Organic mulch, such as wood chips, chopped leaves (not whole), shredded bark or the like, offers many benefits to trees. A layer 2 to 4 inches thick spread over the soil under the crabapple tree--but not up against its trunk--discourages weeds. For more information see Using Mulch.
Of course, when they are first planted it is essential to water crabapples frequently and deeply to help them become established. However, established and mature crabapple trees do not usually need supplemental watering if they are in soil that has some organic content to help it hold moisture and/or they are mulched. In times of drought when regular rainfall is sparse, it is very important to water even well-established trees. Every week or 10 days check the soil under the mulch. For more information see the file About Watering Equipment
Do not overfertilize since this could increase the incidence of disease. Crabapple trees need only one light feeding a year during their early years. In the spring sprinkle all-purpose, slow-acting granular fertilizer on the soil under the tree out to 1 1/2 feet beyond the tips of the branches (the dripline). Do not allow the fertilizer touch the trunk. Normally about 1/2 pound or less of fertilizer for each 1/2 inch of trunk diameter, measured at its base, is sufficient. Do not fertilize at planting time, nor during a tree's first season. After the first 4 or 5 years, annual fertilization is not necessary, especially if the crabapple tree is mulched. Trees grown in the lawn have to compete with the grass for soil nutrients and the grass usually takes more than its share, so they may need fertilization every couple of years. Trees that are mulched are spared this competition and benefit from the nutrients the organic mulch material provides as it decomposes. For more information see Fertilizing Trees and Choosing Fertilizers
Winter mulch helps moderate fluctuating soil temperatures that sometimes cause soil to heave and disturb roots of newly planted young trees.
Crabapple trees require some routine pruning. Start establishing well spaced branch architecture soon after planting. Do this with restraint; with crabapples overpruning is worse than none at all, because it stimulates so much growth. Some crabs tend to develop water sprouts that crowd the interior of the leaf canopy and look unattractive. Cut off these weak, vertical shoots that pop out all along major limbs. If your tree is a grafted type, it may send out suckers from the roots that can, left unpruned, develop into a different type of crab and overwhelm the one you bought. Prune suckers in late spring after the main growth spurt is over. Watersprouts (shoots arising in a vertical fashion from the arching branches or trunks) should also be removed on an annual basis, as they will crowd the interior of the tree with crossing branches.
Suckers (shoots arising just below the graft collar at the base of the trunk, or directly from the roots near the trunk) should be removed on an annual or semi-annual basis, as they represent the rootstock (an entirely different Crabapple or Apple), not the grafted Crabapple cultivar. For more information see Pruning Shade and Flowering Trees and Choosing Pruning Tools
Also, be alert for the occasional diseased or damaged branch and remove it with a clean cut promptly. Be sure and disinfect tools if they have been used to cut diseased limbs or twigs. Dip pruners, loppers or saw in a solution of hot water and household bleach. Cut out branches that cross one another, rubbing their bark off and risking disease. Prune the trees when they're leafless and dormant during the winter. Branches removed before flowering in spring can be forced to bloom indoors, otherwise anytime after flowering is fine.