With blooms that range from white to pink, crab apple trees put on a beautiful show in springtime, but their fruit is also useful so it is helpful to know how to tell when crab apples are ripe. Despite the significant difference in the size and taste of the fruits, crab apples are closely related to standard apples. But because of the tartness of the fruit, it is typically used for purposes other than a nutritious afternoon snack. Crab apple trees are often planted close to other types of apple trees to serve as pollinators. Some apple varieties are self-fruitful and don't require cross-pollination, but they will produce a healthier and more abundant fruit crop if they are cross-pollinated with crab apple trees.
When crab apples are ripe, their seeds turn brown, so you'll have to cut one open to determine ripeness. Crab apples start to ripen in August or September. Most varieties turn at least partially red when they are ripe and produce a nice, flowery scent. The ripe fruits are much smaller than standard apples, usually less than an inch in diameter. Taste isn't a reliable indicator of ripeness since all crab apples are tart. As a result, they are mostly used to make jelly.
Regular pruning will encourage a healthy crop of crab apples every year. Remove any sprouts, or suckers, at the base of the tree in early spring, before flowers appear. This is also a good time to trim any limbs that are dead, damaged, rubbing against each other or growing inward. Also thin out limbs that may be blocking light from lower limbs. Don't trim more than 25 percent of the tree's canopy per year, however. Healthy crab apple trees don't usually require fertilizer, but if the tree is less than 5 years old and isn't growing by 5 or 6 inches per year, apply a 20-5-10 fertilizer in early spring according to label directions. The trees rarely require supplemental watering, except during a drought. During an extended drought, water deeply every two to three weeks, applying 2 to 6 inches of water each time.
Bark beetles and borers commonly infest crab apple trees. A soil drench around the tree, using a product such as Bayer Advanced Tree and Shrub Insect Control, is an effective way to control these pests. The apple maggot can be a potentially serious crab apple pest. They tunnel into the fruit, leaving behind dark tracks through the crab apple's flesh. Apple-shaped traps coated in a sticky poison are effective at eliminating an apple maggot infestation. Peeling bark is common among crab apples and not necessarily a sign of an insect problem. As for diseases, crab apples sometimes become infected with apple scab and cedar-apple rust. These fungal diseases can cause premature leaf drop and should be treated with a lime-sulfur spray during winter and again after they start producing new leaves in the spring. If the problem persists, a fungicide called Captan is usually effective in killing any remaining fungus, according to the North Dakota State University Extension Service. If you have a lawn surrounding the tree, don't use turf fertilizer within 20 feet of the tree's base. The nitrogen in the turf fertilizer will cause more foliage growth, but it will inhibit blooming and fruit production.