Botanically speaking, there is little genetic difference between the trees commonly called apples and crabapples. Both members of the genus Malus, modern apples and crabapples often have blended, hybrid genetic lines. Any apple tree that never produces fruits or develops fruits smaller than 2 inches in diameter is colloquially dubbed a crabapple by horticulturists. Crabapple fruits are edible, but often do not have the sweetness and flavour of apples, which were bred and selected specifically for the largest, highest-quality fruits.
Scab Disease Issues
Crabapples ripen on the tree just like larger apples. The small size of crabapples may make monitoring ripeness more difficult. One advantage is that usually there are many more crabapples on branches, which allows you to pluck one fruit to taste it to gauge ripeness without greatly diminishing the tree's crop. However, while most apple trees are bred with resistance to apple scab disease, many crabpples are susceptible. Scab on tiny fruits leads to dry, scablike brown skin and less juicy flesh to eat. Try to pick crabapples before fruits become too infested with scab.
The precise timing of crabapple fruit ripening varies by climatic region, weather and the tree variety. Usually by late summer, the crabapples are blushing a contrasting colour, making them stand out from the green leaves. Once birds are seen plucking crabapples from trees, they're near or past ripeness. Knowing which tree cultivar you have helps you know what colour to expect in ripe crabapples. Some trees produce red or yellow fruits, while others are a mottled blend of many colours, just like with apple trees. It's also helpful to know the mature size of the crabapple and then gauge the fruit development as it approaches the expected mature size.
Ripe crabapple tend to snap off rather easily from the twig. Expect crabapple trees to bear ripe fruits just prior to and at the same time as regular apple trees. Crabapple cultivars that yield large fruits -- such as 1 to 2 inches in diameter -- are the easiest to monitor, pick and use. Expect to harvest anytime from late August to early November. Taste a crabapple to gauge an acceptable harvest time. A ripe crabapple will almost always be much more tart than an apple. However, crisp fall nighttime temperatures help acidic juices become slightly sweeter. The crabapple should be crisp but soft, not rock hard, as you bite into it.
Gardeners must balance the ripeness and taste of crabapples with the condition of the fruits when harvesting. An infestation of worms may render the crabapple crop unusable some years, or a pesky flock of hungry birds may maim the tree's fruits. If scab or other diseases are beginning to manifest on fruits, a premature harvest may be recommended. Cut off the diseased or damaged parts of the crabapple, and use the clean flesh for making jam or jelly. Deformed or scabby fruits may still be pressed to produce cider.