Never amend the soil near a manzanita. The nutrients will cause the mycorrhiza to detach from the roots, leaving the plants open to attack by pathogens. Manzanitas strongly associate with the ectomycorrhiza in the first four or so inches of the soil. Unless your house is in a newer subdivision, where the top of the soil has been recently scraped away and then bulldozed back into place, there is likely mycorrhiza in your soil. This is particularly true if you have other plants that have mycorrhizal associations. If you have several of these plants in the ground, you may have a mycorrhizal "grid" in your soil. I took out a couple of diseased Myrica californicas and planted a Howard McMinn manzanita in between the two ground-down stumps back in March. The Howard McMinn grew from the size of my hand to 22" x 30" in seven months. That said, I don't see how inoculating one's soil with more mycorrhiza would hurt. The best way to enhance and maintain the mycorrhiza population in your soil is to mulch heavily with bark from plants that have mycorrhizal associations. The bark provides nutrients to the fungi. Oak, pine, fir, and Doug fir bark mulch all work really well. Spread a 3" deep layer in a 3' diameter around where you're going to plant. Water heavily a few times over the course of a couple of months, and your soil should have a strong mycorrhizal population. Add fresh mulch every spring. It's also important to note that it is possible to under-water a manzanita in its first year (I've done it). Manzanitas planted in the spring will probably need water every 10 days or so during that first summer. From June-October, stick your finger a few inches down into the soil. If it's dry, water it (around the perimeter of the root ball). If the soil is moist, check again the following week. In addition, some ESTABLISHED manzanitas do best with at least some summer water. Some of the A. hookeri cultivars do best with water every 10 days or so. On the other side of the spectrum, A. glauca Big Berry prefers no summer water. Some species, such as A. densiflora, have cultivars with very different water tolerances (Howard McMinn can take once-a-week water, while Sentinel doesn't like more than once-a-month water). Be sure to water in the early morning, when it's cool outside. Watering at 2 pm when it's 102 degrees will promote root rot! Below is a link that contains water requirements. Here is a link that might be useful: Manzanita requirements

Ian, it looks like manzanita gets ratings that range from moderately deer resistant to highly deer resistant, depending on which manzanita. Unfortunately, I find deer don't read well, and it's hard to tell what they might snack on or decide to taste in any given year. I've had them shear my evergreen azaleas - only once, but all of them and while standing on hellebores. Another season, allium flavum was tasty. I haven't figured out what the attraction is to my neighbors boring laurel, but deer push right in up to their shoulders and happily strip off entire stems. Elk are a larger problem, literally larger...just their weight alone can cause more damage in a garden. Have you seen elk or signs of elk near yours? BTW, I find elk infinitely better tasting than venison - I prefer ground elk over beef.

In southern Arizona, manzanita only grows where the elevation reaches a certain height (couldn't tell you what it is, but town here is about 2,000 feet and it's probably a thousand feet or so higher than that before you see any in the mountains).

Zone 9 where? Really, the Sunset zone would provide a much clearer picture of your conditions. I live in Zone 9/Sunset 23. Clay soil can be a problem with Arbutus Marina. The lack of drainage makes them even more susceptible to Phytophthora. Even with good drainage, Phytophthora can be a heart breaker with these lovely plants. I have lost three growing in huge pots; one tree, in the ground (sandy soil), remains healthy (knock on wood). Arctostaphylos prefers sandy/rocky soils. Emerald Carpet also prefers acid soils. I haven't tried Emerald Carpet, but in my area (south Orange County CA), A. edmundsii ('Little Sur') and A. uva-ursi 'Point Reyes' are thriving in very bright shade/protected sun. The former beside a shed, and the 'Point Reyes' under Iceberg rose bushes. Not sure they would be thriving in full sun here. These are being irrigated with drip. Howards have not been easy in our garden. We have lost three out of four of the 1 gal. plants tried. But our watering system is drip, and these guys are known to hate drip. How are you planning to irrigate?