how dry am i?

This post may be mainly for the math nerds among you, but I think it could be inter­est­ing to any gar­den­ers liv­ing in drought-prone parts of the world.

In my last post I men­tioned that I’d used instruc­tions in Olivier Filippi’s The Dry Gar­den­ing Hand­book to fig­ure out the drought stress index, or hydric deficit, for where I live in San Diego.

USDA zones are useful for dealing with minimum temperatures. For gardeners in the western U.S., Sunset zones provide more finesse, combining temperature with other climate conditions. The the drought stress numbers, however, are useful if you want to concentrate on understanding how many months a plant might be subjected to severe drying conditions due to lack of rainfall.

Filippi writes in his book that "everyone's drought is different," so be sure to consider factors other than this single number, things like total rainfall, humidity, the sun exposure a plant might get or the amount of wind your site experiences. The technique presented in The Dry Gardening Handbook derives from work of plant geographer Henri Gaussen.

Figuring out hydric deficit is pretty straightforward but will take a few minutes of your time. Either use a spreadsheet program like Excel or a sheet of paper. First, go to a site like World Climate where you can find your area's monthly total rainfall and monthly average temperatures. On the spreadsheet or paper set up a column with the months of the year, January to December. Next fill in a column with the monthly average rainfall in millimeters, and another column with the average monthly temperature in degrees Celsius.

Now you have two options: Fol­low the instruc­tions in the book, which isn’t that hard but requires mak­ing a graph with three dif­fer­ent axes. Or use my sim­pli­fied tech­nique, which requires some cal­cu­la­tions but no graph­ing. I’ll send you to the book for the some­what more pre­cise method, but here’s my eas­ier method: In a fourth col­umn, divide the rain­fall num­ber by the tem­per­a­ture and mul­ti­ply by 2. That’s where the math comes in to play.

Here’s my result for San Diego:

Month Rain­fall (mm) Tem­per­a­ture (Celsius) 2 x (Rainfall/Temperature)
Jan 55.6 14.1 7.890
Feb 41.3 14.7 5.62
Mar 49.9 15.3 6.52
Apr 19.8 16.6 2.39
May 4.8 17.8 0.54
Jun 1.9 19.3 0.2
Jul 0.5 21.6 0.05
Aug 2.1 22.5 0.19
Sep 4.7 21.8 0.43
Oct 8.6 19.8 0.87
Nov 29.5 16.6 3.56
Dec 35.4 14.1 3.62

Count up the num­bers in the fourth col­umn that are less than 1, and that’s your approx­i­mate hydric deficit num­ber. The higher the hydric deficit num­ber, the more severe your dry­ing con­di­tions. For the San Diego Air­port, the num­ber is 6. (If you have a month where the aver­age tem­per­a­ture is below freez­ing, for my over­sim­pli­fied method just throw out that month and con­sider that there’s min­i­mal hydric deficit.)

Now what do you with the num­ber? For one thing, you can use it to com­pare you grow­ing con­di­tions with the drought resis­tance code for a plant in Filippi’s book. For exam­ple, the matil­ija (“tree”) poppy (Rom­neya coul­teri) has a drought tol­er­ance rat­ing of 6–perfect for an unwa­tered gar­den in San Diego. By con­trast, Cean­othus ‘Ray Hart­man’ has a code of 4, and Hid­cote Blue laven­der (Lavan­dula angus­ti­fo­lia ‘Hid­cote Blue’) has a code of 3. These other plants would prob­a­bly sur­vive with­out sup­ple­men­tal water, but to look their best the cean­othus might ben­e­fit from a cou­ple months of occa­sional sup­ple­men­tal water­ing, and the lan­der maybe three. You can also use the num­ber to com­pare the dry­ing forces where you live other regions around you, or apply the num­ber to bet­ter under­stand your cli­mate in rela­tion to that of a plant’s origin.

For fun, I added four other sites in San Diego County. You can see how the county offers a huge num­ber of grow­ing con­di­tions, from dry coastal con­di­tions, moun­tain mead­ows, back­coun­try chap­ar­ral, and full-on desert.

City Hydric deficit
San Diego Airport 6
La Mesa 5–6
Cuya­maca 1
Campo 3
Bor­rego Springs 7


And then a few other cities in Cal­i­for­nia. You can see a gen­eral moist­en­ing the far­ther north you go, and a gen­eral dry­ing as you head east towards the deserts.

City Hydric deficit
Los Ange­les 6
San Bernardino 4–5
Vic­torville 6
Santa Bar­bara 5
Mon­ter­rey 4
San Jose 4–5
Santa Cruz 3
San Fran­cisco 4
Rich­mond 4
Sacra­mento 4–5
Fresno 5
Yosemite National Park 2
Eureka 1 2
Red­ding 2


I’d never played with map­ping in Google Maps, but thought this might be a fun first lit­tle project. I took the num­bers above and applied them to a map. The results are pretty impres­sive for some­thing that’s not hard to do. So far the blips are in Cal­i­for­nia only, but I might work on the map some more to include other loca­tions. Take a look…


View Hydric Deficit Map in a larger map

May 21 2009 | Categories: gardening • landscape | Tags: • • • • • • | 10 Comments »

the dry gardening handbook

Olivier and Clara Fil­ippi have been gar­den­ing in the south of France for well over a quar­ter cen­tury. Theirs is a mediter­ranean cli­mate, and their nurs­ery, Pépinière Fil­ippi, located near Mont­pel­lier, spe­cial­izes in plants adapted to the dry-summer/wet-winter cycles that you find in only five large regions on earth: the Mediter­ranean zone, proper; South Africa; the south­west cor­ner of Aus­tralia; Chile; and much of California.

Cover or The Dry Gardening Handbook

When I picked up Olivier Filippi’s recent The Dry Gar­den­ing Hand­book: Plants and Prac­tices for a Chang­ing Cli­mate, I was expect­ing it to be a dif­fer­ent sort of book than it is, maybe some­thing about gen­eral drought-tolerant plants, or a vol­ume ded­i­cated to help­ing your gar­den adapt to using less water. What this is, how­ever, is a straight book on mediter­ranean gar­den­ing and plants suited to mediter­ranean climates–something that prob­a­bly shouldn’t come as a sur­prise since that’s the focus of the author’s nursery.

There’s a brief intro­duc­tion to what con­sti­tutes a mediter­ranean cli­mate, fol­lowed by notes on the strate­gies plants use to sur­vive and thrive in it. Good advice on plan­ning, plant­ing, estab­lish­ing and water­ing a new mediter­ranean gar­den comes next. Then Fil­ippi gives us the heart of the book, a list­ing of over 400 mediterranean-adapted species, con­tain­ing com­mon and sci­en­tific names, approx­i­mate mature plant sizes, and notes on cul­ti­va­tion and prop­a­ga­tion. (If you can begin to read French, you can check out the online cat­a­log at the author’s nurs­ery, which closely mir­rors the list of plants rec­om­mended in the book. There you’ll also find some of the advice that’s offered in the book, although with­out the nice pho­tos in the book.)

Olivier Fil­ippi gar­dens in France, and the plant list def­i­nitely Euro­cen­tric: lots of dif­fer­ent laven­ders, cis­tus, phlomis, for exam­ple, with rel­a­tively few plants from other the other great mediter­ranean regions. In fact, many of the non-Mediterranean mediterranean-friendly plants listed are drought tol­er­ant selec­tions from sev­eral non-mediterranean cli­mates. For gar­den­ers in dry cli­mates that don’t undergo mediter­ranean cycles, these sug­ges­tions might be some of the best options to try. But those plants might not the be great­est of dis­cov­er­ies: Pho­tinia, heav­enly bam­boo (Nan­d­ina domes­tica), red-hot poker (Kniphofia sar­men­tosa) and Amer­i­can gaura (Gaura lind­heimeri), for instance, are prob­a­bly already com­mon offer­ings in many Amer­i­can nurseries.

One of the book’s most out­stand­ing fea­tures is the use of a “drought resis­tance code” that assigns a num­ber from one to six to each of the species in its plant list. Based on work by plant geo­g­ra­pher Henri Gaussen, the num­ber quan­ti­fies the num­ber of months of the year a plant can be expected to sur­vive under drought stress. The book also con­tains instruc­tions on how to cal­cu­late the cli­matic pro­file of where you live. (I fig­ured out that my coastal San Diego loca­tion exerts a 3.5 to 4 drought stress fac­tor. (Edit May 20: I oopsed on my fig­ur­ing for coastal San Diego. My revised num­ber is a much dryer drought stress fac­tor of 6.)) All that’s a really use­ful way to under­stand drought.

When you see plants sold in nurs­eries and cat­a­logs as drought-tolerant, the descrip­tion can be mean­ing­less. A vari­ety that would go fine for two weeks with­out water could turn into sea­soned kin­dling if sub­jected to six or seven months of con­tin­ued dry­ing. Real­iz­ing that a “drought-tolerant” chamomile plant has a drought resis­tance code of 2 would begin to tell you that it wouldn’t thrive in the same con­di­tions that would suit California’s more “drought-proof” Rom­neya coul­teri, which has a drought resis­tance code of 6. Hav­ing that infor­ma­tion could help you plan com­pan­ion plant­i­ngs, as well as help you avoid plants alto­gether that would only lead to expen­sive mistakes.

Com­ing at plant­i­ngs from a mediter­ranean focus leads the author to say some choice things about lawns:

You don’t have to be a vision­ary to see that the tra­di­tional lawn is an absur­dity in mediter­ranean cli­mates. If you nur­ture a deeply rooted feel­ing that you can’t be happy with­out a vast, lush lawn, then per­haps you ought to con­sider going to live in Corn­wall… Peo­ple often imag­ine that they need a huge expanse of lawn, but all too often the only per­son who walks over a tra­di­tional lawn in its entirety is the unfor­tu­nate indi­vid­ual who has to mow it every Sunday.

The author’s solu­tion? Land­scap­ing that pays atten­tion to where you live. For those of you in mediter­ranean cli­mates, this book can help you develop a deeper under­stand­ing of what’s unique about your envi­ron­ment. It can help you come up with good plant choices com­pat­i­ble with what your loca­tion offers. Along the way, it could help you save water, reduce pes­ti­cide use and maybe even free up some of those Sun­days you spend mow­ing the lawn.

May 16 2009 | Categories: gardening | Tags: • • • | 7 Comments »