Planted in Oct 2015 they are not established. It will take up to 2 years to established.

Once established it is typical for them after flowering to shed some leaves. Mine has shed every year and it is 8 years old and over 20 feet across. Depending on your soil etc it should need water twice a week.

Post a picture it might help us help you more.

Are you on a hill or slope? Is the drainage good? Have you had some heat waves?

One thing comment I was given recently at a native plant nursery is the problem generally can be watering when it is too warm, rather than just watering per se. Heat + water creates the problem--bacterial/soil pathogen rather than just water, because soil pathogens are generally much more active in warm soil than in cold. Food for thought--I do not know if this applies in your case.

Every Ceanothus I've lost has been due to not enough water, rather than too much, but I have light soil, not clay, and slopes.

Here's a picture of one of the plants in question:

It had even more yellow leaves earlier; many of them have fallen off. I haven't watered it since I made the original post--been afraid to. The yellow leaves aren't appearing as frequently as they did that last time I watered it, but new yellow leaves show up periodically.

It's in full sun, not a slope, but good drainage. And it has been quite warm recently.

The soil is also completely bone-dry, yet yellow leaves still appear.

I've had a ceanothus concha for about 6 years, and some leaves do turn yellow and drop off after the plant flowers, but the plant stays mostly green. Summer watering is not advised. In the link below, there's another link specifically about yellow leaves.

Ceanothus

I should add though that since I planted them in October, this is the first summer, and the link says "except irrigation in the first summer". Since it's the first summer, I probably should water it every now and then in the summer, no? (Not with drip irrigation, of course).

Yes, you need to water them through the first summer.

How often should I water them in the summer, given an average summer temperature in the 70s and mostly full sun for the plants?

Because it seems like any time I water them, even if it's been a long while since the last watering, a whole slough of leaves turn yellow and fall off not long after watering. But if I don't water them, it looks like the leaves may turn brown...ugh...don't know how to please these plants.

I have had my ceanothus for two years. They are on a drip systems. This summer both plants have had their leaves turn yellow; & have dropped their leaves. I would like to determine if I should water them more; or take them off the drip system entirely. Thanks for any assistance.