How to Grow Dahlias

How to Grow Dahlias

Dahlias are a bushy, herbaceous plant native to Mexico. Dahlias produce tubers (part of the root system) which were originally grown by the Aztecs as a food source similar to modern day tapioca or cassava.

To get a jump start to the flowering season, you can plant the tubers in pots indoors 4-6 weeks before transplanting into the garden. Once the shoots appear, please provide ample light with indoor grow lights or south-facing window.

When the plants are 30 cm (12″) tall, pinch off the top of the main stem.  This will stimulate branching producing more stems and flowers.

To prevent disease, keep the foliage of dahlias as dry as possible. Water deeply once or twice a week, allowing the 2.5 cm (1″) top of soil to dry out in between waterings.

To grow dahlias, here are the basics to keep in mind:

• Dahlia tubers need to be stored in a cool, but not freezing and high humidity environment through the winter months.
• The tubers should be planted in full sun and well-drained soil with lots of organic matter after the risk of frost is over.
• Make sure you do not over water dahlias after they are planted. They need very little water until the plants are at least 6”-12” tall.
• As dahlias grow it is best to stake them so they to not fall over in the wind.
• After 12-16 weeks they will start to produce flowers and will continue to do so all the way until your first frost.
• The best way to extend the vase life of dahlia flowers is to make sure you keep them in clean water.
• In the fall, after the plants die back, you can dig up the dahlia’s tubers to store them for another year.

Dahlia Tubers

• Some might have long and skinny tubers, where others are short and fat. Some might produce lots of tubers per clump, and others might only be practical to propagate from cuttings because they produce few, if any viable tubers. Some store well, while others are notoriously difficult to store without them rotting or dehydrating. In short, when working with mother nature, you cannot look at tubers as an ‘off the shelf’ widget. Each one will be unique and there is no ‘standard’ shape or form for dahlia tubers across different varieties.

Size doesn’t Matter (mostly)

• One common concern people have when they receive a dahlia tuber is, ‘This is so small. Will this produce a quality dahlia plant?”
• When it comes to tubers, size doesn’t matter – to a point. A dahlia tuber is a tuberous root of a dahlia plant. It is a starchy body that contains food, water, and nutrition for a dahlia plant to grow until it establishes a root system that will then provide food for the plant.
• As long a dahlia tuber is large enough to accomplish what I described above, it is big enough. So how big is that? I want a tuber to be at least the size of my little finger (about the size of a AAA battery). Tubers smaller than that will produce a plant, but it might not produce as many flowers. It should create quality tubers that can be divided for the following year. When I am shipping out tubers, regardless of the variety, I try to make sure the smallest tubers anyone receives are the size of a AA battery. Keep in mind that a tuber that is skinnier than I described is still OK if it is longer. The battery measure is more about total volume of the tuber than the thickness or length.

I also want to add that there is such a thing as “too big”. If a dahlia tuber is too large, it could backfire on its ability to produce more quality tubers or develop a large root system. You can think of it as the dahlia sitting on a reservoir of so much food and water that it doesn’t need to seek out more. The top of the plant might have been productive but when you dig these in the fall you can find a single mother tuber without any new tubers. How big is too big? In general, I don’t like it if they are too much bigger than your average size coffee mug.

I should also add here that some dahlias take longer to eye up than others. For example, the variety Nicholas commonly takes six or seven weeks to eye up where something like Linda’s Baby can eye up in a week or two.

When handling a tuber with an actively growing eye, sometimes the eye will break off. This is not something to worry about. Dahlia tubers will send up more eyes from where the original broke off.

Planting Dahlias

Dahlia tubers should be planted in the spring after the risk of frost. The earliest varieties of dahlias take about two weeks before you will see a sprout. The latest can take up to 8 weeks to emerge after planting. Dahlias also prefer to be grown in full sun, although you can successfully grow them in areas that receive sun 75% of the day. They can be grown in pots but make sure you are using a container large enough for the plant. Most varieties should be grown in a 5-to-7-gallon pot.

It is best if soil temperatures are above 55˚ F when planting. In addition, your soil should be moist, but not wet. Just like when storing dahlias, if there is too much moisture around a dahlia tuber, they are likely to rot. In most cases, you don’t need to worry about watering dahlias until they are at least 6”-12” tall and over watering at this stage can inhibit strong root growth.

When planting dahlias, it is important to allow them enough space to grow. Some varieties can become quite large and planting too close can cause plants to not receive enough light or air flow which can lead to mildew issues later.

We plant our dahlias at a depth of 4″ in the ground. If you are planting an especially large tuber, you can go a little deeper. Dahlias should be planted on their side with the eye facing up. If there is no eye visible at the time of planting, you can just place the tuber in the ground lying flat and the eye will find its way to the surface.

When planting in a single file row, you can plant as close 8”. When planting in a double row, it is best to stagger the planting and we recommend planting at 10” spacing. When doing a triple row, it is again best to stagger the planting and plant at 10” spacing. For walking paths around your flowers, it is good to leave at least 45” between rows of plants. When the plants are full size, you will have a hard time walking down your paths if they are planted any closer.

If you receive a tuber or pull one out of storage and it does not have an eye actively growing on it, don’t panic. Let’s first talk about the structure of a dahlia tuber. There are three important parts of a dahlia tuber. The tuber itself, the neck, and the crown. In the image below, I have color coded the different parts of a tuber: Green = Tuber, Purple = Neck, Pink = Crown. the tuber is what contains everything the dahlia needs to grow. The crown is the only area that eyes will grow from. The neck is the part of the tuber that connect the other two. If you are missing any of these parts on a tuber, it is not going to be viable.

When I am training others how to divide dahlia tubers it is normally shortly after we dig and there are rarely eyes on the clump. What I tell everyone is that if you can keep 1 cm² (about half the area of a dime), of crown attached to a tuber, there is about a 95% chance that there is enough crown for an eye to form. When it comes to mother nature, 95% is about as good as it gets. When planting my own dahlia tubers, I am more concerned with the amount of crown attached to the tuber than whether there is an eye or not.

No need to reinvent the wheel! Michael at SummerDreams Farm wrote a very detailed blog about storing, dividing etc with great pictures.

The above are excerpts from his excellent blog https://summerdreamsfarm.com/dahlia-tuber-and-splitting-guide

Check it out to read more.