Hydrangeas yield pink or blue blooms, depending on the soil pH levels. Adjusting pH levels of different containers creates a colorful indoor hydrangea garden. The Hydrangea macrophylla is grown the most in the United States, according to the U.S. National Arboretum. Hydrangeas require moist soil with medium levels of light. Plants prosper indoors, but demand regular pruning and transplanting provide space for the roots.
Combine 4 parts organic potting soil, 1 part pine bark chips, 1 part peat and 1 part sand in 10-gallon bucket per two hydrangeas. Mix well.
Check the bottom of the 3-gallon containers to check that the drainage holes are open. If not, open the holes. Place a layer of small pebbles in the bottom of the containers, for soil drainage.
Fill the containers half full with the potting mixture.
Remove the hydrangeas from their original potting containers. Squeeze the root balls to loosen the roots. Place the plants into the container. Backfill with potting mixture. Secure the plant by pressing the soil firmly around it. Pinch off any leaves touching the soil.
Place the containers under lightly running tap water until water flows out of the bottom of the pot. Allow to drain for two hours. Water the hydrangeas weekly with 1/2- to 1-inches of water or when the top 2 inches of soil becomes dry. Reduce watering by half from July to January, during the hydrangea's resting period.
Place hydrangeas in sunny location to receive four to six hours of direct daily sunlight. Or, place hydrangeas 1 foot underneath a plant grow light for six to eight hours. If the plants are leggy, give them less light. Reduce the light to two to four hours daily from July to January--the hydrangea's resting period.
Place an oscillating fan in the room with hydrangeas for increased air flow to reduce chances of powdery mildew and root rot.
Use a pH soil test kit for containers. Add 1/4 tsp. of aluminum per pot to adjust pH to 5.5 or lower for blue blossoms. Add 1/4 tsp. of lime or phosphorous to adjust pH to 6.5 or higher for pink blossoms. Adjust pH to 5.5 to 6.5 for purple blossoms.
Leech hydrangeas in July to prevent fertilizer buildup in containers. Submerge container in a full sink of tepid tap water. Allow the soil to become soggy. Drain the sink and allow the container to drain for 30 minutes. Submerge the container again. Drain the sink and allow the container to drain two hours. Return the hydrangea to its growing area when the pot is completely drained.
Transplant hydrangeas to larger containers in July. Examine the bottom of each indoor hydrangea. Roots protruding from the drainage holes indicate that you need to transplant to a larger container.
Pinch off faded flowers during the growing season. Trim off the top quarter of the hydrangea in July. Pinch off or trim any stems or leaves touching the potting soil in July.