The flowers come from corymbs , and have five petals , which may be white , pink or red , with usually red stamens that produce lots of pollen , and an ovary which is below them ; flowering takes place in the spring after 50 â `` 80 growing degree days ( this varies a lot according to subspecies and cultivar ) . The flowers are borne in corymbs , and have five petals , which may be white , pink or red , and are perfect , with usually red stamens that produce copious pollen , and a half-inferior ovary ; flowering occurs in the spring after 50 â `` 80 growing degree days ( varying greatly according to subspecies and cultivar ) .