1 - Pine needles are not toxic and make excellent mulch.
Q. Several years ago you wrote that it was safe to put pine needles in the compost. Now, I wonder if it is equally safe to use pine needles as mulch around roses and in garden paths.
A. Pine needles make excellent mulch for most garden plants. The advantage pine needles have over some other organic mulch materials (grass clippings, straw, hay, etc.) is that they decompose much more slowly. This is because of the resins that they contain. The resins resist the absorption of water necessary for decomposition, so they remain undecomposed, serving as mulch longer. This means they do not add organic matter to the soil as quickly. For some plants, that may be a positive characteristic and other plants that need the organic matter will be well served if you add organic matter before planting. However, pine needles also do not absorb water as readily. That means irrigation and rain water reach the soil and serve to grow plants rather than decompose the mulch material. Pine needles also provide a pleasant (to some people) fragrance in the garden, especially when the sun shines on it. Many people fear that pine needles, spruce needles, and juniper needles are toxic to plants. This is because little grows under the thick mulch these trees produce. Many plant seeds need light to germinate (especially many weeds), so they do not germinate under these trees. It is not toxicity that results in limited growth of competing plants under these trees. The lack of growth is due to the fact that these trees shed water to the drip-line region, so it is too dry under their branches for many things to grow. The thick layer of mulch produced by the needles prevents light from reaching the soil. These characteristics along with the fragrance and slow break-down of pine needles make them an excellent mulch material for our gardens where a persistent mulch and slow decomposition is desired.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: email@example.com, office: 505-865-7340, ext. 113.
For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!