( ( ( ( ( ( A weed ) ( is ( ( any plant ) ( growing ( where ( it ( ( is n't ) wanted ) ) ) ) ) ) ) , ) but ) ( usually ( these ( are ( ( the plants ) ( that ( ( emerge naturally ) ( following ( soil disturbance ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT A) (NN weed)) (VP (VBZ is) (NP (NP (DT any) (NN plant)) (VP (VBG growing) (SBAR (WHADVP (WRB where)) (S (NP (PRP it)) (VP (VBZ is) (RB n't) (VP (VBN wanted))))))))) (, ,) (CC but) (S (ADVP (RB usually)) (NP (DT these)) (VP (VBP are) (NP (NP (DT the) (NNS plants)) (SBAR (WHNP (WDT that)) (S (VP (VBP emerge) (ADVP (RB naturally)) (PP (VBG following) (NP (NN soil) (NN disturbance))))))))) (. .)))
A weed is any plant growing where it isn't wanted, but usually these are the plants that emerge naturally following soil disturbance.
(ROOT (S (NP (NP (NNS Plants)) (SBAR (WHNP (WDT that)) (S (VP (VBP grow) (SBAR (WHADVP (WRB where)) (S (NP (PRP you)) (VP (VBP do) (RB n't) (VP (VB want) (NP (PRP them)))))))))) (VP (VBP are) (VP (VBN called) (NP (NNS weeds)))) (. .)))