WordNet sense 1 (weak from exhaustion):

flagging

WordNet sense 2 (depleted of strength or energy):

tired

WordNet sense 3 (hanging down (as from exhaustion or weakness)):

droopy, sagging

WordNet sense 4 (not taut or rigid; not stretched or held tight):

lax

WordNet sense 5 (having branches or flower heads that bend downward):

pendulous, nodding, cernuous

WordNet sense 6 (not upright in position or posture):

unerect