Good Morning,
Crabapples do not need to be sprayed to flower.  
In the spring, the showy blossoms make their appearance before the lilacs bloom. Although the actual time of bloom will vary from year to year, depending on temperature, a total bloom period of up to four weeks can be expected. Very-early blooming crabapples may flower as early as the star magnolia, while late bloomers may coincide with the black locust. The bloom period of an individual cultivar may vary from two days to almot two weeks depending on weather conditions.
Crabapple buds may be pink, white or red, and open blossoms may be white to dark purplish red, with many variations in between. Most crabapples have single flowers, but a few have semi-double or double blossoms.
Some cultivars bloom heavily only every other year. Avoid these alternate-bearing trees. The majority of crabapple cultivars produce consistent flower displays each year.

Crabapple Selection Guide - MSU Extension

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