10 Questions About Growing Dahlias

If you are growing dahlias this year, you may have a few questions. Though these summer-blooming bulbs are not difficult to grow, they can be puzzling. Even to gardeners with many years of experience. Read on for answers to some common questions about growing dahlias.

My dahlia never came up. What happened?

When you are planting dahlia tubers, you may or may not see a sprout. Often, the tubers don’t sprout until they have been in the ground (or in a pot) for 4 to 6 weeks. It takes tubers longer to come out of dormancy when they are planted early in the season and the weather is cool. Sprouting happens more quickly when you’re planting late and the soil is already warm.

If your tubers have been planted for more than a month and there’s still no sign of growth, it’s OK to do a little investigating. Use your fingers to very gently dig up a tuber. Take care because if there is a sprout, it’s very easy to accidentally snap it off.

If the tuber feels firm, it’s probably fine and just off to a slow start (some dahlia varieties grow more slowly than others). Carefully replant the tuber and wait a few more weeks. Should you find the tuber is soft, it may have rotted because the soil was too wet. Unfortunately this is not reversible. Dahlia tubers contain their own source of moisture and should be given little to none until they are in active growth.

Remember that no matter what sort of plants you are growing, a 100% success rate is rare. Most dahlia tubers eventually sprout, but it’s possible to get a dud without a viable eye. As long as you start with good tubers, don’t overwater, and keep slugs and snails at bay (more on that below) you can expect a 80-90% success rate. This means 1 or 2 no-gos out of 10 is relatively normal.

Something ate one of my dahlias. What do you think it was?

For slugs and snails, young dahlias are a delicacy. If you know you have these pests in your garden, don’t take any chances. As soon as the sprouts begin to emerge, surround them with slug bait (use an organic option such as Sluggo). Refresh the bait weekly and after rain. Once the plants are 6” tall, the stems will be tougher and more resistant to nibbling.

Rabbits, groundhogs and deer may also munch on young dahlias. If you know your dahlias could be at risk, protect young plants with garden fabric, a scent deterrent, or a temporary fence. Should a stem get broken off or chewed, wait a couple weeks to see if the plant sends up new sprouts. They often do. You may also want to grow a few replacement plants in pots and keep them at the ready.

Some of my dahlias are tall and leggy. What should I do?

There are hundreds of varieties of dahlias with many different growth habits. Some are short and stocky. Others are naturally tall and lanky. Most fall somewhere in between. Though pinching your dahlias isn’t absolutely necessary, it helps to level the playing field – and most people who grow dahlias recommend it.

Dahlias should be pinched once when the plant is between 6 and 12” tall. Use your fingers or scissors to remove the center shoot just above the third set of leaves. If there are already multiple stems, you can remove all of the center shoots.

A dahlia that’s pinched produces twice as many branches from each leaf node. This gives you a stronger, bushier dahlia with more flower stems. Though pinching slows growth for about 2 weeks, the plants quickly catch up.

Dahlias can also get leggy if they are not receiving enough light. In most parts of the country, these high energy plants should be grown in full sun. If you garden in one of the central or southern states, your dahlias will appreciate some shade to help keep them cool during the hottest part of the day.

I planted my dahlias weeks ago. Why are they growing so slowly?

Knowing where a plant comes from can help you understand how to help it grow. Today’s dahlias are hybridized versions of plants that are native to mountainous regions in Mexico and Central America. At these elevations near the equator, the soil is always warm and air temperatures are constant, ranging from highs of 75°F during the day to lows of 60°F at night.

There are only a few areas in the US that offer similar growing conditions. We just have to do our best to make our dahlias happy. Most importantly, don’t plant too early. If the soil temperature is much below 65°F, dahlias grow slowly, if at all. Too much moisture also slows their growth. If you live in a cool climate with heavy soil and wet spring weather, consider growing your dahlias in raised beds filled with lighter soil. Another strategy is to plant the tubers so the eyes are no more than 2” below the soil surface. Once the plants are 12-18” tall, you can pull more soil up around the base of the plant for added support.

Even when growing conditions are good, it takes time for dahlias to find their stride. When you transplant a potted annual or perennial, the plant already has an established root system. Dahlias have tubers to support sprouting and the very first stages of growth. But then they have to create an entire root system from scratch — one that will support a 4′ plant that flowers non-stop for 3 months straight. Once your dahlias make the shift from roots to shoots, you’ll be impressed by how quickly they grow.

My dahlias are getting tall and starting to lean/bend/break. What should I do?

Depending on the varieties you are growing, some of your dahlias could get to be 4 to 5 feet tall. Because these plants have hollow stems and big flowers, breakage can be a problem — especially in wet and windy weather. The best way to avoid heartbreak is to stake your dahlias. And the best time to do it is before it’s needed.

If you have less than a dozen plants, it’s easy to support each one individually. You can use stakes and ties, a tomato cage or a cage made from concrete reinforcing wire.

With more plants, you may want to use the corral method. Insert 5’ wooden or bamboo stakes about 3 to 4 feet apart along the sides and at the ends of the bed. Use twine to connect the stakes and contain the stems. You’ll want to have several rows of twine and can crisscross from one side to the other for extra reinforcement.

Should I be watering my dahlias?

Not at first, but once your dahlias are at least a foot tall and growing strong, they usually benefit from getting about an inch of water per week. But there’s no hard and fast rule. With dahlias, too much water is worse than too little.

If the soil is sandy and the weather is hot, dahlias get very thirsty and should be watered deeply once or twice per week. But if the soil is heavy and the weather is cool and wet, you may not need to water them at all.

A dahlia’s feeder roots are very close to the soil surface. In most parts of the country, dahlias benefit from being mulched with shredded leaves or straw. This helps to protect the roots, retain soil moisture and minimize weed growth. But if you live where summers can be cool and wet, keeping the soil bare will encourage good air circulation and minimize slug and disease problems.

Do I need to fertilize my dahlias?

This depends on the fertility of your soil. Dahlias grow best in rich soil with lots of organic matter. If possible, enrich the soil with compost before you plant. Six weeks later, once the plants are in rapid growth, you can top dress with more compost or begin applying a low or no-nitrogen fertilizer.

While some people never fertilize their dahlias, others feed their plants as often as once a month. Many dahlia growers recommend using MorBloom (0-20-20), an odorless liquid concentrate made from fish and other organic ingredients.

I think something’s wrong with my dahlias. What could it be?

Dahlias have far fewer problems than roses, but like most plant, they can be troubled by certain pests and diseases. Common pests include earwigs, tarnished plant bugs, potato leafhoppers and spider mites. Dahlias can also suffer from several fungal diseases and viruses. We will have a new article about dahlia pests and diseases available very soon and will add the link here.

Do I need to deadhead my dahlias?

Yes! Removing spent flowers encourages dahlia plants to continue producing more buds. It also helps keep the area clean, which minimizes problems with pests and disease.

When you cut off a spent flower, don’t just snap off the head. Take time to remove the entire stem right back to a main stalk. This encourages the plant to produce longer stems and also promotes good air circulation within and around the plant.

When do I cut the flowers and how can I make them last?

Unlike tulips and daffodils, dahlias do not open up after they are cut. Flowers should be picked when they 3/4 open and before the back petals begin to soften.

Cut your dahlias early in the morning while the blossoms are cool and well hydrated. Bring a bucket of clean water out to the garden. To avoid crushing the stems, make your cuts with a sharp knife rather than scissors. As you cut, trim off the lower foliage and remove side buds that could steal water from the main flower.

Flower farmers are divided as to the best way to condition cut dahlias. Some recommend standing cut stems upright in a bucket filled with 2-3” of very hot water (165-170 degrees F). Leave them there for an hour as the water gradually cools down. The stems may get discolored, but don’t worry about it.

An equally popular technique is to submerge freshly cut dahlias up to their necks in a bucket of cool water for 1-2 hours. If you are holding cut dahlias for an event, keep the flowers cool, at 55-60°F. Colder temperatures may damage the blossoms.

Ready to learn even more about growing dahlias? Here are some additional articles available on our website:

All About Dahlias, 8 Tips for Growing Better Dahlias, Dahlia Tubers: What You Need to Know, How to Grow Dinnerplate Dahlias, How to Grow Border Dahlias, Tips for Growing Dahlias in Hot Weather, How to Pinch and Stake Dahlias, How to Overwinter Dahlias, and Dahlia Flower Types and Sizes