I'm looking for soil recommendations for a 22" tall container for a dwarf Pieris japonica 'Little Heath'. In recent years I've read convincing evidence that the old idea of putting rocks or other coarse material in the bottom of containers doesn't actually improve drainage, but with very tall pots filling the entire container with potting soil seems expensive and rather a waste since the roots will never reach down that far. Since the drainage issue seems to be an abrupt transition from fine to coarse, would a gradual change in materials be a good alternative. I'm thinking a layer of small rocks or bark nuggets, followed by a layer of coarse bark mulch, a layer of mixed coarse & fine mulch, a layer of fine mulch mixed with potting soil and a 12" layer of good potting soil. Would that work? If so, what sort of soil is best for the top layer? I want to avoid the 'container mixes' that are mostly peat, but what should I be looking for instead? If I try to match the coarser mix of the nursery pot, will there be enough nutrients or will I need to fertilize regularly. Would a coarser layer of potting soil with 2" of compost mulch on top be better.
When I have a deep pot with shallow-rooted plants, I just use my sandy garden soil at the bottom. This works well, but it weighs a lot; so it could be mixed with some light-weight material like vermiculite, perlite, or pumice if lighter soil is desired or the garden soil drains poorly. For the top layer well-rotted (composted) manure mixed in with any potting soil that drains well should provide plenty of nutrients, but probably will make the soil less acidic. Pieris like somewhat acid soil; so, you might want to add some sulfur to the mix.
Same potting medium from top to bottom. If the plant is not going to fill at least half of the container with roots the container is bigger than needed.
Of course the container is deeper than needed, but for esthetic reasons I want the plant at a certain height. Even in a garden not all plants send roots down as deep as the available soil
That is certainly a possibility if I can find an inner pot of suitable size and figure out a stable support system to keep it level at the correct height. I could try something like gravel in the bottom for as a counterbalance to the weight of the full pot at the top, followed by a middle section of empty milk jugs or something else that would support weight and fill up most of the space. I remember following the 'layer of styrofoam chunks or peanuts' suggestion for a few years and it was a real pain. They squashed down or broke and got inextricable mixed with the soil at the bottom-making reuse or composting a problem, and did nothing for the drainage. I'm still wondering about the idea of gradually transitioning soil media as a way to ensure better drainage
He didn't say whether transitional layering had ever been tried, or why he believed it wouldn't work though Just his usual expert opinion that doesn't require any justification.
