• Some plants are listed with a collection number, for example, "IB244" or "SBH6286". These numbers refer to a particular collection (responsibly collected as seed or cuttings usually from the wild) of a species made by a particular person at a particular place and time. These numbers should be retained if you are keeping records for your garden, as they frequently represent a specific distinct form of a species with unique characteristics.

• Names of registered plant cultivars are given in single quotes, as is standard practice in the field of horticulture. Names in brackets, such as Arctostaphylos x media [Mt. Walker], are not actual registered cultivars, but they still indicate a particular distinct form or collection of a plant. Like collection numbers, these names should be retained for gardeners concerned with accurate record keeping.

NOTE: This concept has been temporarily "shelved," but it is still valuable, and I'm just leaving it here for the time being. I may someday attempt to reincorporate it into the plant descriptions.

We have now added drought resistance codes to each plant description, which may provide you with a rough idea of how much drought each plant can comfortably tolerate. Their use can be compared to the way plant hardiness zones are intended to function, in that they can be applied both to plants and to locations. The code operates on a scale of 0 to about 6 or 7, 0 being the least droughty, where (on average) enough rain falls to sustain plant growth at any time of the year temperatures are warm enough; and 6 or higher generally being considered a desert-type climate.

This system, believe it or not, is not something we just made up; nor are we the first to use it to assess plant adaptability in the context of gardening. It is based on the method of determining hydric deficit developed by French plant geographer Henri Gaussen; and has been used in at least one book, The Dry Gardening Handbook by Olivier Filippi. It is calculated as the number of months for which the average temperature (calculated as the average of daily high and low temperatures, not just the average high) in degrees Celsius exceeds half the average precipitation in millimeters. We are incorporating it here because we think it has the potential to be very helpful for assessing plant water needs, and deserves much wider use.

In the future we hope to provide more details about how the system works, including helpful graphics and maps to see what hydric deficit "zone" you are in. For the present we have only made these calculations for a few select locations, and so a brief summary will have to do. If you're in the greater Seattle or Portland area, your hydric deficit zone is probably 2. Not until you get to about Eugene do you go up to 3, and Medford is 4. In the Olympic Rainshadow, Port Townsend and Port Angeles, and probably much of the San Juan Islands, are in zone 3; Sequim is barely within 4. East of the Cascades, Spokane has a hydric deficit factor of 4 and Wenatchee is 6. Looking farther afield, the system becomes somewhat less useful outside of Mediterranean climates, but it is safe to say that anywhere from about east Texas on up to the entire Atlantic seaboard has a hydric deficit factor of 0 because of the relatively even distribution of rainfall throughout the year. Of course, the system is based only on averages and does not take into account exceptional droughts that occur rarely. Even in Forks, which gets a rating of 0 based on averages, summers can be dry some years, and gardeners may wish to plant based on a hydric deficit factor of 1.

Use of the drought resistance codes assumes no supplemental irrigation beyond natural rainfall is provided. It also assumes plants are established: we all know that plants generally still need to be watered to some degree until they are well established. Even so, results will vary based on growing conditions; particularly, the amount of sun vs. shade the plant receives, and the ability of the soil it is growing in to retain moisture. If you are planting in a dry and difficult "hell strip" or have exceptionally sharply draining soil; or wet, heavy soil; you may wish to "adjust" (for gardening purposes: not that you are climate truly changes) your hydric deficit factor up or down accordingly (but probably not by more than one zone). Another consideration is that gardeners will have differing standards for what they consider to be "resisting" or "tolerating" drought for a given plant. If a plant continues to survive, grow and flower but at a reduced rate from what it would do under moister conditions, is it drought tolerant? This is a subjective question; but, in general, we find it best to consider how much drought a plant can tolerate "comfortably," that is, without substantial setback in growth or performance, but allowing a little wiggle room for varying conditions from one garden to the next. Finally, we are not always entirely certain just how much drought some of the plants we offer can or cannot take in stride. We acknowledge that our drought resistance ratings are often based on educated guesswork, and we welcome your feedback regarding their accuracy. For this reason we suggest using these ratings as a general guide only - but, hey, it's a place to start!