How to Grow and Care for Dahlia Natal

Dahlia imperialis, often known as the Tree Dahlia or Dahlia Natal, is a striking plant that commands attention in any garden. This impressive species originates from Central America, including Mexico, and extends into parts of South America. Unlike its more common, bushy relatives, the Tree Dahlia is celebrated for its towering stature and its unique ability to produce an abundance of blooms late in the growing season. Its imposing presence and distinctive flowering period make it a captivating addition for dramatic height and late-year color, creating a truly unique focal point.

Unique Characteristics

The most distinguishing feature of Dahlia imperialis is its remarkable height, typically reaching between 10 to 12 feet, with some specimens growing up to 30 feet under optimal conditions. This tree-like growth habit is supported by hollow, cane-like stems, often described as bamboo-like, that can be up to 4 inches in diameter and feature swollen nodes. These robust stems are adorned with large, dark green leaves up to 60 centimeters long, characterized by their tripinnate or bipinnate structure, adding to the plant’s lush appearance.

The blooms of the Tree Dahlia are equally distinctive, appearing later in the season, from late summer through early winter, or specifically from late October onwards in some regions. These bell-shaped flowers often nod downwards, measuring between 3 to 6 inches across. They typically display shades of pink, white, lavender, or purplish-pink, frequently accented by a contrasting orange center, providing a vibrant display when many other plants have faded.

Planting and Care

Selecting the right location is fundamental for successful cultivation of Dahlia imperialis. This plant thrives in full sun, requiring at least six hours of direct sunlight daily for optimal growth and flowering. It can tolerate partial shade, particularly in hotter climates, but full sun ensures the best bloom production. The soil should be well-draining and rich in organic matter, with a loamy texture and a slightly acidic pH range of 6.0 to 7.5. Good drainage is important to avoid root issues and promote healthy growth.

Planting of tubers is best done in the spring, after the last risk of frost has passed. Place them approximately 4 inches deep and space them 28 to 40 inches apart to accommodate their mature size. Consistent moisture is important, especially during the active growing season; the soil should remain moist but not constantly saturated to prevent rot. Tree Dahlias are heavy feeders and benefit from regular fertilization. Apply a balanced fertilizer, such as a 10-10-10 formulation, every four to six weeks during the growing season. Fertilizers lower in nitrogen but higher in phosphorus and potassium, like a 4-10-10 or 5-10-10, can encourage more prolific blooming, as can well-rotted cow manure.

Due to their impressive height and hollow stems, providing adequate support is essential for Dahlia imperialis, especially in areas prone to strong winds. Staking helps prevent stem breakage and maintains the plant’s upright habit, ensuring its structural integrity. Light pruning can be done in late winter or early spring to remove any dead or damaged branches and to shape the plant, encouraging new growth. Deadheading spent blooms also helps to prolong the flowering period and maintain plant vigor.

Protecting Through Winter

Dahlia imperialis is a tender perennial, generally hardy in USDA Zones 8 through 11, with some reports of survival in Zone 7b when well-mulched. However, it is not tolerant of hard frosts, which can damage the plant. In colder climates outside of Zone 8, the tubers must be lifted and stored indoors for the winter.

Overwintering begins after the first frost causes the foliage to die back. Cut back the stems, and carefully dig the tubers from the soil. Gently wash any remaining dirt from the tubers. The tubers need to be dried or “cured” for several weeks indoors, in indirect light, with good air circulation, which helps prevent fungal issues during storage.

Once cured, store them in a cool, dark, and dry, frost-free environment, such as an unheated basement, garage, or closet, maintaining temperatures between 40-50°F (4-10°C). Tubers can be stored in containers like milk crates, cardboard boxes, or paper bags, packed in materials like peat moss, vermiculite, or dry compost, ensuring good air circulation. Periodically check the stored tubers throughout the winter for any signs of rot or shriveling to ensure their viability for the next growing season.

Addressing Common Issues

A lack of blooms can be attributed to insufficient sunlight, as the plant needs at least six hours of direct sun daily to flower optimally, or to early frost damaging developing buds. Stem breakage can occur due to the plant’s height and hollow stems, particularly in windy conditions or if adequate staking is not provided, potentially leading to plant collapse.

Pests that may affect Tree Dahlias include aphids, spider mites, thrips, slugs, snails, caterpillars, and mealybugs. Simple solutions like hosing off the plants, using insecticidal soaps, or hand-picking larger pests can help manage infestations effectively. Diseases such as powdery mildew, gray mold (botrytis), and stem rot can also pose problems. Improving air circulation around the plants, avoiding overhead watering, and promptly removing any infected plant parts are effective preventative measures to maintain plant health.

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