Plant a good cover crop in the area where you'll be cultivating marshmallow root, a year in advance of planting it. Use manure to fertilize the soil and ensure that the growing area can be deeply irrigated throughout the growing process. After all danger of frost has passed, plant marshmallow root from seeds or cuttings the following spring.
Dig holes about one-eighth-inch deep and approximately 15 inches apart, to grow marshmallow from seeds. Gently press marshmallow seeds into the soil. Keep the area moist.
Wait until late autumn when the top growth of the plant has died back, to grow marshmallow from roots. Dig out mature roots by hand or with a root digger. Remove any rootlets from the main root. Plant root divisions in moist soil about 1 to 2 feet apart.
Dig out mature plant roots and remove any rootlets, to harvest marshmallow root. Wash the root, peel off the bark and dry the root at 92 degrees Fahrenheit to avoid mildew. Examine the leaves for rust fungus, and discard any infected leaves.
Consult a doctor before using marshmallow for medicinal purposes. Using this herb is considered helpful in the treatment of several diseases, including Crohn's, asthma, gastroesophageal reflux and gastritis, but its effects have not been substantiated by human pharmacological studies or clinical trials.