You can grow wild apple trees if you live in a cold climate. In fact, it is the colder climates where the apples prefer to grow. If you have found a wild apple tree and want to nurture it, chances are that it already has established a good root system. Wild apple trees are great for cider apples as well as for the wildlife in your area. It has probably grown from seed which makes it a hardier tree than nursery grown grafted trees. The care for your wild apple tree is pretty much the same as for cultivated apple trees.
Pruning the wild apple tree will probably be your first task. Apple trees will produce good fruit if they are taken care of and all the energy of the tree is not just going into producing branches. Cut out all the dead branches. If the tree has a double trunk, cut out the weaker one.
Clear out the area under the tree back to the drip line. You do not want any other plants crowding out the tree. Cut larger plants back with your pruning shears and then mow the area under the tree. Keep the weed growth controlled during the growing season.
Thin out the tree by a third. This means you want to cut back the growing branches by a third. Remove the sucker branches that grow straight up as they are not the fruit producers like the short stubby spurs that grow off the trunk. Don't cut off more than a third of the wood or you may shock the tree too much.
Apply a fertilizer around the drip line of the tree. According to Michael Rochester, adding a liquid solution of calcium nitrate or ammonium nitrate. Use one pound for small trees, three pounds for medium trees and five pounds for larger trees.