The airspace issue is the critical one I think. The other points you raise might be fixable.

Bark breaks down too. I am not sure that is a unique attribute of peat. Organic materials tend to break down in general.

Regarding breaking down, the studies I have seen so far suggest that if you keep the organic fraction of a soil to 30% or less, the nonorganic materials that make up the other 70% will maintain the structure. There is a commercial potting soil named Top Pot that keeps the organic component as peat to 30% and the reports I hear on that are that it maintains stability and does not compress.

I found one study with peat moss showing that addition of as little as 10% Turface completely changed the hydrophobic nature of the peat. See the chart below. The WDPT is the time it takes to rewet the peat.

The airspace issue is the one that might be decisive, but I just do not understand the mechanics of that if you have a soil with 30% peat or less how will that impair rhododendron root growth.

In any case, I am just studying various soil mixtures - including ones that use peat - and wondering about the special needs of rhododendrons. So far I like how they grow in bark and bark + Turface.

Are you growing rhododendrons in containers? Or are you planning to plant them in the ground with a special soil mix? I use regular soil when I plant rhododendrons in the ground. Them seem to do ok.

@ctgardenguy I am containers only. We have hard clay soil here and I don't think rhododendrons would do well in those.

I was under the impression that growing rhododendrons in containers was a bad idea. There are lots of rhododendrons that I would like to grow, but they wouldn't survive a zone 6 winter. Do you have any advice? Do you store your container in a garage during the winter? I imagine you don't have that problem.

Note that original poster is located in USDA 9, where winter cold will almost never be a problem for any but tropical rhododendrons in containers. Otherwise lots of them are produced commercially in pots but plastic tunnels or other shelters will be used to overwinter them in perfect condition, in those areas where that seems necessary.

Digging special beds for heath family plants requires that these be big enough for the plants to not be subject to adverse soil water conditions where the amended area interacts with the existing soil around the edge. And as always when planting long term plants in amended soils there will be a problem in time with the organic component having broken down. Same as when potting soils break down and plants have to be re-potted.

So we are left with it really being necessary to remove and replace unsuitable existing soils entirely. Or placing suitable soils on top of existing soils, without blending the two levels together. Using soils that have suitable proportions of mineral components present within them, rather than relying on incorporation of transitory organic materials to provide a lasting correction.

@ctgardenguy I am blessed by California weather, and we would rarely have a freezing day here. I guess there are varieties of rhododendrons that can survive down to 0 Fahrenheit, so we are well above the minimums here in zone 9.

ctgardenguy: it is not a bad idea to grow rhododendrons in containers. I have been doing it for many years for mostly species that would not be hardy enough planted in the open ground. What is important is to realize that there is much less margin for error in the small volume of planting mix in a container. This means close attention to watering and repotting every 2 to 3 years. The so-called 5-1-1 mix is a good place to start for containers. This will remain open and not break down too quickly. In sunny areas, be cautious about using black containers that can overheat and damage roots. Controlled release fertilizers applied sparingly each spring are probably the best choice for fertilizer. A thorough watering by immersing the container then allowing it to drain is a much better approach than sprinkling frequently. Keeping the container elevated for drainage and preventing the bottom of the container from resting on soil are important ways of avoiding diseases.

akamainegrower - thanks for the info re containers. I am growing Japanese maples in containers using the 5-1-1 mix so I will do the same with rhododendrons. Is it best to use a container that is wide vs deep to allow the roots to spread out? I store my container JMs in the garage during the winter with very little water while in storage. Do the same for rhododendrons? Do you grow vireyas in containers? Thanks.

@ctgardenguy At least 18 inches deep and wide is probably a good strategy. I am having luck with oak barrels.

Container depth is an important factor in providing good drainage. Rhododendrons will adapt to the horizontal dimension of a container within reason. If you have to move the containers into another area for the winter, weight also becomes an important consideration. I would not want to be wrestling with an 18 inch by 18 inch oak wine barrel filled with moist planting medium. If there's easy access to the winter storage area and you can use a hand truck for the moving, things would be easier. I use the terra cotta colored Panterra plastic pots available from Greenhouse Mega Store.These are a pleasing shape, light in weight and last a long time. Don't be tempted by putting a small rhododendron in an overly large pot to save steps. Repot into a larger size in gradual increments. My largest rhododendrons are in 12 or 14 inch pots. These include r. cinnabarium, r. sanguineium and the beautiful hybrids, Coudburst and Orange Marmalade. Roots can be trimmed at repotting time when the plants need it. Good for their overall health and enables them to keep growing in a manageable sized pot. No vireyas. These would require too much artificial heat in the winter. Rhododendrons are kept in a lean-to greenhouse for the winter maintained at just above freezing. They are allowed to go dormant, as much as rhododendrons ever go truly dormant, outside before being moved in.

Just to deal with real dimensions, my oak barrels are about 18 inches tall and 26 inches wide. As shown in the photo below, I placed them on leveled pavers, to increase the lifespan of the container and prevent the soil below from rotting away the oak. Since I leave these containers outside during the California Winter, I worry less about the weight. I do not think these containers could be easily moved.

westes - that's a nice looking setup you have for your rhododendrons.

akamainegrower - thanks for the detailed response. I am definitely going to buy some rhododendrons to try it out.