You only need a mobile seed, what evolutionary advantage being a "sentient seed" brings into the picture? Maybe if you add a parasitic component to it, like immobilize/kill some animal and bring the body into an appropriate place before seeding it? At least this way some elements of world perception and tactics may be necessary.
Another problem: "sentient" vs "intelligent" - if you slide into asking even low level elements of "intelligence", you have the problem of passing the knowledge between generations: the progeny of mammalians learn behaviors for their parent(s) - well, their mum mostly.
May work if you allow the seed a longer period of growing within a social group before seeding - something like the first 30-50 years of the human life.
Something like the fertile period of life is long but the entire energy from metabolism doesn't allow for higher brain functions, so the fertile period is spent in a vegetative state, like a tree.
It is the "mobile seedling" phase when learning and "societal advances" occur, when all the "territorial wars" happen, when "intelligent choices" (or just "hormonal fitness") about the place where to "live happily thereafter" are made. Lots of opportunities for "coming at age" stories, telling about individual transitions to a part of the life when "ignorance on world matters is a bliss" and "one is living the dreams for the future offsprings".
Oops: I just realized I almost described the Pequeninos in the Orson Scott Card's "Speaker for the dead" - the only difference is the totally deep slumber in the sexually fertile period of life.
Depending on how you do it, you may need to ask Card's permission to play with his toys.