Fall flowers, especially yellow and white chrysantemum are a typical flower used for decoration of graves during All Saints Day celebrations in the beginning of November. Chrysantemum is my personal favorite, some yellow chrysantemum are just gorgeous!. Unfortunately - it is almost specifically used for All Saints celebration and for funerals. Because of that these flowers are completely inappropriate for any other occasions like birthdays or weddings. Also other fall flowers like asters are used for All Saints celebration, but not exclusively.
The most popular flowers for funerals are also chrysantemum and kalia lily (latin Zantedeschia aethiopica, Polish: Kallia or kalia). Check selection of Polish funerals flowers. The only time when Poles really use wreaths - is funeral or All Saints Day (maybe also Harvest Feast called in Polish dozynki which is celebrated in the countrysite in during Fall after harvest). For Poles - wreath at the front of the house - would mean that somebody die - whereas wreaths in the USA are used commonly for Thanksgiving.
Christmas does not have its flower in Poland, since this is a time when there are no any naturally growing flowers. So called Christmas decorations, Polish: stroiki, consist of the piece of branch of the coniferous tree decorated with pines and other decorations - like Christmas balls and candles. See Christmas stroik. Check Christmas articles.
The most popular flowers for every occasion are roses, carnation, freesia, orchids, gerber, tulips. When I was a child in 60-es and 70-es carnation (lat. Diathus, pol. gozdzik) were very popular, in 80-es they were partly replaced by freesia and orchids.
Poles decorate their houses with simpler and cheaper flowers - like jonquil, hyacinths, daisy, marguerites etc.
Of course the variety of available flowers and their prices depends on the season. In the past Poles celebrate so called "International Women Day" on 8th of March. This is not a good time to buy cheap flowers, yet, the sale of flowers was unusually high. The most popular and the cheapest flower for this day was definitively a carnation (clavel, in Polish: gozdzik). Since the Women's Day was replaced by Valentine's Day - carnation is not that popular anymore!
Read about Flowers for Weddings and First Communion