Choosing what to plant under a crab apple tree depends in part on the effect that you hope for. Select plants that do well in the partial to full shade and slightly acidic soil that you have under a crab apple tree. There are ground covers that fit your needs well if you appreciate leafy green plants that do well all summer long with little care. On the other hand, you may prefer flowers for extra color and cheer. Even some fruits provide a good ground cover under a crab apple tree.
There are some plants that produce few flowers, being mainly appreciated for their foliage. Of these, hostas grow well in the shade under a crab apple tree. Another ground cover option that spreads easily and requires little care is pachysandra. Pachysandra ranges from dark to light green, and some varieties have white stripes. Vinca minor plants have dark green foliage, and small white or purple flowers. If you wish to plant an evergreen under your crab apple tree, try pachistima canbyi.
If you prefer to plant colorful flowers underneath your crab apple tree, choose flowers that grow well in partial or full shade and slightly acidic soil. Bleeding hearts, impatiens, and begonias are all good options. Another good choice would be hardy geraniums. Planting any of these flowering plants underneath your crab apple tree will provide you with flowers all summer. Lily of the valley or violets also grow well underneath a crab apple but will flower more in spring rather than all summer long.
There are some berries that do well in the shady, acidic soil found under a crab apple tree. For example, lingonberries, which grow to less than two feet in height, can survive very low temperatures in winter and provide fruit in the fall. Only plant lingonberries under a crab apple if you live in a region with warm summers. Another berry plant to consider is the bunchberry. Related to dogwoods, bunchberry plants produce bright red fruits after flowering in the spring, and its foliage turns yellow and red in the fall. Bunchberries grow to about 10 inches high.