With its distinctive irregularly cylindrical crown and reddish brown bark, ponderosa pine is an iconic tree of forests in the western United States. There is much to like about ponderosa pine – it is drought resistant, fire-resistant, provides great habitat for wildlife, makes great lumber and is an attractive tree to have in the landscape anywhere a large tree fits (in other words, don’t plant this tree under telephone lines, power lines, or within 20 feet of houses and other permanent structures).
Two of the biggest complaints about this species are centered on its needles. Ponderosa pine needles are large – up to seven inches long, growing in groups of three to a fascicle, and these trees drop thousands of needles every year. The typical needle retention is three to four years, which means that one-quarter to one-third of the needles in a ponderosa pine tree come down every year, mostly between late August and October.
Down needles can be a concern to small forest landowners because they can become tinder dry very quickly. While loosely piled and well oxygenated on the forest floor, a fire can burn through them swiftly and intensely.
Ponderosa pine trees grow in slightly acidic soils and the needles themselves are acidic. Because of this, many people believe that pine needles will acidify their soil or damage their plants if used as mulch or compost. Ponderosa pine needles are also covered with a thick cuticle that is decomposition-resistant. Many people who have put ponderosa pine needles into their compost have concluded that pine needles at best do not make good compost. Some believe that they cannot be made into compost at all.
To address these concerns, we set up a research and demonstration project at the WSU Spokane Extension office to determine how different treatments of ponderosa pine needles might make of them a useful carbon source for compost. In this recently concluded project we compared three different treatments: 1) current year (most recently fallen needles from the previous autumn) intact needles, 2) current year, shredded needles, and 3) needles that had been shredded and then placed in a large pile for a year. Needles for the treatments using current year needles were bone dry and showed no evidence of decomposition. “Old” needles treated by shredding and piling for a year were slightly discolored, lacked the rigidity of fresh needles, felt comparatively soft to the touch and generally appeared to have already undergone some decomposition. We shredded some needles to test our hypothesis that decomposition rate would speed up if the waxy cuticle was broken down and the needle surface area increased. The increase in surface area would allow greater microbial action on the needles. Using “old” needles added a time dimension and an option of a year of “passive composting” as a possible composting technique landowners could employ.
Our study began on the Fourth of July weekend, 2015, which as some of you may recall featured temperatures in excess of 100oF. For all treatments we mixed 122 pounds of needles with 110 pounds of coffee grounds, 67 pounds of freshly cut grass and 23 pounds of dried leaves from hardwood trees (mainly Norway maple). We did four replicates of each treatment. “Ingredients” were chosen based on their availability to most local homeowners. The mixtures were placed in GeoBins designed for home composting and available from the Spokane County Extension Office among other sources.
Our most important findings were that ALL THREE treatments resulted in usable compost within three months and the final pH level of for all treatments was very close to neutral. There were clear differences between treatments and their rate of decomposition. Treatment 3 (“old” needles that had been shredded and piled for a year), melted almost like butter and was usable as compost within six weeks. The newer needle, shredded treatment attained higher temperatures and decomposed more quickly than did intact needles. Both current-year needle treatments took longer to decompose, but were usable as compost by summer’s end.
Our suppositions about the composting process seemed to be accurate. Ponderosa pine needles are carbon rich, probably having a carbon : nitrogen ratio of about 110:1 and they make great compost. Breaking down the waxy outer surface of needles appeared to be helpful in hastening decomposition although we do not have a way to determine the relative importance of that as the shredding treatments also served to increase the area of surface exposed to microbial action which also contributes to more rapid decomposition.
It appears that, in effect, the battle to turn pine needles into compost requires an investment of energy that will break down cuticle walls. This energy can take the form of shredding needles, turning piles, and adding water and nitrogen-rich materials to them. The treatment of shredding and aging saves labor because it allows normal biological activity to works on needles over time. The “best” approach to composting needles will depend on which of the following are most available to you: time, or energy to turn piles coupled with nitrogen rich materials. Your desired timeline—how quickly you hope to have materials go from needles to compost—may influence your decision about which approach is best.
Our conclusion is that you can reliably turn ponderosa pine needles into garden ready, pH-neutral compost. This means you do not need to send needles off site for disposal, but can utilize needles to improve the tilth and fertility of your soils. Decreasing fire danger around your homes by gathering ponderosa pine needles and turning them into compost creates a win/win situation and it might even make for an alternative product to sell locally. Many gardeners will pay for the high quality, herbicide-free compost that many small forest landowners are ideally situated to provide. Many landowners have paid to bring in soil so that they can garden on their forested property. The thin, low organic matter, rocky soils usually found in these areas requires improvement to become productive garden or crop land. For the patient land owner, composting pine needles to build high quality soils in place is viable option.
By Steve McConnell, Regional Extension Specialist, Forestry, WSU Extension, firstname.lastname@example.org; and Tim Kohlhauff, Urban Horticulture Coordinator, WSU-Spokane County Extension, email@example.com