Hello,
How you harves the plant depends on what you want to use the plant for. I harvest some of the flowers in mid-late summer (in the NE) and put them in a jar. I compact the flowers pretty well and fill the jar with olive oil. I leave the jar in the sun for 6 weeks and after the oil has turned red I remove the flowers and use this oil as a rub for musle pain/spasms. It can also be used as a sunblock and to treat sunburn. I find it to be very effective, and it does not kill the plant.
Amy