Buying a potted evergreen to serve both as a Christmas tree and a yard tree is possible, though a bit of a challenge. Most trees do best if they are planted soon after purchase and during the cool months of autumn. But that doesn't stop gardeners from making this plan work just fine.
The key to success is timing. Keep the tree outside as close to Christmas as possible, and keep in indoors for as brief a time as you can. It is also important to prepare a planting spot outdoors before the ground freezes so hard you can't dig.
1. If you want a Christmas tree that can live in your yard, buy a ball-and-burlap or container tree. You can keep it indoors for 7 to 10 days if you give it a cool spot near a window. Choose a manageable size; root balls are heavy.
2. When you buy the tree, place it in a garage or a shed for a few days to adjust to the warmer air. Keep the ball moist while in the house, but not in a tub full of water. Display it in a watertight tub and place ice cubes on top of the root ball as needed to keep roots barely moist and cool. You don't want the ball to dry out completely, but by the same token it shouldn't be soggy all the time either. Just moist. You can wet it thoroughly, but then don't water again until the water is almost gone.
3. Choose a mild day to plant the tree.
4. In cold-winter climates, dig the planting hole in late fall, before the ground freezes. Make it twice as wide as the root ball will be. Then, fill the hole with mulch and protect the excavated soil with a tarp.
5. After Christmas, acclimate the tree to cooler air by placing it back in the garage or shed for a few days. On a mild day, place the tree into the hole. Remove the burlap. Backfill with excavated soil and tamp gently. Water deeply, then mulch heavily. In harsh climates, evergreens are vulnerable to wind damage during their first winter. Protect your tree with a screen such as the one shown, which is made with old pallets and draperies. This should only be needed in our area if the winter is unseasonably cold.