Zinnias vs. Dahlia: What’s The Difference?

At first glance, it’s easy to mistake a zinnia for a dahlia. After all, they’re both brightly colored flowers that bloom in the summertime. They’re also both very popular flowers to grow in a cutting garden. However, there are some key differences between these two plants.

The main differences between zinnias and dahlias are their propagation methods and lifespans. Zinnias are annuals that are only grown from seed, whereas dahlias are most commonly grown from tubers that can be replanted year after year as a perennial flower.

Other differences include bloom shape and color, though these elements are more subtle as both flowers can produce large blooms in an array of colors. Here’s an overview of how these two beautiful garden flowers compare.

	Zinnia	Dahlia
Propagation	seed	tuber or seed
Lifespan	annual	tender perennial
Colors	most colors, including red, yellow, green, and more	many colors, including reds, pinks, whites, and more
Height (inches)	6-48	15-72
Hardiness	annual: zone 2 and up	annual: 3-7 perennial: 8 and up
Bloom types	5 types, including single, semi-double, double, cactus, dahlia	14 recognized bloom shapes, including ball, cactus, collarette, dinnerplate
Best use	Cutting, companion planting	Cutting, landscaping

This table shows you at a glance the differences between zinnias and dahlias, but there are a few more details that help distinguish these flowers and why both deserve a place in your flower garden.

Are zinnias and dahlias the same flower?

Zinnias and dahlias are not the same flowers, though they are both members of the aster family (Asteraceae), which also includes daisies and sunflowers. Although they both have large, brightly-colored blooms, they have different propagation methods and growth habits that make them distinctly different. 

But based just on the blooms, it’s no wonder dahlias and zinnias can be mistaken for the same flower! Just check out these two cactus-style blooms, one from a zinnia and the other from a dahlia.

Originating from Mexico and the southern United States, both zinnias and dahlias are warm weather-loving plants that do not tolerate frost well. They also have similarly-shaped, large, unscented blooms that work well as cut flowers; however, their similarities end there.

Dahlias are large plants that can grow over six feet tall and offer larger, flatter blooms that can range in size from a few inches to up to 14 inches wide.

The shapes of dahlia blooms are more varied than zinnias, with 14 different recognized forms of dahlias on the market today. On the other hand, color is more limited in dahlias, which come in yellow, purple, orange, pink, red, and dark purple. 

Finally, although dahlias can be started from seed, they are primarily grown from their tuberous roots, which can be dug up and stored indoors over winter in colder climates. In zones 8 and up, dahlias can be left in the ground to overwinter as perennials.

On the other hand, zinnias are smaller plants overall, ranging in height from a few inches tall (for border varieties) to a little over four feet. 

Though more limited in shape and smaller than dahlias, zinnia flowers boast a broader range of colors, including yellow, white, off-white, pink, lilac, pale green, gold, red, purple, and orange.

Hardy to zone 2, zinnias are annual plants that grow only from seed and do not have tuberous roots. The plants die back each fall and need to be replanted each spring.

Zinnia basics

Zinnias’ cheery, colorful flowers are exceptionally popular among gardeners and cut flower enthusiasts. And with good reason. Not only are zinnias stunning additions to garden beds, but they are simple to grow once you understand their basic growing and care requirements.

When choosing where to grow your zinnias, select a location with full sun and moist, nutrient-rich soil. If your soil is poor, be sure to amend your beds with good quality aged compost before planting, which can help to ensure your zinnias bloom earlier in the season.  

After the danger of frost has passed, plant your zinnias in rows or clusters and cover them lightly with a quarter inch of soil. Germination will occur within four to seven days after planting as long as the soil is warm enough. 

For longer bloom times, succession plant zinnias over several weeks so that your plants mature at different times.

Want more details on how to do this? Check out this article, Do Zinnias Bloom All Summer? (Tips For A Summer Of Flowers).

Once your zinnias have germinated and are at least three inches tall, thin your seedlings to 6-18 inches apart, depending on the variety. Thinning your zinnias is incredibly important because these plants are susceptible to powdery and downy mildew and good air circulation is critical in preventing its spread. 

Watering your zinnias in the morning and only watering at the base of your plants can also help prevent mildew issues.

Zinnias bloom roughly 60 to 90 days after planting, depending on if you’re growing dwarf varieties or standard ones. Applying a small amount of phosphorous and potassium-rich fertilizer when buds start to appear can encourage your zinnias to produce more blooms. 

After your zinnias begin to flower, deadhead old spent blooms and harvest cut flowers regularly to encourage your plants to produce more. When properly tended, zinnias can bloom from late spring until fall. 

Zinnias are not frost-hardy and will die back after the first frost of autumn. Prolific self-seeders, zinnias will come back year after year if you allow them to.

While zinnias are naturally deer-resistant, they can attract mealybugs, spider mites, and caterpillars and are vulnerable to mildew in certain conditions, so inspect your plants for signs of pest damage. 

Popular among pollinators and other beneficial insects, zinnias are often interplanted in vegetable gardens to increase pollinator activity and boost harvest yields for plants like tomatoes and pumpkins.

If you need some ideas on how to interplant your zinnias, you can read about them here: Zinnia Companions: 7 Flowers & Vegetables To Plant With Zinnias.

Zinnia flower categories

While dahlia blooms come in a wide range of shapes and sizes, zinnia blooms fall into just a few categories.

Single Flower

Single-bloom zinnias have a single row of petals encircling a prominently displayed yellow center.

Double Flower

Featuring a double row of petals, the center of double-bloom zinnias is often partially or entirely hidden from view. Popular varieties of double-bloom zinnias include ‘Benary’s Giant’ and ‘State Fair Mix.’

Cactus Flower

The petals of cactus-form zinnias are long and roll in upon themselves, giving the petals a thin, tubular appearance and making the overall bloom look spiky and star-like.

Dahlia Flower

As the name would suggest, this zinnia type looks very similar to a dahlia. This particular flower type contributes to the confusion between the two flowers. 

Dahlia basics

While zinnias may have a wider variety of colors, dahlias’ larger flower heads and showy blooms are popular among gardeners and florists. 

Although they can only be grown perennially in zone 8 and above, with proper care, they can be grown as showstopping annuals in the flower garden, overwintering the tubers indoors in colder climates. 

Dahlias are most commonly started from tubers which can be ordered online from seed companies or purchased at home and garden shops. Starting dahlias from tubers guarantees what your dahlia flowers will look like, avoiding the variance from seed-grown dahlias (more on that shortly!)

An ideal location for a dahlia bed will have full sun exposure and protection from heavy winds, and rich, well-drained soil, which is essential for preventing tuber rot.

When you receive your tubers, be sure to inspect them carefully for signs of wrinkling or soft areas indicating rot; unlike potatoes, do not cut dahlia tubers before planting.

For a wealth of information about buying and growing dahlias, be sure to check out Swan Island Dalias’ website. They’re a well-reputed company with high-quality dahlia tubers and learning resources.

When the danger of frost has passed, dig holes for your dahlia tubers about 8-12 inches deep and amend the soil with compost. Backfill your holes with several inches of dirt and then plant your tubers, sprout side up, and cover them with three inches of soil. 

Depending on the variety and purpose, tubers should be spaced between one and three feet apart. Dahlias grown specifically for cutting can be spaces 12-18 inches apart, whereas plants grown to be part of the landscape need more room to grow. 

If you live in a particularly cold climate, you can start dahlia tubers in pots indoors to extend your growing season.  

After planting your dahlias, do not water your plants until sprouts begin to appear above your soil line. When your plants have spouted, begin regular watering and apply organic, low nitrogen, liquid fertilizer at the base of your plants once a month. 

Once your plants are about 15 inches high, pinch off the terminal bud to encourage your plants to branch and grow fuller. Blooms should begin to appear approximately eight weeks after planting, usually around the middle of July. 

As dahlias can grow quite large, they frequently benefit from staking. 

Cutting dahlia blooms encourages your plants to produce more flowers so be sure to regularly harvest dahlia flowers throughout the season. Dahlias will continue to bloom from mid-July to the first hard frost of autumn.

Dahlias can be susceptible to powdery and downy mildew, so good air circulation in your flower beds is essential. Other common pests include deer, rabbits, slugs and snails, as well as rodents that frequently feed on dahlia tubers. 

To prevent pest damage, consider planting your tubers in bulb cages, baskets, or flower pots. In all of those scenarios, be sure to provide enough space for the dahlia’s roots to spread out in the soil.

Dahlia flower categories

Like zinnias, dahlias come in different shapes and sizes. Below are some of the most common dahlia forms available today.

Single

Single-bloom dahlias have a single row of flat or slightly cupped petals encircling a flat, disk-shaped center. Popular varieties include ‘Happy Single Romeo,’ ‘Magenta Star’ and ‘Happy First Love.’

Pom-Pom and Ball

Pom-pom and ball dahlias have spherical blooms with symmetrical, inward-curving petals. Varieties include ‘Little William’ and ‘Bantling.’

Cactus and Semi-Cactus

The curled petals of cactus variety dahlias make for flowers that appear spiky and star-shaped. The unique shape of cactus-variety blooms means these varieties often require less staking as they are more resistant to heavy rainfall. Popular examples of this dahlia include ‘Doris Day,” Ryecroft Pixie’ and ‘Dame Deirdre.’

Waterlily

As the name suggests, waterlily dahlias resemble water lilies with wide flowers and broad petals that narrow at the tips, such as ‘Caballero’ and ‘Karma Choc.’

Anemone and Collarette

Anemone and collarette dahlias boast a row of large, flat petals encircling a cluster of smaller, central petals. Varieties include ‘Impression Fortuna’ and ‘Night Butterfly.’

Dinner plate 

Any variety of dahlia, from waterlily to cactus, can be classified as a “dinner plate dahlia” as well. Dinnerplate refers to dahlias that produce very large blooms, usually 12-14 inches in diameter, regardless of shape.

Can I grow dahlias from seed?

While traditionally grown from tubers, dahlias can be grown from seed. Dahlias grown from tubers are identical to their parent plant; however, dahlias grown from seed are one of kind. This genetic diversity is caused by pollinator activity which crosses the genetic information of two different dahlias creating unique seeds. 

Part of the joy of growing dahlias from seed is the surprise of what sort of dahlia you’ll end up with. The majority of dahlias grown from seed will either be single-bloom or semi-double bloom varieties; however, any shape of dahlia is a possibility.

Dahlia seeds are started indoors six to eight weeks before the last frost date and transplanted outdoors after the danger of frost has passed. Blooms will appear 100 to 120 days after sowing your seeds. 

At the end of the season, the dahlias you started from seed will have formed tubers which can be dug up and overwintered indoors.

How do I overwinter my dahlia tubers?

If you live in zones 8 and higher, dahlias can be grown as perennials, and the tubers can remain outdoors all winter long. Dahlias can also frequently overwinter outdoors in zones 6 and 7; however, the foliage should be cut back, and a thick layer of mulch should be added to help insulate your tubers against cold winter temperatures.

If you live in a very rainy area you can also cover the mulched bed with a tarp to avoid having waterlogged soil. In springtime, remove the tarp to avoid a huge slug or snail population that will feast on your dahlias as soon as they sprout (ask me how I know!)

In colder regions, dahlias can only be grown as annuals, or they will need to be dug up in autumn and the tubers overwintered indoors. If you choose to dig up your dahlias, cut your plants back, leaving only about 2-4 inches of stems remaining. 

Starting about a foot away from your plant, carefully dig your tubers out of the soil, lifting them gently to prevent damage. Brush away any loose soil, and then allow your tubers to dry in a shady location for several days. 

Tubers can then be packed in sand, peat moss, sawdust, vermiculite, or perlite and stored over winter. Once a month, check your tubers for signs of shriveling and adjust moisture levels accordingly.