We all love the wonderful weather that we have in San Diego, including the more than 265 days of sunshine a year, the mild winters, and the TEN inches of heavy drizzle we call rain.
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 in 2014 we have had a paltry 5.1 inches of rain.
We have been looking at the drought’s effect on trees in San Diego and what we are seeing ranges from trees in decline to those that are completely dead. We are cleaning 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 that we consider drought-tolerant are quietly turning brown.
Even trees on irrigation systems are finding it hard to meet their water needs. 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 providing nearly enough water for a large tree.
So what happens when a tree can’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. When transpiration stops so does every other important physiological process in the tree: photosynthesis stops, food production and storage stop, 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 it will slowly die from starvation.
What can you 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. Do this as infrequently as once a month, or perhaps twice a month in mid-late summer.
- 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 will help shade the soil beneath the tree and slow evaporative water loss.
- Do not apply fertilizer (unless the tree has clorosis) because it will stimulate growth and increase the tree’s need for water.
- When installing new trees, choose species that can survive in our semi-arid climate.
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.
~ Peter Green