Bigleaf hydrangea is often called "florist hydrangea" because many floral shops sell it in pots for people to give as gifts. This type of hydrangea, hydrangea macrophylla, is bred for beauty and not for hardiness. The plants can be considerably weaker than those with smaller leaves and flowers. That doesn't mean you can't plant florist hydrangea, just that you have to pay very special attention to its care to ensure its survival.
Scout out a spot to plant. You will need to find an area of your yard that gets sun in the morning and shade in the afternoon. Your plant's leaves and buds may burn if it's subjected to full noontime sun. Note that this shrub can grow to as large as 4-feet square.
Check the soil quality in the planting spot. Florist hydrangea needs soil with adequate drainage (i.e. no pooling water) but the soil does have to be moist. The recommended pH is 4.5 to 6.5. Hydrangea does well without fertilizer, but if you want to enhance your soil, add 1 to 2 cups of compost or manure.
Dig a hole that is twice as deep and wide as the pot from the florist. Use your trowel to loosen the soil in the bottom of the hole so the roots will be able to spread easily.
Tip the pot upside down at a 45 degree angle to the ground so that the hydrangea plant falls out. Run your fingers through the soil surrounding the roots to loosen them up. Place the plant in the hole so that the roots are underground but all of the stems are above ground.
Fill in any gaps in the hole and pat the soil down flat at ground level. Water the plant, stopping when water starts to pool. Surround the plant with bark mulch, laying a 1-inch layer in a circle with a 5-inch radius with the plant as the center of the circle. That will help keep the soil damp and cool.