How fortunate that your grandmother has such lovely, mature Camellias. You should know, however, that some of the activities you mention could result in significant damage to those plants.

They have very shallow roots (as do most woody plants, but camellias especially so), which do not like to be disturbed in any way. This means not only avoiding digging into the root system, but not altering the soil system as well (grading, compaction, etc.).

I urge you to consider not planting anything under them at this time, but to spread a two to four inch layer of mulch instead. Yes, the roots exceed a 20 foot spread, but I doubt that you would kill one by severing roots at that distance. Just don't pile a bunch of construction debris underneath them, pile ANY soil all over the root system, etc. If contractors are to come onto the site, you will need to erect something around the entire root system to protect the plants and root/soil system.

I really can't speak to the removal situation, since you didn't mention how far apart these plants are to each other. Some methods of removal would be less invasive than others.

Regarding the allelopathic properties of Camellia: I know for certain that such chemicals exist (strongly) in Camellia sinensis, but don't know about other Camellia species. That would be interesting to find out! Now, THERE'S an inducement to avoid underplanting, lol!

Just remember, the more mature a plant is, the more sensitive it is to change. Disturbing the soil/root system by underplanting is pretty aggressive.

Why would I want to spread the mulch? To prevent weeds? Or aid the Camelias? There's about 12 feet between the Camelia that is the most questionable and the others - they are in a square area.

Thanks for the information already!

Mulch is the single most beneficial thing you can do for the root/soil system of woody plants, and ought to be considered essential for those that are especially shallow rooted. Aesthetically, the mulch will set off the camellias as specimens, making you less anxious to plant something under them.

A side benefit is that mulch can prevent weeds and keeps the darned mowers and weed-eaters away from the trunks of trees and shrubs! ;-)

Thanks