daylilles maybe?

How big is this tree? My mother has a 40+ft tall Norway spruce in a SE corner of the yard next to a chain link fence and she had me plant a couple forsythia divisions along the north-facing fence partly under the canopy of that tree and literally about 6ft from the trunk. It was a JOB to dig between those darned roots but they have been there at least 7 or so years now and established just fine, covered in blooms each spring. There are also a couple rose of sharons near and under the same tree canopy, although those had been planted earlier in the tree's life.

The problem with the site is that it can get dry under there. Some other things that have handled it though include pachysandra as a ground cover and a bulb bed with daffodils and tulips.

If you actually have a pine - my mother has one on the opposite side in the NE corner and that also has some pachysandra growing around it and had bamboo along the fence on that side that happily spread and popped up under the pine canopy (the bamboo is now being removed). It seems that side is even drier than the spruce side though.

The daylily suggestion could work because in between those trees are, daylilies all along the eastern wall, some having been there for decades. In addition, there are roses behind those along with some rose of sharons. In order to get the sun for them, the trees were gradually limbed up over the years to open the canopies as they now overlap between the 2 trees that were originally planted about 30 ft apart.