Although you may initially assume that the months with the hottest average temperature pose the biggest threat to plants, other factors also contribute to plant stress levels.
Let’s look at a table of average weather in Phoenix:
Temps of 100 plus are common June through September.
July and August pose additional challenges for plants, with average nighttime lows in the 80s. Plants that use CAM respiration can struggle and growth may stop, leading to root rot.
Monsoon officially starts in June, but Phoenix typically starts to benefit around mid-July.
So what factors affect plant stress during the hottest months?
- Daytime high temperature
- Nighttime low temperature
Daytime temperatures break the century mark and may even soar into the 100+ degree zone. Low humidity, lots of sun and little rain make this an extremely challenging month, especially for non-desert adapted species. Cell damage can occur at about 115 degrees for broader leafed plants under these conditions. Applying extra water during June may not cure the stress because plants are not able to function at a level where they can overcome evapotranspiration.
In addition to low rainfall, deserts are characterized by a high rate of water loss from the ground (evaporation) and through plants (transpiration). Together this is called evapotranspiration. Potential evapotranspiration is the amount of water that would be lost through evaporation and transpiration if it were available.What Is a Desert – DesertUSA
Protecting non-desert adapted plants with a 40% or 50% shade cloth will help, especially during the first year. Another helpful practice is to plant in fall instead of spring so there is more time for plants to get established.
The first couple of weeks can be similar to June, depending on when the Monsoon engine gets up and running. Rising humidity levels provide a break from the sun’s rays and allow plants to get a foothold against evapotranspiration.
Clear dry air transmits about 90% of available sunlight to the ground on a typical desert day compared to 40% in a typical humid climate.What Is a Desert – DesertUSA
Cloudy days and rainfall also provide relief for stressed plants, resulting in new growth and flowering. Nighttime temperatures may be high enough to cause issues for CAM plants, resulting in root rot – this mostly affects non-Sonoran desert plants.
CAM is an adaptation for increased efficiency in the use of water, and so is typically found in plants growing in arid conditions. (CAM is found in over 99% of the known 1700 species of Cactaceae and in nearly all of the cactii producing edible fruits.)Wikipedia
Monsoon continues through August. Average rainfall and nighttime lows are similar to July and average hours of sunshine fall by about 7%.
Monsoon officially ends September 30th, so humidity, clouds and rain may benefit plants throughout the month. If the days turn arid, though, high daytime temperatures can pose issues similar to June. Average daily temperatures are down a few degrees from June/July, hours of sunshine are down about 10% and rainfall averages begin to decline.