Dahlias are among the most beloved garden flowers, celebrated for their incredible diversity in color, size, and petal formation. These stunning blooms, ranging from tiny pom-poms to dinner-plate-sized blossoms, are a popular focus for gardeners worldwide. Despite their popularity, there is frequent confusion about how they are propagated, specifically whether they grow from a bulb or a seed. The perennial portion of the dahlia is neither a true bulb nor a typical seed, but rather a specialized root structure.
Dahlia Tubers Not Bulbs
The confusion often arises because the dahlia’s underground storage organ is frequently sold alongside true bulbs like tulips and daffodils. This perennial structure is correctly identified as a tuberous root, commonly referred to simply as a tuber. A tuber is a swollen, nutrient-storing part of the plant, functioning similarly to a potato. Dahlia tubers are enlarged root tissue that stores starch and water for the plant’s survival.
A true bulb, such as an onion or tulip, is made up of layers of modified, fleshy leaves surrounding a central stem plate. The dahlia tuber lacks these concentric layers and the protective papery skin found on many bulbs. New growth, or an “eye,” must be present on the crown—the area where the tuber joins the old stem—to produce a viable plant. Without an eye, the tuber, despite being plump and healthy, will not sprout.
Planting and Initial Care for Tubers
Dahlia tubers should be planted outdoors in the spring only after the last risk of frost has completely passed. The soil temperature should ideally have warmed to a consistent range of 55 to 60 degrees Fahrenheit before planting begins. Tubers thrive in well-drained, fertile soil, as sitting in cold, soggy ground is the primary cause of rot. If your soil is heavy clay, amending it with compost or planting in raised beds will help ensure proper drainage.
To plant, dig a hole approximately 4 to 6 inches deep, laying the tuber horizontally with the eye facing upward. Deeper planting can delay emergence, while shallower planting risks exposure. After covering the tuber with soil, water very sparingly, or not at all, until green shoots begin to emerge above the soil line. Overwatering the dormant tuber before it has started to grow can quickly lead to rot.
When to Use Dahlia Seeds
While tubers are the standard way to grow a specific, named dahlia variety, the plants can also be grown from seed. Growing from a tuber produces an exact genetic clone of the parent plant, ensuring the resulting flower is identical. Seeds, in contrast, result from cross-pollination and carry a mix of genes from two parents, making the outcome a genetic mystery.
This genetic variability means that seed-grown dahlias will not reliably reproduce the parent’s characteristics, offering a wide range of colors, sizes, and forms. Growing from seed is an economical way to produce a large number of plants, often grown as annuals, and is primarily used by breeders seeking to create entirely new varieties. Plants started from seed will generally take longer to reach blooming maturity than those started from a tuber.
Preparing Tubers for Winter Dormancy
In climates where the ground freezes, dahlia tubers are not cold-hardy and must be lifted from the soil to survive the winter. The best time to begin this process is immediately after the first hard frost has blackened the foliage. This final cold snap signals the plant to send its remaining energy down into the tubers for storage.
After cutting the stem back to a few inches, carefully dig around the plant, lifting the entire clump of tubers gently with a garden fork to avoid damaging the delicate necks. The tubers should then be allowed to cure for several days in a cool, dry location with good air circulation to harden the skin. Once cured, store the tubers in a dark, slightly humid, and cool location, maintaining a temperature between 40 and 50 degrees Fahrenheit, often packed in peat moss or wood shavings to prevent shriveling or rot.