Rhodies: how mulch w/o raising soil level to to where won't bloom
I have several rhodies on my 1 acre property here in a suburb of Seattle/Tacoma. Some existed when I purchased the home a few years ago, some I planted (heights from 3' up to 15'). I put them all on a drip system when I purchased the home and I do twice weekly deep waterings when dry weather (e.g., late spring through summer into fall). Most of the plants look pretty healthy but some don't bloom at all, some have very few flowers. They're in a mix of conditions from partial sun to partial shade a few full sun but I see no pattern to lighting conditions which are/aren't blooming and suspect soil conditions.
I've read in an Ed Hume book if rhodies are not blooming 85% chance that soil level too high (he states shouldn't be above rootball level) so I'm questioning comments I read on this forum about mulching rhodies with 3-4" of mulch -- wouldn't this raise the soil level too high and cause it to not bloom even if healthy plant? How close to the trunk are people suggesting mulching rhodies??
The soil level was higher than the root ball on SOME of my rhodies, I removed the extra soil but I also suspect the rhodies aren't planted in great soil nor have they ever been fertilized/mulched based on what I know of previous owner who left landscaping an unmaintained/unirrigated weedy jungle.
I would like to improve the soil by beginning composting and/or fertilizing on a periodic basis, but am concerned about raising soil level to where they won't bloom.
Ed Hume suggests poking 8-10" deep holes in soil to put fertilize down into roots which makes sense to me vs. just throwing it ontop of soil. But I'm particularly wondering how to add compost to soil without raising soil to where rhodies won't bloom. Any advice on this plus timing of when to fertilize/add compost greatly encouraged!
Thanks in advance!