It's not horseweed. I pull that out all the time. Leaves/stems are not hairy, but have a slight sheen (I took the above picture in the rain, so ignore the shinier raindrops). Stem is square in cross-section. Leaves are opposite.

This first sentence is the answer: "If it is really hot, then watering, and spraying the leaves, every day may well be necessary." I'd simply add that you're dealing with transplant shock and that it is very normal under your current conditions for the entire tree to defoliate but to also, come back. It is entirely possible the tree may re-foliate this year even. Or, it may send most of it's energy to the rootball at this time. It's the tree's choice basically. I believe that keeping up on water will keep it alive on both top and bottom. When, or if you notice the buds dying, then expect the birch to sucker and as mentioned above, you'll need to remove any portions that do not re-foliate next spring and select new emerging growth for the style you wish for (clump or single trunk). If you have a cultivar of River Birch, then the new suckers that come up may not be be true to cultivar form but since you did not mention this, I won't assume. And this only applies for grafted trees. Trees produced from cuttings (I'd have to look River Birch propagation up to know for sure) - will produce suckers true to form. Yes, they are produced from cuttings. Dirr and Heuser (2006) confirm this. Dax Dirr, Michael A and Heuser, Charles, Jr. ÂThe Reference Manual of Woody Plant Propagation: From Seed to Tissue CultureÂ; (Second Edition 2006); Varsity Press, Inc. ISBN: 0942375092 or ISBN: 13: 978-0-942375-09-1

It has been a long while, but I remember the white berries didn't have any blue, and were larger in size and had a different flavor. If the white part was rubbed off (not easily done), or the berry opened (easier), the skin was not colored. There were blue berries that did form a whitish film, but they were obviously blue underneath. Bushes were consistent in which color berries they formed year after year. The largest plants always seemed to be of the dark-berried type, but that could have been a coincidence. My observations spanned many years, but I had no formal botanical education at that time. The area burned in several wildfires, and I have not revisited where the light colored berry bushes were seen. But since the discussion was continuing, I thought I'd add my recollections. If blackrab is willing to share a more detailed description of the location, it may be possible for me to check the plant she photographed during a future trip to San Diego County. Of course, the birds may have eaten them all or something, but throwing it out there as a possibility. I have a trip planned in 2 weeks, but I will have other people along, and don't know where the location of the plant is in conjunction to where I will be going.

There is some general similarity to Weigela although I won't say it's a perfect match. If you think it might be the same as one of these other shrubs nearby try going and holding it against them, seeing if it looks exactly the same in leaf and twig characters. Then you will have a larger, more developed example to use in order to arrive at an identification. One that is showing the adult growth habit, has been producing flowers and maybe fruits. These are all features that can be used for recognition.