Two years ago, UCCE Placer/Nevada embarked on a study to evaluate pruning and mulching in five local mandarin orchards. In a blog posted in June 2017, I discussed the beginnings of the study, the specific treatments and what the study was hoping to accomplish in the coming years. As the study enters the third year and data analysis begins, there are some observations that we should share.

The environment in our orchards is changing. We are experiencing unpredictable temperature swings and extreme periods of rainfall. Soil moisture levels this past winter and spring reached saturation levels for a prolonged period.

Roots need both water and oxygen. In saturated soils, oxygen in the soil pore spaces is displaced by water. This year, tree stress increased as the roots struggled to breathe. When root cells can't get oxygen, they die. Without them, the rest of the tree starts to die too. Many of us suffered tree loss due to the suffocation of root systems.

As orchards and fields began to dry, temperatures began to swing from high to low in unpredictable patterns. Stress was further introduced into the system. In the past few weeks, recorded temperatures in the five orchards have gone from lows in the 40's to recorded highs of 105 in the first few weeks of June. Citrus has a tendency to drop fruit that the tree cannot support and increased tree stress can increase fruit drop. 

Field observations over the past few weeks show that drop is occurring at a much higher rate and volume in treatments that haven't been mulched. The picture on the left is from underneath the canopy of a tree without mulch (Control) and to the right is a mulched tree. These two trees are in the same orchard, approximately 18 feet apart.

Soil moisture levels are being monitored under both of these trees. Moisture levels under the mulched tree are consistent with what one would expect. The soil profile is able to retain moisture with minimal depletion at a 6-12” depth. Most moisture depletion is occurring in the top 6” of soil and is minimal compared to the Control. Moisture levels in the Control deplete at a more rapid rate throughout the soil to a depth of 12”.  Once again, tree stress is on the rise. The soil moisture in the Control profile becomes much harder to replenish, resulting in longer irrigation cycles.

Soil temperature at a depth of 9” is also being monitored. Data from the Control show rather significant increases and decreases in temperatures in short periods of time. Data from the mulched tree reflect more temperature stability with the highs lower than those in the Control soil. Again, it seems that the Control may be under more stress due to the rapid temperature swings that the root mass is encountering.

It would certainly seem that a cursory examination of the data with field observations shows that mulching is a beneficial production practice in an orchard. However, there are some considerations to be had before one just charges ahead with this practice.

First, as we learned, timing is everything. Mulch should only be applied when the moisture in the soil profile is at capacity. Experience has taught us that applying the mulch when soil moisture is low results in playing catch up. I have observed orchards that were never able to replenish the full soil moisture content during the irrigation season. 

Second, monitoring soil moisture is important. Mulching will change your irrigation timing and frequency.  In our study, the period between irrigation cycles in mulched orchards has increased.  Maturation appeared to be slowed in orchards that chose to stay with their old programs and not effectively monitor the soil moisture. Mulch reduces moisture loss from the soil, so if irrigation was not adjusted, the soil under the mulch stayed too wet. While less dramatic than the saturated soil situation this spring, stress was induced by the tree roots struggling to obtain oxygen and maturation slowed.

Certainly, mulching requires a little more effort on the grower's part.  But is the not the reduction or control of the stress induced in the growing system beneficial to the entire orchard system?  As a grower, I know that adding a little bit of stress into my trees at the right time and conditions is good for the fruit. Adding that stress at a time when it will not damage the tree or cause crop loss is key.  Mulching certainly seems to mitigate stress in the growing system when stress is detrimental to the crop and system. Further data analysis will also provide us with more insight into the benefits that the mulch provides to the soil biology and composition.  We will share those insights as they become available.