Avoid Five Common Dahlia Growing Mistakes

By Steve K. Lloyd

Copyright © 2024 All Rights Reserved

Dahlias have taken the flower world by storm in recent years. From their origins in the mountains of Central America, they were introduced to Europe in the 1700s. They later traveled to North America, where they were hybridized to create the amazing colors and shapes of the dahlias we see today.

Modern dahlia growers can choose from a vast array of flower colors and shapes. Knee-high bedding varieties or enormous dinner plate dahlias that can easily reach seven feet tall, and flower colors that include every shade except blue and black, dahlia growing offers the home gardener a chance to truly personalize their garden space.

Like other garden plants, dahlias need a few essentials: Enough light, nutritious soil, adequate water, and a bit of TLC. They are relatively easy to grow, but dahlias can challenge new growers who may need to learn some trickier aspects of raising robust, healthy, beautiful dahlias.

We’ve all made our share of dahlia-growing goofs! To help you get the most out of your gorgeous dahlias without learning these lessons the hard way, here are some mistakes I’ve seen.

Dahlias can be grown from seed, but because of their peculiar genetics, seed-grown dahlias won’t look exactly like the parent plants. Flower style, color, plant height, and other characteristics will differ considerably from the dahlia that produced the seed. As a result, the most common way to grow dahlias is to plant a tuber from a known variety. 

Tubers, a part of the dahlia's root system, will always produce a plant that is a genetic copy of the parent plant. This enlarged root contains starches and other chemicals that will nurture the new dahlia as it makes top growth which will become stems, leaves, and flowers. At the same time, it builds a new root system underground, which anchors the plant and allows it to draw water and nutrients from the garden soil.

Dahlia tubers should be planted horizontally, not upright, to permit all these essential functions to happen correctly. Each tuber needs at least one viable eye, the growth point from which the plant will make a new dahlia. Roots will grow from the other end of the tuber, and after four months or longer in the ground, the plant will create new tubers to nurture the next season of growth for its offspring.

If a grower plants the tuber too shallow, it's hard for the new plant to create the robust root system necessary to support itself. In addition, since new tubers are typically formed near the stem of the dahlia, shallow planting can position these tender new tubers too close to the surface, where they are susceptible to damage.

When planted too deeply, it can be difficult for the emerging shoot to find its way to the surface without a struggle. The newly formed roots may be too deep to take full advantage of the water and fertilizer they need for peak performance.

Delivering the correct amount of water to growing dahlias is critical to their care. Let's begin at the beginning—when you first plant your dahlia tubers.

Dahlia tubers are self-contained nutrition pods for the new plant. They contain starches, sugars, water, and all the genetic information that will determine the growing dahlia's size, foliage, flower shape, and color. All it takes is the correct soil temperature and a small amount of moisture to awaken the sleeping tuber, triggering the growth of a stalk and the formation of roots.

Before these roots are established, too much water can cause a dahlia tuber to rot. To avoid this unhappy result, don’t water newly planted dahlia tubers until you see a couple of inches (5 cm) of growth peeking above the soil. Then, moisten the surrounding soil slightly every couple of days with a sprinkle from a watering can or garden hose. Before surface growth appears, the dahlia tuber has not yet formed roots and, therefore, cannot take up water from the ground. 

Watering too much at first can doom your tuber before it even has a chance to sprout.

Once your new dahlias grow, they become thirsty plants, especially in dry climates. If you rely on a lawn sprinkler for irrigation, a deep watering of 45 to 60 minutes three or four times per week may be needed. If you use a soaker hose or drip irrigation system, each dahlia should receive two to four quarts/liters of water every few days.

Adjust these guidelines according to how quickly your soil drains and remember that hand watering alone is not enough to maximize dahlia growth and flowering.

Many dahlia varieties grow to about 30" (75 cm) or shorter and look great, spreading their colorful flowers in containers or along a garden path. However, the most popular dahlia varieties grow considerably taller and need to be supported to keep them standing tall. The time to think about a support system is when you first plant the tuber or seedling.

Many dahlia growers use wooden stakes about 4-5 feet (1.2-1.3 m) long near each dahlia. I prefer the #4 steel rods used for concrete reinforcing (rebar), which weathers to a pleasing rust color, nearly invisible in the garden. 

Whatever you choose, pound the stake in the ground about 6" (15 cm) away from the tuber. Installing the stake before you have scooped the planting hole before placing or covering the tuber is best. If you wait until the plant is tall enough to need support, you risk stabbing the tuber and ruining your dahlia just when it's ready to grow.

As each dahlia grows, use twine or foam-cushioned garden wire to create support points between the pole and the plant, about every 2 feet (60 cm). These anchors will help the dahlia resist wind and allow the stems to stand tall as the beautiful flowers mature.

Like all plants, dahlias rely upon the soil to supply all their needs. The food and water dahlias extract from the ground fuel growth, allowing them to draw energy from the sun and create lush foliage and beautiful blooms.

Top dahlia performance must begin with carefully prepared garden soil. The planting location should be sunny, with a soil temperature of at least 55F (13C) when the tuber is planted.

Dahlias enjoy a rich soil with lots of organic matter, which you can add as well-composted manure, rotted leaves, or bagged compost. The soil should be dug at least a foot deep and be free of large rocks to enable the tubers to grow. If your soil has a high clay content, some lime can be added along with rich compost. I throw a handful of bone meal in the bottom of each tuber-planting hole—dahlias love it!

You can add fertilizer through the dahlia's growing season, but you can never go back and change the soil once your dahlias are up and growing. Take the time to make sure your dahlia beds contain all the ingredients your plants will need to grow their very best.

Expert gardeners know dahlias get most of their everyday nutritional needs from the soil. But dahlias go through distinct growth phases, and a little extra targeted food will maximize dahlia performance.

When the tuber is first planted, there are no roots to take up garden nutrients. We recommend a little bone meal sprinkled in the planting hole, providing a head start for developing roots in your dahlia's early weeks of growth. 

Once your dahlia is growing and has a bit of top growth (usually 2-4 weeks after you planted the tuber), apply some balanced vegetable fertilizer such as 5-10-10 or 4-10-7 to help your dahlia produce strong leaves, sturdy stems, and beautiful flowers.

Dahlias don’t need a high-nitrogen fertilizer, which can promote lots of leaves at the expense of blooms. For that reason, the N or nitrogen content of dahlia fertilizer, which is the first number on the fertilizer package, should always be about half or less of the percentage of P (potassium) and K (phosphorus), which are the middle and last numbers, respectively.

Apply a bit of balanced fertilizer every 3-4 weeks through your dahlia's growing season. It will help keep it healthy and resistant to damaging insects or diseases. 

Even better, a well-nourished dahlia will reward you with a steady stream of vibrant, colorful blooms, where the real magic of dahlia gardening happens. 

Armed with insights into proper tuber planting, watering wisdom, sturdy support, soil preparation, and fertilization finesse, your dahlia garden will flourish. 

May your days be adorned with the magic of vibrant, colorful blooms, and your gardening journey be as delightful as the dahlias themselves. Happy dahlia growing!