How much dirt can I safely add on top of an established pine tree's roots without harming the tree? I want to add berming for landscape interest around and near the pine, which is 29 years old so rather large. [Edit] If I add a mound of dirt in a few small areas near the drip line - just enough to tuck in a few young plants, not over the entire root area - do you think that would cause any problems with the tree? I'm talking about a 2' diameter planting in a couple of spots near the edge of the root area which is 22' in diameter. Also want to place a few 1to 2 foot rocks here and there within the drip line, would that be an issue? Thanks for your input!
Many people would say that you can't do this safely. If the roots are showing the tree is responding by bringing the roots to the surface to get oxygen since the soil is poorly draining or if the top soil is very shallow. If you do put dirt on top ( 2 inches ) of the roots and it doesn't kill the tree, then you may find the roots rise up again so that you're back to stage one.
What you can do is put a mulch down which you keep away from the trunk to prevent rotting the bark. But you won't be able to plant in it as if you can establish something growing within the drip line, you'll be depriving the tree roots of water and oxygen as you are giving it competition.
I would say zero for clay soil. however; my large pines ( 4 ft. D , 100 ft tall) had up to 6 in, of sand put around them with no problem ; done 22 years ago. Porous material like mulch should not be a problem. Big old pines like this "self" mulch up to a foot deep close to the trunk with old bark scaling off.
No more than an inch of soil or mulch over the root bed beneath the canopy of your tree is safe. Any more and your tree will be put in shock, it will try to grow feeder roots that are higher and are able to access the water but mostly your tree will be damaged.
The bed of soil a tree grows in is sacrosanct. Those roots are at a level they need to be to get water and chemistry up into the plant itself. The bulk of roots of all plants is within the top 4 to 6 INCHES of soil. Why? Because that is the depth that plants are able to get at the water and chemistry and AIR they need. Roots that grow deeper are not for transport of water and chemistry. Those roots are for support. You cover the roots of a tree within its canopy and you stand a great chance of killing or weakening that tree. You can expect disease and naughty insect invasions. Partial kill of the canopy. Weakened branches that will break off in a breeze.
Graham is correct. Forget growing anything else within the 'drip line' of the canopy. We need to talk about 'tree wells' soon.