Assalamu Alaykum wa Rahmatu Allahi wa Barakatuh

If the Hawl is due, i.e.,  the Zakah is due on your father before he died, the amount of Zakah for that year becomes due on him. In this case, the heirs must pay it like other debts to other persons. Some scholars even argue that the debt of Allah is stronger in the requirement to be paid than the debts to people. Also there are scholars who say that now the property belongs to heirs and they are not responsible for the neglect of the deceased. Well it is true technically! But He is your father and even if you make Sadaqah on his behalf, he will be rewarded for it, Zakah is definitely more important.

Wa Allahu A’alam