Dahlia Tuber Planting Instructions
Prior to planting, amend your beds with as much aged organic material, i.e., compost, manure, peat moss, etc. as possible. Locate your beds where your dahlia plants will get a minimum of 4 to 5 hours of sunlight per day. Six to eight hours would be even better. Do not plant close to structures with a southern exposure, as the reflected heat will harm your plants.
Plant after your last expected day of heavy frost. Here in the Pacific Northwest we can usually begin planting about the 1st of May, but adjust your planting schedule to your climate conditions. It is also best to plant during a dry period to avoid potential rot.
All standard sized dahlias (3 feet and higher) need to be staked to keep them from blowing over during wind and rain storms. Stakes should be placed in the ground before planting. Five-foot steel fence posts are great for this purpose, especially in heavy soils. Some people use rebar cut to five-foot lengths or you can use cedar stakes.
Spacing of your stakes will depend on your individual needs. Larger varieties (6 inch bloom or larger) are generally placed 30 to 36 inches apart while smaller varieties (under 6 inch blooms) will be fine at 24 inches apart. Most people plant double rows (two tubers or plants per stake). Some people, however, plant staggered rows with only one tuber or plant per stake. When planting staggered rows, plant your tubers offset four to six inches from the center of the row and space your stakes 18 to 24 inches apart. The nice thing about staggered rows is that your watering hose can be laid down the middle of the row. You can also place more tubers or plants per row with this method. Space your rows four to five feet apart so that you have plenty of room to get in around your plants to groom and tend. It also helps promote better air circulation, which helps prevent powdery mildew.
Once your beds are prepared, your stakes are in place, and all danger of frost is past, it is time to plant. For average soils, most people plant their tubers six inches deep. Plant a little deeper in lighter soils and shallower in heavy soils. Dig a deep hole and back fill to appropriate depth. Lay the tuber horizontally in the hole with the eye end closest to the stake. The tuber eye should point up and be about four inches from the stake. Cover with a couple of inches of soil (leaving a depression) and gradually fill in after the plant has started growing. Once the dahlia has started growing, you can sprinkle a tablespoon of a time release fertilizer (14-14-14) around the soil surrounding the plant. You don't need to water your dahlia tubers until the plants are several inches high and only then if the soil is dry.