Don't know anything scientifically backed up but judging from the structure and a quick google search I'd say it should be fairly stable in pure, pH neutral water (gut feeling: half-life of a day or two?). But that is just an impression when looking at the structure... it contains a lactone ring (an intramolecular ester) which is prone to hydrolysis especially in basic or acidic solutions (it may regenerate spontanously but I wouldn't bet on that).
They speak about 'environmentally unstable' which comprises also metabolism, sun and oxygen exposure, soil adsorption etc.
Best might be to prepare a stock solution in an aprotic solvent (principally no water and no alcohols) such as DMSO, acetonitril or acetone (isopropanol as a sterically hidered alcohol might work too) and store it in the freezer .