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Since the bulb of the lily would be difficult to plant in cold, hard soil, wait until after the last frost date before transferring it outdoors. Consult local nurseries or the Old Farmer's Almanac to find out the last frost date for your area. Take a shovel or trowel and dig a 6 in (15 cm) deep hole for each Easter lily you want to plant. The deep hole will give the plant space to develop its root system. If there are any wilting or dead flowers left on the Easter lily, pluck or trim them off. Place the plant into the hole. Avoid cutting off the stem and leaves of the lily until fall. Spread well-draining soil around the sides of the lily to fill in the hole. Mound soil around the stem of the plant and press down firmly to remove any pockets of air. Water the lily immediately after you plant it and water it thoroughly every day for at least 1 week. After a week, you can begin watering the plant every few days until the root system is established. Check the soil around the lily if your region is going through a dry period. Water the plant to keep the soil moist.

Summary:
Keep the Easter lily inside until the danger of frost has passed. Dig a 6 in (15 cm) deep hole. Cut off the old flowers and put the bulb into the hole. Fill in the hole around the plant with soil. Water the plant every day.