The invention relates to the use of a water-soluble metal salt as a tracer substance in a treatment product for cut flowers, said water-soluble salt being a salt of a metal selected from lithium, molybdenum, boron, cerium, silver, selenium, strontium, tin, cesium and lanthanum.The invention also provides a method of determining the concentration of a treatment product in water that cut flowers are placed in, said method comprising the steps of:• providing a treatment product containing a known amount of a tracer element selected from lithium, molybdenum, boron, cerium, silver, selenium, strontium, tin, cesium and lanthanum• applying the treatment product to water that cut flowers are placed in and• quantitatively analysing the concentration of the tracer element substance.
