Black Dahlias: Plant Care and Growing Guide

While all types of dahlias are popular garden flowers, black dahlias are something extra special. With dark maroon blooms that almost appear black, these flowers add a bit of mystery and intrigue to the garden. Like all dahlia plants, black dahlias grow from underground tubers. Although these plants require a bit of patience and care, their gorgeous dark flowers make them worth the work. This guide will take you through everything you need to know about how to grow black dahlias at home.

How to Grow Black Dahlias

Growing black dahlias is similar to growing any other type of dahlia. By following these steps, you’ll be on your way to alluring, dark blooms.

When to Plant

Although black dahlias are technically perennials, they’re treated as annuals in much of the United States. That’s because neither the plants nor the tubers can tolerate extended periods of below-freezing temperatures.

If you live in an area where winter temperatures dip below freezing, you should plant your dahlia tubers each spring then dig them up in the fall. If you’re planting your tubers directly in the ground, wait until the danger of frost has passed. You should also wait until the soil warms to 60ºF.

Another option is to pre-sprout the tubers indoors four to six weeks before you plan to plant the tubers outdoors. This will allow you to get a jump-start on the season, which means your dahlias will bloom sooner.

To start the tubers indoors, place them in a shallow crate filled with potting soil. The dahlia potting soil should be just a little moist. Only about a quarter to a half of the tubers need to be under the soil.

Next, place the crates somewhere warm. The tubers should begin to sprout in a few weeks. Once outdoor conditions are favorable, you can plant the tubers outdoors.

How to Plant

Once the soil has warmed, it’s time to plant your dahlia tubers.

Dig a hole that is about four to six inches deep, depending on the size of the tuber. When you place your tubers in the soil, it’s important to follow the proper orientation. 

If you pre-sprouted the tubers, the shoots should be facing up. If you did not pre-sprout, look for a round eye and place it upwards.

Next, cover the tubers with a few inches of soil.

If you’re planting compact black dahlias in a container, you’ll follow similar steps to those described above.

Soil

Black dahlias prefer rich, well-drained soil. You can amend your soil with compost to increase drainage and add nutrients if you’re dealing with heavy clay.

To amend your soil, use a digging fork or shovel to loosen about a cubic foot of soil. Add a few handfuls of compost to the soil and mix to incorporate. At this point, your soil is ready for dahlia tubers.

As far as pH goes, black dahlias prefer a neutral to slightly acidic pH. Aim for a soil pH between 6.0 and 7.0.

To lower the soil pH, you can add elemental sulfur. And if you need to raise the pH, you can add garden lime.

Light

All types of dahlias, including black dahlias, prefer full sun. Therefore, you should plant them in an area that receives at least eight hours of direct sun each day.

Although black dahlias can handle a bit of shade, they will produce fewer flowers in shady conditions.

Watering

You should not water your dahlia tubers until after they sprout. If the soil is wet before the tubers are actively growing, the tubers may rot. This is especially true when the soil is still cool.

After your tubers have sprouted, you should water dahlia flowers about once a week. The goal is to prevent the soil from drying out while avoiding saturated soils.

It’s better to water deeply and infrequently than to water shallowly and frequently.

Fertilizing

Although dahlias may grow without any additional nutrients, they will benefit from regular fertilizer applications. However, choosing the correct type of fertilizer is essential if you want loads of healthy flowers.

To encourage flowering, choose a fertilizer with a higher amount of potassium and phosphorus than nitrogen. This means the first number in the NPK ratio should be lower than the last two numbers.

Some products to try include Espoma Organics Flower Tone and Happy Frog Fruit and Flower Fertilizer (via Amazon)

Fertilize your black dahlias about a month after you plant them outdoors. When you fertilize, follow product instructions to apply the proper amount of nutrients. You should continue to fertilize your plants once a month throughout the summer.

Staking

Since healthy black dahlia plants can grow over five feet tall, they may benefit from staking. This will help the plants remain upright and prevent stems from breaking.

To avoid damaging dahlia tubers, it’s best to insert your stake just after you plant your tubers. However, you can still opt to stake your plants once they are a few feet tall. Just do your best to avoid spearing the tubers with your stake.

When it comes to staking, you have two main options: singular stakes and cages.

If you wish to use a single stake, you can choose a material like a bamboo stalk or metal t-post. After inserting the stake next to your plant, use pieces of string to contain the branches.

An alternative method is to contain your dahlias using a tomato cage. Simply place a cage around each dahlia plant at the time of planting.

Pinching

If you’re aiming to produce the most flowers possible, you may want to pinch your black dahlia plants. While this will delay initial flowering, it will increase the total number of flowers each plant produces.

Pinching – also known as topping – dahlias involves removing the terminal bud when the plant is about a foot tall. At this point, the plant should have at least four sets of leaves.

Use your hands or a pair of scissors to remove the top bud. You should only remove the bud and not any of the leaves.

By topping your black dahlia, you will encourage your plant to send its energy into side shoots, which will create a bushier plant. In time, this will lead to a greater number of flowers.

Pests and Diseases

Black dahlias don’t experience too many serious issues with pests or diseases. However, it would help to keep an eye out for the following.

• Slugs and snails can quickly consume dahlia foliage, especially on young plants. Handpick the pests and/or sprinkle diatomaceous earth around the base of your plant.
• Earwigs can chew on flower buds, leading to tattered petals. You can trap earwigs by filling shallow containers with fish oil or vegetable oil with a drop of animal fat.
• Powdery mildew is a type of fungus that coats dahlia plants in white powder. It thrives in humid environments with low airflow. Ensure adequate space between plants to prevent this disease.

Black Dahlia Growth Expectations and Flowering Season

Since the term black dahlia refers to numerous types of dahlias, these plants have numerous growth forms and heights. Some black dahlias are compact varieties that only grow a few feet tall. Other types of black dahlias can grow over five feet tall.

You’ll want to give your black dahlia plants ample room to grow to be safe. When planting, place tubers four to six feet apart. However, if you know your black dahlias will only grow a couple of feet wide, you can plant the tubers closer together.

As long as you plant your tubers when the soil is above 60º, the plants should begin to produce shoots within a few weeks. While the time to flower will depend on the exact variety, you can expect your dahlias to produce flowers a few months after you plant the tubers.

If you provide your plants with the proper care, they will keep blooming until the first fall frost arrives.

Are Black Dahlias Edible?

Both the tubers and flowers of black dahlias are edible, but you should exercise caution before consuming them.

Since these plants are thought of as ornamentals rather than edibles, the tubers are sometimes treated with harmful chemicals. Therefore, you should always be sure about the source of your tubers if you wish to consume them.

While black dahlias are edible to humans, they are toxic to dogs and cats. Therefore, you should prevent your pets from eating any part of the dahlia plant.

Popular Varieties of Black Dahlias to Consider

With thousands of varieties of dahlias available, it can be challenging to figure out which ones you want to grow. If you’re looking for black dahlias, here are some popular types to try.

• Arabian Night
• Black Jack
• Black Touch
• Black Wizard
• Bridgetta Alidia
• Fidalgo Blacky
• Karma Choc
• Kenora Macob
• Mingus Alex
• Natal
• Hollyhill Black Beauty 
• Nut d’Ete
• Sam Hopkins

Wrapping Up

If you’re looking to step away from bright yellow and pink flowers, black dahlias may be just the plant you’re looking for. Remember to plant the tubers in early spring, provide the plants with full sun, and fertilize regularly.

For more, see our in-depth guide to cutting dahlia flowers for a vase or bouquet arrangement.