Question: Why are some of my plants guaranteed and others are not?
Answer: I called around to several nurseries and the answer was the same: they can't guarantee plants because they have no idea what will happen to them once they are off with landscapers or designers (who get discounts or buy wholesale). There are a few nurseries who do offer guarantees (usually for woody plants and usually for a year), but it will be directly to the customer and not to a professional gardener. This probably means that the nursery grows the plants itself and won't be hugely out of pocket for a replacement. Some professional gardeners will offer guarantees by raising the wholesale price, while others offer plants at slightly less than most nurseries to cover the possibility of a plant not surviving the first year after planting. After that, you are on your own.