I have had 15 large red tip photinia bushes ( 12-13 feet tall and very sick) removed, stumps were grinned, but many roots are still there. I have a plan to plant Savannah Holly's there again for privacy fence, it is along a back fence in a backyard. My question is , would the roots that are left interfere with my new plants, would they be able to grow? The new plants are in a 15 gallon container. Thanks to all for helping out!
I don't think these remaining roots are a significant problem at all. For a year, they will still be some a kind of physical obstacle, but in two years they will become softer than the soil around them, and in three years they will become pockets of powder in the soil - believe it or not. They will become a source of food for many underground creatures, and this will contribute to the richness of surrounding soil.. Perhaps in five years one could not find them at all. You may notice some mushrooms in the area next two years - that is normal during intensive root rotting, and you do not need to remove them at all, only if you wish.
If there are lots of roots left behind, the difficulty might be actually being able to plant the area, because when you dig to make a hole, they're in the way. If that's the case, you will need to remove roots as necessary just to get your new plants in. It would be helpful to replenish the soil as you go by working in some organic material as you dig (composted manure, good garden compost you've made yourself, leaf mould, anything like that). As long as there's sufficient soil around your new plants that isn't solid with roots from the Photinias, it should be fine. The old Photinia roots will eventually rot down, but that takes quite a bit of time if the roots are large.
The only risk from leaving in large, woody roots is possible honey fungus; there will inevitably be lots of mushrooms and toadstools appearing over time as the wood breaks down, but honey fungus is the only one to worry about.