Plants dying right after mulch laid... nitrogen robbing or?
I would love some input on a problem I'm having. About a month ago, I planted some new purchases from the garden centre (namely astilbe, bleeding heart, impatiens and miniature hostas) in average to somewhat poor soil in a garden that is mostly shaded. Before planting, I amended the soil with a pretty respectable, but not excessive, amount of compost. Aside from what I planted, the garden already contained large hostas, Canadian anenome, and snow-on-the-mountain. I left these plants be, other than the occasional watering, and they flourished.
A couple weeks ago, I covered the ground around my new plantings with cedar mulch. Now, literally all of the new plantings, as well as one large hosta in particular and a good bit of the snow-on-the-mountain, in suffering dramatically. A couple astilbe are completely dead, having shriveled up and turned brown; the bleeding hearts are yellow and pathetic-looking, as are my hostas on the leaf edges; even the impatiens (we all know how indestructible they usually are) are suffering.
I did a search in this forum and it seems most of you are of the view that nitrogen robbing by mulch is a myth. But I just can't think of anything else that could be causing this! Another reason that I suspect nitrogen robbing is that the yellowing is starting at the oldest leaves.
To try to fix the issue, I have raked back the mulch for the time being, scuffled a small amount of blood meal with additional compost into the first inch or so of soil, and done two foliar feedings, first with kelp emulsion and then with vermicompost tea, as well as a vermicompost tea soil drench.
I'm scared to apply the bone meal at the regular application rate- or do anything else for that matter- before I am sure I've correctly identified the issue, as I find overfertilizing in times of stress worsens the problem.
What should I do, guys? Lime? Leave well enough alone? This house is being put on the market within the week and I'm really hoping to work some miracles here...
Appreciate any and all feedback!