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 plant dahlia tubers, there may or may not be a sprout. Often, they don’t sprout until they have been in the ground for a couple of weeks.
If you planted the tubers and have been waiting three weeks or more, it’s time to investigate. Use your fingers to gently dig up the tuber. If it feels firm and you can see a sprout, it’s just off to a slow start (some varieties grow more slowly than others). Don’t see a sprout? It’s possible there wasn’t a viable eye. Though this is relatively uncommon, sometimes you get a dud. If the tuber is soft, it probably rotted because the soil was too moist.
No matter what sort of plant you’re growing, a 100% success rate is rare. With dahlias, as long as you start with good tubers and don’t overwater, you can expect 80-90% of them to grow. That means 1 or 2 no-gos out of 10 is pretty 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 naturally 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 multiple stems, you can remove all of the center shoots.
A dahlia that’s been pinched will produce twice as many branches from each leaf node. The result is 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. For best results, grow these high energy plants in full sun. If you garden in one of the central or southern states, your dahlias will appreciate protection from the hot afternoon sun.
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 relatively constant, ranging from 75° during the day to 60° at night.
This is why it’s important not to plant dahlias too early. If the soil temperature is much below 65°F, they simply won’t grow. 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 that are filled with lighter soil. And when planting, position the tubers so the eyes are no more than 2” below the soil surface. Once the plants reach a height of 12-18”, you can pull soil up around the base of the plant for added support.
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?
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 they can be troubled by certain pests and diseases. Common dahlia 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!
Do I need to deadhead my dahlias?
Yes! Removing spent flowers encourages the plants to continue producing more buds. It also helps keep the area clean, which reduces problems with pests and disease.
When you cut off a spent flower, don’t just snap off the head. Take the time to remove the entire stem right back to a main stalk. This encourages longer stems and 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, How to Pinch and Stake Dahlias, How to Overwinter Dahlias, and Dahlia Flower Types and Sizes