One of the best tips for planting comfrey seeds is to start the seeds in a greenhouse before the growing season starts. Starting the seeds in the greenhouse makes regulating temperatures possible, even in frost-heavy late winter and early spring. Comfrey seeds can be started outdoors with high germination rates, however. When starting the seeds outdoors, one of the best tips is to wait until the last frost date has passed and the soil starts to warm up.
Comfrey seeds germinate reliably when the soil temperature remains between 60 and 70°F (about 15 to 21°C). Soil temperature can be regulated by keeping the seed tray in a temperature-controlled room or greenhouse, or by using a heating pad. Heating pads that are specially designed for horticultural use go under the seed flat. Not only does a heating pad allow for constant temperature regulation, it also provides bottom heat, which can shorten germination time and increase germination success rates. Comfrey seeds take between 10 and 15 days to germinate when kept within this temperature range.
One of the advantages to starting comfrey seeds indoors in a greenhouse is that the seeds can be started early in the growing season. By the time the last frost date has passed, the seedlings are ready to plant out into the garden. It is critical to wait one to two weeks after the last frost date before planting, as tender, greenhouse-grown seedlings are highly susceptible to late spring frosts.
The seeds can also be started outdoors after the last frost date. Planting the seeds directly into the garden bed saves labor and greenhouse space. Once the seedlings begin to grow, they are already in place and don't need to be transplanted from the seed flats. Comfrey seeds can be sown out into a prepared seed bed in spring when the soil temperature warms up above 40°F (about 4°C) and commonly take anywhere from 20 to 30 days to germinate.
Comfrey seeds should be placed in the soil, in the seed-starting flat, or in the seed bed about 1 inch (about 2.5 cm) apart and covered with 1/8 inch (about 3 mm) of soil. Keeping the soil damp — but not wet — is critical for reliable seed germination. Once the seedlings germinate, they can be transplanted into 2-inch (about 5-cm) pots, or thinned to a spacing of 6 to 8 inches (about 15 to 20 cm).
