Top cut off:
If the top of the Hadas [its branch and its leaves] has been cut off, some opinions rule the Hadas nevertheless remains valid. Others rule it is invalid. Practically one should be stringent to use another Hadas, if it is available. If another Hadas is not available then one may use it with a blessing. If the top leaves alone have been cut (or fell) off but its stem is still intact, it may be used even initially.
 The reason: Now although whenever the head of one of the other Minim have been chopped off the species is invalid, nevertheless by a Hadas it remains valid. The reason for this is because the leaves of a Hadas cover over the top branch that was cut off and it is hence not noticeable that the Hadas has lost its head and its Hadar is thus not readily effected. [ibid]
 The reason: As since the head of the Hadas is not Hadar the Hadas is invalid. [ibid]