or deadheading to encourage more flowers.
• Most importantly, enjoy them.

Dahlias do not need to be perfect to be beautiful. Every variety grows a little differently, and every season teaches us something new. With a little care, your plants will provide beautifully untamed flowers from summer until the first frost.