When used excessively as mulch, pine needles may increase soil acidity. This is beneficial to some plants, but lawn grass that prefers alkaline soil may gradually brown and die over time. Covering grass with pine needles also blocks out light that they need for growth.
Pine needles are dry and take longer to decompose than many other mulch materials. This makes them inefficient mulch when gardeners need to make an immediate impact, but Curtis Smith of New Mexico State University recommends them for their slow decomposition and release of nutrients.
Gardeners can maintain soil pH by raking away excess pine needles. Instead of throwing them away, gardeners can recycle them as mulch for plants, especially those that prefer acidic soils.