Dig a planting hole that is double the width of the nandina's roots and just deep enough to position the shrub at its former depth.
Replace half of the garden soil with organic compost, and place the roots into the hole carefully. Cover them well, and water deeply to settle the soil and to get rid of air pockets.
Spread 2 to 3 inches of organic mulch under the nandina plant. Keep it at least 3 inches from the plant's stem to minimize the risk of disease.
Provide 1 inch of water each week during the growing season, unless rainfall has been sufficient.
Feed once in late winter with rhododendron fertilizer. Apply the fertilizer in the amount instructed on the package, and water it deeply after feeding.
Prune in early spring if the nandina plant develops a leggy appearance. Trim half of the oldest, woodiest branches all the way back to the ground, and remove damaged, dead and unattractive growth.