Poppy flowers live on their plants for a period of days to a week or more depending on the cultivar. When the spent flowers are left on the plant a seed head will form that can be decorative and used for propagation. Poppy blooms can also be cut for use in floral arrangements provided they are conditioned properly to counteract the effect of the milky sap contained in the stems.
Allow poppy flowers to age and die back while on the plant and do not deadhead them if your want seed pods to develop.
Prevent the pods by cutting the spent flower down at the bottom of the stalk between the foliage with secateurs. Poppies will not re-bloom with deadheading so it is not done unless seed heads are not wanted or you feel the plant looks untidy.
Condition poppy flowers for use in arrangements by singeing the cut end of the stem with a match or lighter until blackened to prevent the sap from callousing over and inhibiting water uptake. Alternatively, immerse just the end of the stem in clear boiling water for 20 to 30 seconds. Place into the vase of tepid water immediately.