There is a greater number of young, fine roots farther from the trunk. Water and dissolved nutrients can only be absorbed by those fragile root tips....sometimes called the 'feeder roots '. A well established root system will extend far beyond the drip line.

I prefer to water at the dripline or even further out with the goal of forcing the roots to grow out to get the water.

For young trees in the first two years, I water from the trunk all the way out about a foot past the dripline. A good slow soak once every two weeks when we're having no rain makes them very happy. (Amount of water sometimes dependent on the tree.)

I use 5-gal. buckets with very tiny holes drilled into the bottom. Fill 6-8 buckets per tree, set over the rootzone, and the buckets drain within a couple of hours.

i think you guys are missing the forest thru the leaves....

i THINK the OP is asking.. and i have often wondered the same ...

if i have a tree with borers... and want to use a systemic DRENCH [as OP said] ...

why apply it at the trunk AS PER INSTRUCTIONS ...

rather than worry about doing it at the drip line ...

wherein of course you would need a billion more gallons.. on a good sized tree ...

how can the tree suck up the systemic.. at the trunk.. where all the mature roots are.. and there are no feeders ...

OP?? .. is that the crux of your question???

and even if it isnt.. answer it for me.. please ...

ken

I applied my EAB treatment at the trunk. Bet it is a compromise of how much imadichloropid (sp) you woukd have to use at the drip line. Also at the drip line the ash would be competing with the dogwoods for it where at the trunk it is only the ash and a couple weeds.

When watering new transplants at first I just do the planting hole. Later if all is going well I also try to encourage the new growth of roots by watering outside the drip line.

FWIW, when I water the larger trees which is only in time of drought, I just plop down the sprinklers and soak the whole area for six hours or so.