It is considered special if we offer flowers grown at our homes or pluck flowers from other places than buying them outside.
Buying flowers for money from a shop is called krayakreetam, an indifferent act. We can purify the store-bought flowers by chanting ‘bhuH bhuvaH suvaH’ or by chanting Tiruvashtakshara mantra and sprinkling little water only as there is a rule that lots of water should not be sprinkled on flowers.