The crabapple is an ornamental tree that produces small sour or bitter apples. The tree is a popular root stock tree for grafting hybrid apple branches (scions) on. The similar genetic makeup of a crabapple tree ensures that the root stock will not reject the grafted scion branches of an apple tree. For best results, select scions and root stock younger than 5 years old for grafting. Older trees do not graft well.
Sharpen all of your tools with a sharpening tool to ensure that the scions or root stock do not bruise when you cut them.
Measure the diameter of your apple tree scions and crabapple root stock with a measuring tape to determine the graft to use. The size of the scions and root stock determine whether you will use a whip graft, cleft graft or side graft.
Whip-graft a tree in which the scions and root stock are both less than ½ inch in diameter. Make a slashing cut across the bottom of the scion that is at a 45-degree angle and approximately 1½ inch long, using a grafting knife. Cut a tongue into the scion by slicing straight downward through the middle of the scion. The cut should extend from your slash's center to a point level with the slash's end. Make a slash and a tongue in the scion that mirrors the root stock cuts. Sandwich the two cuts together so that the bark layer of the scion and root stock touch and wrap them with grafting tape. Remove the tape once the scion and root stock heal together.
Cleft-graft the scions from an established apple tree that are 1 to 2 inches in diameter onto a smaller crabapple tree trunk, or onto the side branches of a larger crabapple tree. Cut the trunk of the crabapple rootstock tree or side branches of the tree away to create a stub. The location for the graft should be less than 4 feet from the ground. Cut your scion into a blunt V shape 1½ inch long with one end slightly thicker than the other. Create a cleft between the bark and the tree's trunk with a chisel and mallet. Try to avoid splitting the bark. Insert the V end of the scion into the cleft and cover with asphalt water emulsion.
Side graft scions that are between ½ and 1 inch in diameter to root stock branches larger than the scion. Select a smooth location on the understock branch that is at least a foot from the trunk of the tree. Make a slanting cut at a narrow angle that extends almost into the center of the tree. Cut the scion as you did for the cleft graft in a short, sharp wedge with one side thicker than the other. Bend the crabapple branch to open the side cut and insert the scion branch into the cut. Make sure that the bark of the scion and root stock touch. Cover the cut with asphalt water emulsion to protect it as it heals.