Dahlias are tuberous perennials celebrated for their vibrant blooms. Successfully cultivating them in Colorado demands precise timing and attention to local microclimates. The state’s varied elevations and unpredictable weather mean gardeners must move beyond calendar dates to ensure these warm-weather plants thrive. Understanding the specific environmental signals for planting is key.
Determining the Ideal Planting Time
The primary factor for planting dahlia tubers is the temperature of the soil, not the air. Tubers planted into cold, wet earth are susceptible to rotting before they sprout. Gardeners must wait until the ground consistently reaches 60°F or higher, which generally aligns with the period after the last hard frost.
The planting window varies significantly across Colorado’s diverse landscape. For lower-elevation areas, such as the Denver metro area, mid-to-late May is typically the earliest safe time to plant. Gardeners in high-altitude mountain regions must often wait until early June, or later, as the soil takes much longer to warm up at those elevations.
Gardeners should consult local extension resources for frost dates and use a soil thermometer to confirm the 60°F threshold has been met. Planting should only occur when the forecast shows a sustained period of warm, dry weather. Excessive moisture combined with cool temperatures is detrimental to the dormant tubers.
Essential Site Preparation and Planting Technique
Once the soil temperature is correct, selecting the planting site is important. Dahlias require full sun, meaning a minimum of six to eight hours of direct sunlight daily, which encourages strong stems and abundant flowering.
Most Colorado soils, particularly the heavy clay common along the Front Range, require significant amendment to ensure proper drainage. Incorporating generous amounts of well-rotted compost or coarse sand will lighten the soil structure, preventing water from pooling and causing the tubers to rot. Dahlias thrive in a rich, loose medium that allows for rapid water percolation.
When placing the tubers, dig a hole approximately four to six inches deep, laying the tuber horizontally with the eye—the small bud from which the sprout emerges—facing upward. After covering the tuber with soil, avoid watering until the first green shoots break the surface, which prevents premature rotting. Taller varieties of dahlias will need support, so drive a stake or place a sturdy cage into the ground at the time of planting to avoid damaging the developing tuber later.
Managing Dahlias in Colorado’s Climate
After the initial sprout, dahlias require a consistent watering schedule due to the low humidity. While the soil must be well-draining, established plants need regular moisture, often increasing to two or three times a week during peak summer. Watering at the base of the plant is preferable to overhead watering, which can encourage powdery mildew in the dense foliage.
As the plant grows, pinching and deadheading will maximize the bloom period. When the plant reaches about 12 to 16 inches tall, removing the central growing tip, known as pinching, encourages the dahlia to branch out. Removing spent blooms, or deadheading, ensures the plant continues producing flowers.
The short growing season and cold winters mean dahlias cannot remain in the ground. After the first hard frost blackens the foliage in the fall, the tubers must be lifted and stored indoors to survive. Gardeners should cut the stem back to a few inches, dig the tuber clump, and allow it to cure in a cool, dry place for several days. The cured tubers are best stored in a cool (around 40–50°F), dark location, nested in a slightly humid medium like peat moss, vermiculite, or wood shavings to prevent them from shriveling or rotting over the winter.