Growing Dahlias

by Deanna Williams

Dahlias are one of my favorite flowers to grow in the field and they’re gaining popularity fast in home gardens!

One of my favorite things about dahlias are their genetics. They come in so many different shapes, sizes, and colors you could fill your flower garden with nothing but dahlias and never get bored with them. The decorative and ball dahlias remind a lot of us of the illustration from Dr Seuss books that we read growing up. Plants can grow up to six feet tall and tower over you, bringing you right back to your childhood.

Some of the other favorites I see are dinner plate varieties. Blooms can span just over 12 inches with a show stopping number of petals. They’re unique and hardy so it’s no surprise how loved they are.

If you’re new to growing dahlias, you’ll be pleasantly surprised that they’re pretty easy to grow! They’ll take a little bit of care in the beginning to make sure they have what they need to get going – but they hold up really well here in the Inland Northwest!

First things first. You need to make sure that you wait to plant your dahlias until all signs of frost are gone. You can check your zone for your first and last frost dates with the USDA. For North Star, we usually have to wait until early or mid-May if we’re planting them straight out to the field.

Secondly, dahlias need full sun. This means that to grow and produce flowers, they’ll need a minimum of 6 hours of sunlight every day. So make sure you find a nice, sunny spot top plant them.

They also favor heavily amended soil. If you try to pop them into existing flower beds that haven’t had much maintenance over the years, odds are your dahlias won’t grow as well as you hope.

For home gardens, I recommend digging and loosening up the soil where you plan to plant them. It doesn’t have to be anything crazy, but loose enough to be able to get your amendments mixed in. Add 2-3 inches of compost or well-rotted manure, a light dusting of bonemeal, and some well-balanced fertilizer (I really like Down to Earth’s flower mix which has an N-P-K of 4-8-4). Mix everything in to your soil and then you’re ready to plant!

For farmers, I highly recommend having your soil tested and making any amendments needed based on your results. If you’re working with a large growing space that will be managed for production, keeping an eye on your soil is a must. The top things to look at are your N-P-K (Nitrogen – Phosphorus – Potassium) levels, organic matter, and soil health. My favorite lab to use is Ward Labs.

Everything starts with healthy soil. It will make or break your farm or garden. Now let’s get started with the actual growing! If you’re just getting started in your journey and have questions about getting to know your soil, you can read about Soil Health Basics here.

What are Tubers?

Tubers are underground stems that carry nutrients and DNA for a plant. In dahlias, they look like odd-shaped potatoes (but taste more like a water chestnut) and are planted every spring and dug up each fall in our Zone 5.

For a dahlia tuber to successfully grow, you’ll want to look for the following:

Anatomically, you need the tuber itself which is the section that will have the most meat and seem the most like a potato. And you need the crown and the neck. The crown is going to be the section that was cut from the tuber club during division in the fall and the neck is what connects the crown to the tuber body.

On the crown, you will also want to make sure you have eyes. This is where your dahlia plant will sprout from and without eyes, there won’t be successful plant grown from the tuber.

When growing dahlias, planting tubers is the only way to guarantee the variety you are growing. Grown from seed, dahlias will carry DNA from separate parent flowers, but from tubers, it is identical DNA from the flower the tuber was harvested from.

How to Plant Tubers

Planting tubers is super straightforward. Growing dahlias can sometimes feel really intimidating at first, so it might feel a little underwhelming when it’s time to plant them.

Simply find a good growing space that gets plenty of sun, make any amendments you need to your soil, and dig 4 to 6 inches holes spaced about a foot apart.

Place a tuber in each hole horizontally so that the eye is facing up, refill the hole with soil. And you’re good to go!

If you plan to dig up your tubers to plant next year, be sure to add a label so you can keep track of which plant is which. Come fall, it can be hard to tell once all of the blooms are gone.

Growing + Caring For dahlias

Care is also really simple! Dahlias do need to be watered throughout the growing season. Water them enough so that the soil stays moist without drying out. That can be a little bit every day from your lawn or other garden sprinklers, or it could be a couple good soakings two or three times a week.

Be sure not to water your dahlias until after you see them sprout up from the ground – at least a small glimpse of the green shoot. Watering too early could cause the tubers to rot.

This is where it will take a little bit of intuition…you don’t want your soil to become dry and dusty so you might have to give it a little bit of water – but in the Inland Northwest we are lucky enough o get rain in the spring that will keep everything right where it needs to be.

If you want to improve the number of blooms you get each season, remember to pinch your dahlias! Once the plants are about 8 to 12 inches tall, you’re going to want to use your snips or fingers and pinch the tope third of the plant off right about a set of leaves. It will feel like a lot and your heart might tell you not to do it, but trust me, you’ll be happy you did!

The most important thing to remember when pinching, is to always leave at least two sets of leaves (four total, minimum) on each plant. Not long after pinching, you’ll notice small stems starting to branch out. That’s a good sign that you’ll be getting twice as many blooms as you would have without pinching!

Stability + Support

Stability will be another thing to think about. Dahlia plants get tall. Think four to six feet. So they’re going to need a little bit of support to stay upright.

For home gardens, I recommend putting in a sturdy post or stake when you plant your tuber and then as it grows bigger you can tie the main stem to the stake to keep it standing straight. Adding the stake at the time of planting will let you keep it close to the plant without having to worry about accidentally puncturing your tuber or roots if you decided to add it later.

For flower farmers, you’ll have a much larger number of dahlias. Like the picture above you see that I have two rows side-by-side. Here, I use t-posts or fence posts and corral the plants with baling twine to hold them in. I usually have posts spaced 8 to 10 feet apart and have two different lines of twine run. I usually aim to add these right after I plant so that I’m not having to worry about fussing with it during the middle of the growing season.

Harvesting

Dahlias are one of the few flowers that are harvested once they are almost fully open. Once you cut your stems, they’re not likely to open up much more, but you’ll still get around 5 to 7 days of vase life out of them when they’re fresh from your garden or field!

These flowers are considered a cut-and-come-again variety. This means that every time you cut a stem, you get more in its place. So in a sense, harvesting is very similar to pinching. The more dahlias you harvest throughout the year, the more flowers you’ll continue to get.

To encourage longer stems, you’ll want to take a lot of stem during your first few harvests – it will feel like more than you want and you’ll sacrifice some buds, but you’ll be rewarded to longer, healthier stems in the future. The first trues harvests, I usually take two to three sets of leaves with the bloom and in that one stem, I’ll take with it two buds. Sometimes I leave them to add to designs, but keep in mind that it’s one more part of the plant that will take up nutrients and water. For the longest vase life in the intended bloom to harvest, take away all other buds and foliage.

Quick Hits

Dahlias contain many “jumping genes” (transposons) that move from place to place in the DNA sequence – this is part of why there is so much diversity in varieties and variety possibilities.
Originally, dahlias were classified as a vegetable because the tubers are edible. I’ve tried them once and they are like a cross between a potato and a reddish!
The discovery of the dahlia was first recorded by the Spanish in 1525. They somehow disappeared from record until the late 1700’s, and we didn’t see the first double-petal variety until 1815. Prior to that, they were all single-petal with yellow centers.
Some cousins to the dahlia are sunflowers, asters, daisies and chrysanthemums. They’re all part of the Asteraceae family.
Dahlias are not fragrant. When harvested, you might get a little whiff of something that smells like celery, but other than that they have no smell! So, to attract pollinators, they rely on their showy blooms.