Blackberry seeds will remain viable in the ground for 6-7 yrs. When I was a kid they were rampant on the station we lived on. We used to pick them and sell them to blokes who used to come around and buy them. Used big old square kero drums made into a bucket over the shoulder. They got so bad on the station they had to use aerial spraying to get rid of them on some hills were you couldn't get in. If a fire goes over the site of an old bush you will have heaps of new ones next year.