Dahlias and zinnias are excellent garden companions that share a preference for warm, sunny conditions and put on a spectacular, long-lasting display of color. Both flowers are celebrated for their vibrant, voluminous blooms, which range from small pom-poms to large, multi-petaled forms. Planting them together creates a dynamic, layered look in the garden, providing continuous color from mid-summer until the first hard frost. A successful mixed bed relies on understanding their shared fundamental needs and addressing their differences in maintenance.
Shared Environmental Needs
A successful mixed planting depends on meeting the foundational environmental requirements that both these plants share. Both zinnias and dahlias are native to Mexico and Central America, meaning they thrive in similar warm-weather conditions. They require a location that receives full sunlight, ideally a minimum of six to eight hours of direct sun daily, to support their prolific blooming habit.
The quality of the soil is another shared factor; it must be well-drained. Dahlias grow from tubers that will quickly rot in heavy, waterlogged conditions, making good drainage a requirement. Amending the planting bed with compost improves drainage and provides the fertile base both flowers appreciate. Watering needs are similar, requiring consistent moisture, especially during dry spells, but avoiding standing water around the base of the plants.
Designing the Mixed Garden Bed
Strategic placement is important for creating a harmonious and healthy mixed bed of dahlias and zinnias. Both flowers come in a wide array of sizes, so using their mature height to create depth is a practical design technique. Taller dahlia varieties, such as those reaching four to five feet, should be positioned at the back of the bed or interspersed with medium-sized zinnias, like the ‘Benary’s Giant’ or ‘Giant Dahlia Mix’ varieties, which can grow up to 40 inches tall.
Proper spacing is necessary to ensure adequate air circulation, which helps prevent foliage diseases like powdery mildew, a common issue for zinnias. Aim for a final spacing of at least 12 to 18 inches between mature plants in a mixed bed to promote airflow. This greater distance accommodates the bushy growth of dahlias and the dense canopy of zinnias. Staggering the planting of zinnias by sowing seeds every few weeks ensures a continuous flush of fresh blooms that fill in the gaps between the slower-to-establish dahlias.
Unique Maintenance Considerations
While their environmental needs overlap, the gardener must be aware of the distinct maintenance requirements of each species.
Dahlias, particularly the taller, large-flowered varieties, often require staking or caging early in the season to prevent heavy blooms from snapping their stems. These tender perennials also require their underground tubers to be lifted and stored indoors in colder climates before the first hard frost. Dahlias prefer a fertilizer lower in nitrogen, which supports bloom production over excessive foliage growth.
Zinnias demand proactive attention to disease prevention and growth habit. They are highly susceptible to powdery mildew, making overhead watering a practice to avoid, as wet foliage encourages fungal growth. Aggressive deadheading, or cutting the spent blooms, is necessary for both plants to encourage new flower production. Zinnias especially benefit from regular cutting to maintain a bushy shape and continuous flowering. Zinnias are less demanding and thrive with only light, periodic feeding.