Certified GKH Gardening Expert
This is a highly debated subject and pretty much depends on the soil, region, and plants. Pine needles are naturally acidic in nature. As they decompose, which is a slow process, additional acid is added to the soil. However, this may not necessarily be a dramatic increase. Since pine needles are acidic, it goes without saying that acid-loving plants (like azaleas, rhododendrons, hydrangeas, blueberries, etc.) thrive with this type of mulch. Other plants may not respond as well; therefore, pine mulch may not be suitable.