I've had a few conversations recently that revealed to me what a dumb thing it is to call a plant "drought tolerant". Wikipedia gets this right technically, but leaves us without enough to apply the information: "Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions. Well, okay - so when we say a plant is drought tolerant, what the heck does that mean? It means that plant has some degree of adaptability to drought. But here's the thing - how much adaptability? How do I use this information? What does this plant do in response to drought conditions? Saying a plant is "drought tolerant" is like saying there is a temperature outside. Is it cold, warm or hot? See what I am getting at? "drought tolerant" only means that there is some degree to which the plant may tolerate (not die immediately) a shortage of water. More information is needed: enter WUCOLS. I won't bore you with the details and technical stuff behind WUCOLS, they do well enough - read through their website.

Not bored yet? Good! Now read through the rules on WELO, the California ordinance for water efficiency in the landscape. Bet you're bored now. The deal is that these classifications in WUCOLS mean something. Their water needs categories quantify how much water a plant wants in a particular region of California. This does not take into account sun/shade exposure, wind, soil type, microclimates, or any of the other factors involved in meeting a plant's water needs. It is not a perfect tool, but it is the best thing we have going. WUCOLS isn't the only resource, you can get information from nurseries and books also, but it is the most helpful hydrozoning tool I use. The other sources do not correspond to a quantity of water, and they don’t all use the same terminology.

In WUCOLS, the classifications are "very low", "low", "moderate", and "high". It may also say "inappropriate" or "unknown". This is pretty self-explanatory, but what is most interesting here is that these words correspond to a percentage of ETo. ETo is a reference point - a baseline of sorts. So a "high" water use classification means 70-90% of ETo which is based on the water needed to keep a 4" tall cool season turf VERY well watered and super lush, or to put it another way, the "high" water classification is 70-90% of a whole lot of water.

To illustrate the uselessness of using “drought tolerance” as a means of grouping plants, I indulged online - I pulled up an article on "drought tolerant" plants from Sunset magazine. Of their 12 plants, that I looked up in WUCOLS for my home town of Oakland, one was classified by WUCOLS as "very low", one was "moderate", and the rest were "low". So if you go by a judgement of “drought tolerance”, you could wind up with plants that are literally not allowed to be planted together per state law (MWELO) assuming the ordinance applies to your project. Even if it does not, that doesn’t make it a good idea to plant a low water use plant with a high one.

Additionally, Nassella tenuissima / Mexican Feathergrass is listed as an invasive plant by PlantRight and should not be used at all, but WUCOLS isn't about invasiveness, it is about water, and it was in that list of drought tolerant plants. Shame on Sunset! Not only should we keep plants with different water needs separate, we need to not install invasives, period..

Just incidentally, if you absolutely must use plants together that WUCOLS gives different water needs classifications to, then the classifications should be next door neighbors - so "low" and "moderate" is kinda okay (provided you irrigate and do your calculations for the higher amount), but "moderate" and "very low" is no bueno.

Next I went to the "drought tolerant" poster child: succulents. Low water use, right? Drought tolerant?! erm.... no. Again, WUCOLS demonstrates my point. In the plants WUCOLS lists as succulents, 3 classifications are included - very low, low, and moderate (screen shot is just of the alphabetical early birds, I didn't pick favorites here). So much for that assumption.

So really, drought tolerant is anything that will grow with less than 70% of a boatload of water if you include all that Sunset does.. That could still an incredible amount of water, folks. Granted, the lower classifications are pretty good and some plants truly do have a relatively dry footprint, but all plants need extra water during their establishment period, that is a post for another day. Oh - and about those drought tolerance adaptations emphasized in the first paragraph: a drought tolerant plant might go dormant, drop all or some of its leaves, wilt, or shrink in size in times of stress (stress being the drought they're tolerating) as a way of coping .... and if not watered in time, drought tolerant plants die. Tolerance is about not being dead yet - it has nothing to do with living well, being healthy, or looking good, it is about living to drink another day. Thanks for listening!