It's raining here in Phoenix today. Two days ago it was 108 °F, dead dry and windy. Today it's 75 °F, overcast and raining. And not the kind of torrential downpour that accompanies howling thunderstorms during the monsoon season, but a nice steady, look-there's-a-mud-puddle-let's-go-for-a-walk-and-then-bake-cookies kind of rain.
In the Desert Garden, rain is always a welcome event, but even more so in May, which is typically the driest month of the year, averaging around one-tenth of an inch of rain at best. In the Chihuahuan Desert, to the South, the greatest amount of rain falls in the late Summer months; in the Mojave Desert, to the North, the period of greatest rainfall is in the Winter, from October through March. Here in the Sonoran Desert — smack in the middle of the other two — we have the best of both, but are still left with a difficult dry period as the weather really heats up in May and June.
Rain, as distinct from irrigation water, has a magical effect on plant growth. As carefully as we may water with irrigation water, plants respond to rain with noticeably greater effect.
There are a few reasons why plants respond to rain water. While pure water is at 7.0, or neutral, on the pH scale, rain water dissolves atmospheric CO2 as it falls, creating a very weak solution of carbonic acid. This slightly acidic water helps to free up soil minerals that are generally locked away in our alkaline soils. Interestingly, rain accompanied by a dust storm during the monsoon can carry enough dissolved alkaline dust particles to move its pH back above neutral, to the alkaline side of the scale.
Next, rain water is free of the dissolved salts that are present in our irrigation water, and so dilutes soil salinity, which is almost always good for plants.
And, last, rainwater tend to contain dissolved atmospheric nitrogen, so it's as if the entire landscape gets a light dose of fertilizer.
Plus, the plants all get a good shower to wash the dust off, which is bound to make them feel better.