2-Amino-4-(hydroxy) (methyl)phosphinoylbutyric acid, which earlier nomenclature referred to as 2-amino-4-methylphosphinobutyric acid or its metal salts, or acid-addition salts thereof are used as perennial weeds and brush controlling agents. The L-isomer is twice effective than the racemic acid. The perennial weeds and brush controlling effect may be obtained by foliar application over a prolonged period and may be surprisingly enhanced by using the compound with a herbicide or synergist selected from the group consisting of choline salt of maleic hydrazide, a phenoxy series herbicide, a benzoic acid series herbicide, 2,3,6-trichlorophenylacetic acid or a salt thereof, (3,5,6-trichloro-2-pyridyl)oxyacetic acid or a salt thereof, N-phosphonomethylglycine or a salt thereof, ethylcarbamoylphosphoric acid or a salt thereof, 2-(1-allyloxyaminobutylidene)-5,5-dimethyl-4-methoxycarbonyl-cyclohexane-1,3-dione, 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea,3-amino-1,2,4-triazole, choline or a salt thereof, and diethylamine or a salt thereof.