Tag Archives: Olivier Filippi

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

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.