Guide to Hardening Off Dahlias

Dahlia seedlings will produce stunning blooms if properly hardened off prior to planting.

Dahlias sown from seed are typically started indoors. When the seedlings reach the right size, they are transplanted outside. Before they are planted outside, however, the dahlias must be hardened off or they may struggle to survive. The hardening off process acclimates the delicate seedlings to their new environment so they aren't scorched during a sudden transition to a full-sun location.

Timing

Dahlia seedlings should be about four to five weeks old before they are hardened off for transplanting. In addition, outdoor temperatures should be at least 60 degrees Fahrenheit with no danger of late-season freezes. In most areas, this is late March through late April. If there is a high likelihood of strong spring storms, wait until after they have passed. Even if temperatures are warm enough, storms with high winds and heavy rain can damage dahlia seedlings.

Hardening in Pots

Place the seedlings outside in the sunlight during the daytime. Choose a location that is protected from the wind. Leave the seedlings outside for about two to three hours the first day and then either bring them inside or place them in a shady, protected area. Repeat this process for the next six to nine days, but each day increase the amount of time they are left in the sun by about one hour. If a storm or bad weather hits, bring the dahlias inside, but try to get them in the sun for at least part of the day if the bad weather eases or clears up. If the weather suddenly becomes much hotter, decrease the time they are exposed by two or three hours and then add an hour each day to re-acclimate them to the hotter weather.

Hardening in Cold Frames

Instead of moving the dahlia seedlings each day, you can place them in cold frames to harden them off. Open the cold frames during the day, and close them at night. Keep the frames closed during inclement weather. Like hardening in pots, it will take seven to 10 days to harden the dahlias enough for planting. If the weather has suddenly become extremely hot, consider moving the dahlias to a shaded place for part of the day during the first few days of hardening, and returning them to the cold frame in the evening.

Watering

It's vital to keep the seedlings moist during the hardening off process. Because the plants are being placed in direct sun and are typically planted in small pots, the soil will lose moisture quickly. Water when the top of the soil is dry to the touch, but do not allow the soil deeper in the pot to dry out. It should always be moist to a 1/4-inch depth. Seedlings will likely require daily watering during the hardening off process and may even need twice daily watering in especially dry and hot areas.

References

• Swan Island Dahlias: How to Grow Dahlias (Starting Early Indoors & Growing from Seed)
• Washington State University Extension; Hardening Off Transplants; Holly S. Kennell