We all love the wonderful weather that we have in San Diego. We love the beaches, the sunshine and warmth, and the palm tree-lined streets.We love the more than 265 days of sunshine a year, the mild winters, and the TEN inches of heavy drizzle that we call rain.

We tell people that we live in a Mediterranean climate, and we also grow lots of plants and trees from Mediterranean-climate areas from all over the world.

However, if we research the San Diego climate, we find that it is actually semi-arid. We don't get anywhere near the 20+ inches of rain needed to classify our climate as Mediterranean. San Diego receives an average of only 10 inches of rain a year (at Lindbergh Field), and we trick our Mediterranean plants by irrigating them with water from the Colorado River and the high Sierra snowpack.

You have probably heard by now that we are having a bit of a drought in southern California: in 2011, we received our average rainfall of 10 inches; in 2012, we received 7.9 inches; in 2013, we received 6.5 inches; and so far for 2014 we have had a paltry 5.1 inches of rain.


Superficially, the result isn’t too bad for us – more sunny days and fewer rainy-day freeway problems. However, as a tree care company, we have been thinking about what effect the drought is having on the trees in San Diego and what we are seeing ranges from “my trees are looking a little icky” to trees that are completely dead. We are getting calls to come out and clean up healthy looking trees that have fallen over because the soil they were growing in doesn’t have enough moisture content to hold the roots in the ground.  

As the Tree Health Care Manager, I am seeing trees dying from secondary pests because the trees are stressed and weakened from lack of water. Trees that were marginal when they were receiving a full ten inches of rain are now in severe decline. Trees on slopes, trees in poor soils, trees that don’t receive any irrigation, trees that don’t belong in our semi-arid climate, and even some trees that we consider drought-tolerant are quietly turning brown. Even trees on irrigation systems are finding it hard to meet their water needs.

If you are watering five minutes a day for turf and think that your trees should be fine, your trees might be finding the soil-water bank account empty. A large healthy tree can process 400 gallons of water on a daily basis. The three to five minutes of irrigation per day that is keeping the turf green isn’t coming anywhere close to providing enough water for that large tree.

So what happens when a tree doesn’t meet its water needs? Transpiration (water vapor loss through tiny holes in the leaves, called stomata) slows down in an attempt to conserve water within the tree and stop sudden death by dehydration. Unfortunately, when transpiration stops, so does every other important physiological process in the tree. Photosynthesis stops, food production and storage stops, and the tree’s energy reserves become depleted. Over a prolonged period, the tree will not have enough energy to produce buds for new leaves, and will slowly die from starvation.

I can hear you all saying “oh my goodness! That sounds just terrible. What can we do?” We recommend tree-appropriate irrigation and water conservation practices. A tree’s roots are typically in the top 12 inches of soil, so you need to provide enough water to moisten at least that top 12 inches. This doesn’t need to be done very frequently; once a month, or perhaps twice a month in mid-late summer is enough. Installing a large mulch ring beneath a tree will make a huge difference in soil temperature and also decrease the amount of water lost to evaporation from the soil surface. Leaving low hanging branches on trees (as long as they don’t create clearance problems) will help to shade the soil beneath the tree and also slow evaporative water loss. Do not apply fertilizer (unless the tree is suffering from clorosis), as this will stimulate growth and increase the tree’s need for water. Also, when installing new trees, choose tree species that can survive in the semi-arid climate we all love.

In general, I think it is useful to consider this situation in the long term. Many trees fail and are removed when they could have been saved by the application of a bit more water. The higher water bill associated with keeping these trees alive is easily offset by the costs of removing and replacing failed trees. On the other hand, nothing can ever make up for the time that has been invested in growing your trees to a mature and beautiful state.