Cypress Mulch Under Oak Trees

Soil pH

• Cypress mulch will gradually lower the soil pH. A study conducted by the Lee County Cooperative Extension Service showed that cypress mulch lowered the soil pH from 7.37 to 6.47 over a period of one year. This is beneficial for most oak trees, which absorb nutrients more efficiently when planted in acidic soil. The live oak (Quercus virginiana) requires an acidic to neutral soil pH of 3.7 to 7.0. Northern red oak (Q. rubra) requires an acidic soil with a pH between 4.3 and 6.5. Pin oaks (Q. palustris) require an acidic soil pH of 3.7 to 6.0. White oak (Q. alba) can be grown in acidic to slightly alkaline soils with a pH between 3.7 and 7.3. Lime may need to be added to extremely acidic soil to bring the pH up.

Nutrients and Organic Matter

• Cypress mulch breaks down more slowly than other mulches. The up side to this is that cypress mulch needs to be replenished less often than other organic mulches. Unfortunately, among the benefits of organic mulches are increased organic matter and nutrient levels added to the soil by mulch decomposition. Whether this is an issue depends on the organic matter and nutrient levels in the soil and the needs of the oak tree. Some oak trees, such as the sawtooth oak (Q. acutissima) need organically rich, fertile soil, while the southern red oak (Q. falcata) does quite well on sandy soils with poor nutrient levels.

Mulch Application

• Spread cypress mulch over the entire oak tree root zone. The roots spread from the trunk to the outer edge of the tree canopy plus one-third of that distance beyond the canopy. Spread the mulch uniformly at a depth of 2 to 3 inches, but keep it several inches from the trunk. Mulch will keep the bark damp, which encourages disease. It also provides cover for bark-chewing rodents. Make sure the mulch is at least 6 inches from the trunk in the fall to minimize the risk of rodent damage. It can be pushed back to a few inches from the trunk in the spring.

Root Protection

• Oak tree roots can be damaged easily from excessive weight on the soil above the root zone or soil disturbance from tilling or digging. They can even sustain damage from excessive foot traffic. Do not park vehicles or disturb the soil within 6 feet of the tree. Replenish the cypress mulch every two to three years to maintain a depth of 2 to 3 inches. In addition to retaining moisture and limiting weed growth, the mulch provides a cushion that protects the roots.