The roots of a plant extend out just beyond the drip-line (the edge of the canopy), so your moat should probably extend out as far as possible to that point.

I'm not at all familiar with growing conditions in your region, but it sounds like you may be watering too frequently. Good observation on your part (an important quality in gardening, by the way). If it's a newly planted tree (within the past two years), check the moisture level 2" below the surface - if it's moist, hold off on watering, if it's dry, go ahead and water. You can help stabilize soil temperature and moisture level (thereby conserving water) by applying about 2-3" of organic mulch (not rocks) over the root zone of the tree, leaving a little space between the trunk and the mulch.

These are all plantings from this spring. Doesn't that mean the drip line method is not the best? My thinking is that I want the water to enter the root ball, then spread out, rather than trying to spread into the root ball when the water runs off it's domed. Is that good thinking? So I would want the moat to be at the edge of the root ball.

As to moisture level underground, I bought a 36 inch probe. The landscape person said to check 10-12 inches down. I'm on a well, so I don't need to conserve water, and I prefer the natural look, so just natural dirt on top. Doesn't that mean I just need to water a little longer, especially on hot days?

An issue here is the soil. I'm on an ancient riverbed, and the soil has lots of clay and small to medium rocks. When I probe, I can't always tell the difference between the point where soil becomes dry or if I run into a clay layer. Rocks I can tell.

OK, here are links to two websites. The first, from Oregon State Univ. Extension Service, focuses more on fall into winter care, but good info here. The second is from Portland Parks & Recreation. Both should be good, reliable sources. You will see that both recommend mulch. Mulch can look natural, too - undyed wood chips, shredded leaves, finished compost, are some examples. Stabilizing soil temps and moisture levels will benefit any plant, and, even though you're on a well, it is wise to conserve water ... you're drawing that water from the groundwater supply, which has to be replenished and, unfortunately, in many areas we are depleting it more rapidly than it is being replenished. Water from rains (stormwater runoff) is often diverted to surface water supplies (streams, lakes, reservoirs) before it can seep into the ground to replenish the groundwater supply (an aquifer - sort of like an underground river). This is why rain gardens are so helpful; they capture stormwater and allow it to drain slowly (within 24 hours) back down to replenish the groundwater supply. In the process, the growing medium and the plants filter out pollutants. [sigh] Sorry, lecture over, soap-box put away.

http://extension.oregonstate.edu/gardening/how-care-newly-planted-trees-fall-winter

https://www.portlandoregon.gov/parks/article/454952

It is very common to construct moats to hold water around new plantings and of even more importance when planting on a steep slope. I generally recommend the moat extend out several inches from the edge of the rootball but how much kinda depends on the degree of the slope!! As long as the rootball is being adequately hydrated with each watering, the precise distance is not critical and it should be fine.

How often to water is an entirely different issue and one that can't be answered easily. It depends on your soils, your weather and the specific plant in question. It is always best to water infrequently but gradually and for an extended period (the hose left on a slow trickle). This allows water to penetrate the soil profile to an adequate depth (the 10-12 inches suggested by your landscaper). Then allow the soil to dry on the surface 2-3 inches completely before watering again. Even for newly planted trees, daily watering may be too much. I'd check daily and adjust your schedule as needed but 2-3 times a week or so should be about adequate until we hit our long, dry season further into summer. Then you may need to bump up your schedule.

Thanks Gardengal. All my plantings are native, so hopefully, except for drought, I'll need to water just this summer and next.

Thanks for the moat advice.

I just probed all the new trees, and except for one they all have about the same 10ish inch easy push, so I think I'm starting from a good place, where everything is watered. With the cool and showery weather (I got just .3 in the past three days), I'll stop watering until the top soil drys out. Is the only real implication of not using a covering like mulch is that it will dry out faster and need water maybe a day earlier than otherwise?

How about the small plantings, like the ground cover containers (e.g., kinnikinnik (my Mac wants to change that to skinniness :)) that were probably 2 inches square, and the smaller starter plants? Is the concept of letting the top soil dry out some before watering again key, but the lesser/shorter the roots, the less top-drying depth?

Smaller creeping plants like the kinnikinnick have a much shallower root system than do trees or larger shrubs so they can certainly dry out faster and you may have to water a bit more frequently for them. OTOH, kinnikinnick is a real trooper and very drought tolerant so a bit less inclined to overreact to a day or two of being dry :-)

Mulch will certainly help to slow soil moisture evaporation. Depending on the type of mulch covering and its thickness, it could add several days to the between watering interval. And it will also significantly slow down any weed development as well as add to soil fertility as it breaks down. All in all, mulch is a really good thing and even with just native plantings, I would encourage its use. At the very least, it will reduce the watering requirements to some degree and allow you to avoid climbing your slope to weed as often :-))

Thanks! I am considering placing sod around the trees, so the grass goes right up to the trunk. Maybe a darker green color which would contrast with the lighter green of the rural "grass." What do you think of that?