5 Common Mistakes When Growing Dahlias (and What to Do Instead) (2024)

In any garden they grace, dahlia flowers shine with a spectacular combination of beauty and form in a rainbow of hues. They make stunning summer flower arrangements and have an impressive vase-life of seven days or more. You'd never believe these floral divas grow from unremarkable tubers planted in spring to become some of the most anticipated summer stars. But sometimes, dahlia flowers don't debut as expected, leaving you to wonder what went wrong. Here are five key dahlia growing mistakes to avoid and what you can do instead to ensure they produce a full show of blooms for you.

1. Not Enough Sunlight

Dahlias demand bright sunlight for at least 6 hours a day to produce flowers. The more sunlight the better in many regions, with 8 or more hours best in cool areas. In hot regions, such as the South and Southwest, 6 hours of direct sun during the first part of the day is helpful for protecting the blossoms from baking in the intense afternoon rays.

2. Not Watering Enough

Dahlias send up shoots and unfurl new foliage quite quickly. All that growth requires a hefty amount of water. Dry soil stymies vigorous growth and bloom production. Aim to keep soil around dahlia tubers consistently moist after the first leaves emerge. However, tubers are susceptible to rot in saturated soil, so be careful not to go overboard. Deep, thorough watering once or twice a week is usually just right for dahlias. Top the soil with a 2-inch-layer of mulch to help retain moisture between waterings.

3. Using Too Much Nitrogen Fertilizer

Dahlias demand nutrient-rich soil to fuel their rapid growth and exuberant blooms. Fertilizer helps supplement nutrients already in the soil, but it becomes troublesome for dahlias when your fertilizer is high in nitrogen (the first number in the N-P-K ratio on the product packaging). Excessive amounts of nitrogen encourage plants to produce lots of leaves and few, if any, flowers. Fertilize dahlias with a general garden or tomato fertilizer for best results, applying it according to package directions. For example, an N-P-K ratio of 5-10-5 is a good combination for dahlias.

4. Planting Too Early

Dahlias are native to the tropical regions of Central America. They thrive in warm, moist soil and don't begin to push out new shoots until the soil is at least 50°F. Because they need warm soil to start growing, they don't come into full flower production until late summer or even early fall in some northern regions. Plus, planting too early can lead to rot when the tubers are sitting in spring-rain-soaked soil for weeks before they begin growing. Plant dahlias around the time you would plant tomatoes.

5. Not Deadheading

Dahlias, like all flowers, bloom to produce seeds for the next generation of plants. When you remove or deadhead the faded or spent flowers, the plant will want to create more flowers because it still wants to set seeds. Regularly clip off dahlia flowers that are past their prime to encourage plants to produce more flowers, extending the bloom time until the first frost. Along the same lines, the more dahlia bouquets you harvest, the more flowers you can expect in subsequent weeks.

Grab your clippers and get ready to harvest. Easy-care dahlias are sure to put on a bold show when you avoid these common growing mistakes. Fair warning, though: A little success this year might lead to you wanting to plant a few more dahlias next year. Get the vases ready!