Among the brightest and boldest of the common bedding flowers, Dahlias offer a host of uses and benefits and rich symbolic value. With over 20,000 named cultivars recorded, there is a Dahlia for every color, purpose, and placement. This flower’s centuries of selective breeding have resulted in great variations in both height and bloom size, with most dahlia varieties growing anywhere from 12 inches to 30 feet tall. Only the Tree Dahlia rises above five to six feet tall, the limit for most garden varieties. So, whether you’re looking for a tall and stately flower or a short-growing blooming shrub, you can find a Dahlia to match.
|Zones:||Perennial and winter hardy in USDA zones 8 through 11, grown as an annual in zones 3 through 7|
|Exposure:||Full sun for at least 6 to 8 hours a day. Some varieties can take partial afternoon shade|
|Bloom Time:||Mid-summer to the end of fall|
|Soil:||Well-drained, rich soil high in organic material, pH range of 6.5 to 7.0|
|Amendments & Fertilizer:||Add peat moss to help make soil slightly acidic and to hold moisture, fertilizer with a general-purpose flower feed once per month during blooming season.|
|Watering:||One inch of water per week, delivered in one to two deep waterings|
|Diseases and Pests:||Root rot, powdery mildew, caterpillars, stem rot, aphids, and leafhoppers.|
How Tall Do Dahlias Grow?
Photos of fields full of Dahlias can be misleading if there are no size references in the shots. It’s easy to think you’re looking at a field of flowers barely knee-high when they’re actually rising a full 3 to 5 feet above the ground.
Most Dahlias have an average height within this range, although many varieties are bred to either stay much shorter or grow a little taller. The maximum height tends to top out at around 5 to 6 feet, putting some Dahlia varieties in competition with sunflowers for eye-catching blooms.
Miniature varieties are better suited for growing close together in a bed or container since they don’t reach nearly as tall as the average person. They’re also easier to blend with other plants of different heights without having to be the backdrop as the tallest flower.
However, the height of standard Dahlias makes them popular with people who want an impactful and unmistakable landscape feature. They’re also suited for solo planting where the bush can be admired on its own.
Short-growing Dahlia varieties generally tend to feature smaller flowers, and more prominent flowers are found on taller plants. Yet some rare cultivars don’t follow that general pattern, offering big flowers on short stems.
Which Dahlias Are Tallest and Shortest?
One of the tallest Dahlias wasn’t bred that way, but grew naturally from 7 to over 20 feet tall in the mountains of Mexico. That’s the Tree Dahlia, or Dahlia imperialis. It sports a hollow stem once used for carrying water by the ancient Aztecs, but today it remains popular as a tall and unique flowering bush.
For more standard tall Dahlias easily grown as annuals, consider varieties like ‘Reedley”, a popular British variety with salmon pink flowers and stems as tall as 7 feet. ‘David Howard’ Dahlias have glowing orange blooms and dark purple foliage that is as striking as its 5.5 foot height.
Keep in mind that the taller varieties of Dahlias will almost definitely need staking, so plan on one stake per blooming stem. Each Dahlia tuber or bush can produce a dozen flowers if it’s healthy and well-fertilized. Bamboo and cane sections make ideal stakes if cut long enough, but even trimmed branches and saplings can be reused in the garden in a pinch.
On the other end of the size spectrum, the smallest Dahlias are short indeed. Miniature varieties tend to top out at a mere 12 inches tall, although some will stretch to 18 inches in height no matter how you try to keep them shorter.
These varieties tend to work well for bedding and border plantings where bunches of blooms are needed, but longer stems would overcrowd the area. Short Dahlias also need no staking, especially since they are generally planted closely enough to hold each other up. ‘Harmony’ offers lemon yellow ball-shaped flowers and stays under 2 feet in height, while ‘Labella Grande Fandango’ stays under 12 inches in both height and width for super compact planting.
The various colors of ‘Dark Angel’ Dahlia varieties, named after their black, chocolate-colored dark centers, are also a good choice for a 12-inch tall plant. There’s no need to settle for single-flowered Dahlias either just to save space since many of the smallest varieties have doubled or ball-type blooms.
How Else Are Dahlias Measured?
The American Dahlia Society is responsible for setting classifications for the flowers they rank and rate. They don’t use height categories for separating varieties and judging them since the height can vary between individual plants of the same cultivar based on conditions.
Instead, they separate the over 20,000 varieties into 21 categories of flower forms and 9 size groups. These size groups are based on the diameter of the bloom rather than the total height of the stems. Each size group can contain a wide variety of Dahlia heights within it, but most are linked in some way to the plant’s average height. The main ADS categories for flower size are:
- AA – (Giant), over 10 inches in diameter, which tend to grow on plants only three to four feet tall since taller stems flop over from the weight.
- A – (Large), over 8 to 10 inches in diameter, found on plants growing from two to five feet tall.
- B – (Medium), over 6 to 8 inches in diameter, concentrated in the varieties that grow between three and five feet tall usually.
- BB – (Small), over 4 to 6 inches in diameter, generally found on the tallest varieties and many smaller ones.
- M – (Miniature), up to 4 inches in diameter, linked to Dahlia varieties growing four feet and under in most cases.
- BA – (Ball), over 3.5 inches in diameter, a common size for bedding and border Dahlias that grow under three feet.
- MB – (Miniature Ball), over 2 to 3.5 inches in diameter, one of the smallest sizes you’ll still find on tall-growing varieties of up to five feet.
- P – (Pompon), up to 2 inches in diameter, found usually only on the shortest growing Dahlias.
- MS – (Mignon Single), up to 2 inches in diameter, also limited to the shortest varieties most of the time.
Factors Contributing to the Height of Dahlias
Due to their tuberous roots, Dahlias need a good amount of space to reach their total height. It’s easy for plants to grow stunted and shorter than usual when overcrowded. Think about the fact that the plant bushes out and puts out multiple stems when spacing the roots.
Tall varieties can be planted just 12 inches apart since they need a little crowding to keep them upright, even with staking. Border and bedding varieties tend to spread wider, so give them two to three feet per plant to allow them to reach their full height.
Fertilizing dahlias is key for taller blooms, so don’t over-fertilize if you want to keep your plants shorter.
Dahlias can stretch to almost 30 feet tall, but only one species can accomplish that feat. Most of the garden Dahlias you might plant will top out at five to six feet at the most. It’s easy to find shorter varieties and experiment with different heights in the same bed. For more, see our comprehensive guide to growing dahlias at home and cutting dahlia flowers for a vase or bouquet arrangement.
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