AusBonsai

Don't tell him he's bonkers. Rather ask him to substantiate . . . . . Perhaps we learn something. Perhaps not.

Water running down the trunk of a tree? Like when it rains? In nature? Scary!

Cheers,
Mojo

It is possible you or they have got 2 different concepts mixed up.
Many plants do produce substances that inhibit other plants from growing. It is called allelopathy. Many plants produce biochemicals from leaves, bark or roots that are released into the surrounding soil and suppresses the growth of other plants so that the resident has less competition. Obviously it would be counterproductive to produce a growth inhibitor that stopped yourself from growing so the plants are resistant to their own biochemicals. It only affects other species or young seedlings.
Oats, Allocasuarinas, Eucs, Pines are just a few of the plants that produce growth suppressing chemicals to inhibit competition.

IF this thread is referring to allelopathy the effects do not inhibit the plant itself so no problem. If this friend is talking about some other effect, I don't know about it and agree with others - no problem ever observed and all my trees are watered overhead.
Also consider that you may have misunderstood the concept before telling friend he is bonkers (you end up looking a bit foolish yourself)

[quote="Mojo Moyogi"]Water running down the trunk of a tree? Like when it rains? In nature? Scary!
Hi Mojo I am moving to your area if you get rain every day .
Seriously though Legoman if you water your oaks foliage everyday ie down the trunk . You could be creating an environment with high humidity which could cause problems with powdery mildew . There is usually some reason why old wife's tale are born .