We recently moved into a home in Houston, TX that has a 15-20ft magnolia tree that is planted too close to the garage. I have no idea how long the tree has been there. The diameter of the tree, including trunk, branches and leaves is maybe 4 ft. I was wondering what the tree's chance of survival would be if we tried to replant it at another location in the yard.

 

i would say pretty good if you will get as much roots as possible. make the root ball as large as you can. make the transplant hole 1-2 ft. larger in diameter and make sure the edges are angular and not slicked so the roots will grow out and not around in circles. also do not plant the tree any deeper than it is now. planting too deep will kill the tree. backfill the hole with a good rich compost if you can, if not your native soil will work. stake it off and keep it moist. be sure not to drown the tree by water logging the roots. when you water in the first timme, use a rod to poke around the ball to let the air pockets out.
down in the valley, we would use post hole diggers to dig a 3-4 ft. hole in the bottom of the new site, fill it with gravel, then plant the tree. the oldtimmers swear the trees will grow faster because some roots can get deeper faster.

 

The trick will be moving the tree and rootball. I wonder how much that weighs. When I move large plants, I trench around the plant 4" wide and 12 - 18" deep and fill the trench with compost or potting soil. Dig the trench in a line with the canopy of the tree. This encourages the growth of new feeder roots at the new cut line. I would do that now.

In the spring, when the forsythia blooms, move the tree to its new location. Water the tree the day before you move it. The root ball will come up easily because it was trenched in September. Dig out around the tree working up under the root ball as now defined by the trench, and cut it loose.

My rough figures indicate the root ball will weigh on the order of 700 pounds. This is where your friends come in or the local teenagers willing to earn a buck or two. A heavy tarpaulin is handy to move it a short distance.

Be sure to dig a wide hole for the tree 2- 3 times the width of the root ball. Backfill with loosened native soil and some compost or peat moss, if the soil is heavy. As kerry said, don't plant it any deeper than before. In fact, elevating the ball a couple of inches will help prevent any problems if it settles. I believe that the tree should face north the same as before. No research on this, just belief. Water it in well to settle the soil.

I would feed it with some liquid starter such as comes from miracle grow. Keep it watered. Add a 2" layer of mulch over the base to help retard evaporation. I recommend deep feeding the tree as you ordinarily would in the spring.

Hope this helps.