Since last summer, most of my decorative plantings have died - mondo grass, creeping mazus, ivy and even my indestructible bishops weed have all disappeared. I would have suspected vandalism with some herbicide but this is contradicted by the fact that many adjacent plants are still healthy. Any ideas?
When many different species have similar problems, it is usually an environmental issue (not an insect or disease problem). In the case of Maryland landscapes, many plant deaths were caused by the abnormal rainfall last year--about twice normal! This means plants that can't tolerate standing water or saturated soils for long periods all died. Also, plants that can have low-level chronic fungal issues, had a fungal disease overload last year which they could not sustain. (Such as the Bishop's weed. )
So, replant with plants that can tolerate wetter soils. Raise the grade a little by adding top soil, or regrade so water doesn't pool for long periods. Since the weather is totally unpredictable, don't speed up run-off too much, because we may have droughts this year and then you want rain to soak in.
Planting with more adaptable plants--and native plants are a good investment--is always wise. Here is a good online publication with photos and cultural charts for Maryland natives: Native Plants for Wildlife Habitat and Conservation Landscaping.
Also here's a handy plant search site: http://www.nativeplantcenter.net/
Rain gardens can be a good solution to areas that periodically flood. Search 'rain gardens' and 'stormwater management' our on website.
Can you suggest something to replace the Bishop's weed and creeping mazus?
Take a look at our list of recommended groundcovers. https://extension.umd.edu/hgic/topics/groundcover-list
Japanese sweet flag (Acorus) and plumbago (Ceratostigma) are two options that tolerate moist sites.
Use this link to search for additional plant options for your site's conditions.