This isn't really science fiction anymore:
Flowers regenerated from 30,000-year-old frozen fruits, buried by ancient squirrels
While animals are still difficult to clone, cloning plants is a far different matter. If you have a clean piece of living tissue, even a very tiny one, putting it in a sterile petri dish with the right combination of plant hormones can cause it to sprout roots, leaves, or both.
Even before we learned to do that in the mid-20th century, orchardists have been cloning fruit trees by grafting branches onto host trees since before the Roman era. Not relevant to your question, just giving you some background.
But back to the petri dish thing, it's typically called micropropagation. Researchers use it alot when they need to save a plant, or to create identical plants for experimental controls.
Now, one of the neat things about seeds is that the embryo inside of them is basically in suspended animation. In the right environment, they can last decades or even centuries. I think the record for germination was for a seed well over a thousand years old (found in some Middle Eastern tomb). The seed they found in the Siberian cave was 30,000 years old, give or take. It was not viable and would never have germinated on its own... the embryo had died. But we all know that (complex) life on Earth is composed of multiple cells. The embryos in mature seeds aren't in the single-cell stage either. And one of those cells in the embryo must have still been alive and dormant.
After carefully cutting the embryo out of the seed, dissecting it with a scalpel (under a microscope), they sanitized each little piece so that there were no bacteria or molds on them, put it in a dish, and added those plant hormones I spoke of. In a few days it grew larger, until tiny roots and leaves had formed.
Now, that's not quite 70 million years old. But we're very close to that. If a flower or seed were to be trapped in amber, well, we're not even sure how old is too old yet.
And in the fiction of Jurassic Park, they had the ability to piece together DNA... if dinosaur DNA were to survive such a thing, pollen or other plant tissue would surely preserve some plant DNA. Even if chunks were missing, they'd do what they did with the rest, add in plausible DNA from modern plants to fill in the gaps.