Those of us who live in one of the world’s five Mediterranean climate areas can take cheer from the fact that these regions are home to some of the world’s greatest biodiversity. As an example, according to Olivier Filippi’s The Dry Gardening Handbook (translated from the French by Caroline Harbouri), 25,000 plant species grow in the Mediterranean Basin proper (about 10 percent of all the world’s flora), compared to just 6,000 species in all of non-Mediterranean Europe.
These plants have adapted to our dry summers in various ways and to various degrees. We need to recognize that there are many variant subregions within the Mediterranean Climate zone. In general, however, gardeners who regularly water native plants over the summer months will end up killing them. Turn off that irrigation timer!
Donald Trump’s comments to the contrary notwithstanding, California has been in a multiyear drought. That is a fact, based on recorded annual precipitation. But for gardeners and farmers there is another meaning of drought—“physiological drought”—which refers to periods of hydric deficit. Hydric deficit occurs when plants transpire more water than they can take in through their roots. It is not just a function of water but of the relationship between water and temperature. This is critical for an understanding of Mediterranean climates.
We should not think of plants as either “drought tolerant” or not. Instead we should understand them on a scale of hydric deficit tolerance and seasonable adaptability, as this map from Filippi’s book suggests.
It is helpful to create a line graph that enables visualizing the extent and duration of physiological drought conditions in your location. Following is Filippi’s chart for Marseille, France. Note how the graph displays both duration and intensity of physiological drought. (Further below is the graph I have done for my garden.)
I have made such a graph for my garden (further down the page). Here is how to do it:
- Acquire monthly precipitation and high/low temperature data.The chart is based on sixty-five-year averages of temperature and precipitation. This data is not available for my location because we don’t have a weather station that has been keeping track of this data near enough (currently there are some nearby Weather Underground stations, but their data is only from recent years). Unfortunately for the present purpose, conditions vary quite a lot across short distances here. So I acquired data from Richmond, Berkeley, Orinda, and Martinez (google “climate summary [you location]” and look for results from the domain www.wrcc.dri.edu), and estimated my data using those.
- Average the high and low monthly temperatures by adding them together and dividing by two.
- Convert from Fahrenheit to Celcius with the formula (F-32)/1.8.
- Convert precipitation from inches to millimeters by multiplying by 25.4.
- Create a graph with two Y axes. P is precipitation in millimeters and T in temperature in Celcius. T-axis intervals must be twice the P axis intervals, so that, for example, 80 on the P axis corresponds to 40 on the T axis. The X axis is simply the months of the year.
Unfortunately, I don’t know how to create a line chart in Excel with two different Y axes with different scales. So I had to draw the lines by hand, which is not a real precise technique but might be close enough for visualizing. If anyone can tell me how to do this in Excel, I’m all ears.
- Draw lines connecting the data points. Conventional is red for temperature and blue for precipitation.
So there you go. The part of the year where the red line rises above the blue line is the period of hydric deficit. You can see that the duration of drought in my area is from May through September, and that August is the month when hydric deficit is greatest.
Armed with this data you can select plants that are optimized for your region.