You really do not want or need to use a potting soil mix for inground plantings. It's far too expensive to replace garden soil with something like these mixes and they are not intended for this sort of planting situation anyway.

Clay soil is not as bad for planting as everyone makes it out to be - it tends to be moderately nutrient-rich - just somewhat difficult to dig and often slow draining. And many plants are very clay-tolerant, including the photinia. The easiest and fastest way to lighten and improve drainage in clay soils is to add in generous quantities of organic matter - things like compost, composted manures, shredded leaves, lawn clippings or even wood chips.

Your photinia doesn't have a disease - it has a mite infestation. That dotting or stippling on the leaves is extremely characteristic of mites. Applications of Neem oil or horticultural oil should help in their control. And the plant also appears slightly chlorotic, so you might want to check your soil pH - photinias prefer a slightly acidic soil.

These particular hedges have extremely high value to me because they establish the privacy of the front yard garden. Therefore I am willing to invest in replacement soil in order to maximize growth of the hedge. Currently the hedge grows very slowly and looks sickly. I don't mind spending money on soil replacement.

Regarding the compost, manure, and wood chips, you are recommending these as a top soil around the base of the plant?

I look at a mite infestation as a disease. If a human has a parasite, that's normally treated by a doctor as a disease. So there is just semantics there. I'll research the neem oil and any advice on application I appreciate.

Thanks for information on chlorosis. But possible causes of that are poor drainage and root damage, both of which might be happening because of the clay? It could also be nutrient deficiencies of minerals, so maybe I need a fertilizer plan. How should I improve the acidity of the soil?

I am happy to address some of your issues but the Container Gardening forum is not likely to get you much additional input. What is going on with your shrubs has nothing to do with any issues pertaining to container gardening or using or replacing with container soil mixes. If you find my comments not fully helpful, I'd strongly suggest you repost this in the Shrubs or even the Soils and Compost forum.

Redtip photinias, Photinia x fraseri, is known to be tolerant of clay soils - in fact it is included on lists of plants recommended for clay. So I doubt the issues with your plants are confined to their being planted in clay. Unless the clay does not drain adequately. And as you indicate the hedge is established for privacy, I am assuming they have been in place for some time so you would have noticed drainage problems - like puddling or standing water for periods of time - before now.

It is seldom practical or cost effective to remove and replace soil. And typically unnecessary as well. One amends the soil, adding to existing soil whatever amendments/soil additives may be necessary to affect improvement. With clay, that usually means adding the generous quantities of organic matter I mentioned in my previous post, to both lighten or loosen the soil and improve drainage. Using these products as a topdressing can help but it is not as efficient a method or as fast as incorporating the OM into the existing soil. Unfortunately, that means you need to remove the current hedge to do so.

As an aside, if you were to replace the soil, you would need to do the same thing, so I am assuming you are not adverse to that. FWIW, if that's the plan, I'd consider some alternatives to the redtips, perhaps even one of the California native plants commonly used for hedging, as they tend to be well-adapted to both your climate (and drought) and native soil conditions, including pH.. And will generally have less issues than the redtips, which are plagued by some serious disease issues that are significantly reducing their suitability for landscape plantings in much of the US.

Your plants are definitely stressed - the mites and the chlorosis are clear indications. The clay may or may not have much of a bearing on this - it's almost impossible to say for certain without seeing the plants and the site in person. It could just as well be that these particular shrubs are just not well suited to your situation. One thing I would suggest is to bring in a horticulturist for an onsite consultation - your extension service or better garden center should be able to give you some referrals. These folks are experienced and skilled in evaluating plants for both siting and cultural issues as well as diagnosing insects, diseases and nutrient problems. I know these sorts of folks exist as this is a significant part of what I do for a living :-)

If you do care to repost on another, more relevant forum, please include photos of the entire hedge and at least one plant in its entirety, as well as any detail photos. The more we know about the situation, as limited as that might be just via photos, the better the advice you will receive. And include exactly where you are located and how these plants are watered and the frequency.

@gardengal48 would you have any references on this photinia mite disease that you were referring to based on the photos above? This is definitely not Entomosporium Leaf Spot or Powdery Mildew?

Mites are an insect pest (technically an arachnid), not a disease. And they can infest just about any BLE, especially those that are stressed. They also favor warm and dry environments. Sometimes just hosing down the plants periodically is enough to deter the mites as they dislike humidity but treating with a horticultural oil will control them as well, although spraying must be very thorough and get the underside of the foliage and the stems as well.

If you need more info, Google 'spider mites'.

Ditto what mblan said about bowls and raised beds. When I dig a hole I use posthole diggers to 'square' the sides. That is, to make the sides perpindicular to the surface. I believe this vastly improves drainage. Depending on the plants needs and site conditions some of my plantings are holes, raised bed or a combination of both.

Actually, a very wide but shallow planting hole is the suggested method when planting in a clay soil. Sides should never be perpendicular but very sloped or dished out to the widest point at the soil surface and always roughed up so that both water and roots can more easily penetrate the native soil. Then site the plant in the hole high, with the top of the rootball above grade by several inches. Backfill only with what soil was removed from the hole, sloping it up to the edge of the rootball to form a small mound or berm.

This method is advised as it minimizes any possible "bucket effect" from the clay and allows proper drainage with any excess water accumulation being wicked away by the surface exposure and whatever mulch is being used. Deeper, perpendicular planting holes do NOT improve drainage and will remain as buckets.......where else is that water going to go?? Especially if one does not remove the very water retentive potting soil that surrounds the rootball?

Works for me. :) I have massive oaks and other trees planted precisely this way over 20 years ago. Not to mention countless shrubs and perennials in recent years.

@kyle no matter how you cut the planting hole, the drainage stays the same. The drainage is inherited from the surrounding region of soil as the perched water table and the capillary action of water in the surrounding soil. That's why you get the bathtub effect. It took me a while to understand this, but the drainage is not established by the soil that goes into a planting hole that is surrounded by clay walls, nor by how those walls are cut. The drainage is established by the clay soil around the hole.

The physics of a raised planting bed allows it to overcome the perched water table and pooled water from capillary action of soil by sitting on top of those things instead of side-by-side, or inside of them.

I guess many types of plants will survive, establish, and thrive in clay no matter how you plant them, given enough time.

Possibly. I still consider my method the best for me after 40+ years of planting. I do prefer raised bed though wherever and whenever possible.