Trevor Norton, of Hampden, asks :-

We notice that very little growth occurs under a canopy of pine. In places blackberry, tutu and gorse will grow (but not necessarily thrive). After trimming a radiata hedge, the trimmings have been put through a chopper. Is the resulting mix of coarse pine wood chips and lacerated pine needles ok to use around garden shrubs as a mulch and weed suppressant, or being pine, will it have a detrimental effect on the soil or the shrub it is placed around? If radiata pine mulch is toxic, are there other trees too, whose mulch should not be used in this manner?

Jo Wakelin, a horticulturist at Otago Polytechnic, responded.

Yes, you are quite correct that little grows under mature pines. The main reason many plants don’t thrive in this situation is the dense shade and extreme dryness these evergreen trees create, rather than any special toxicity. The trees have dense root systems near the surface and are strong competitors for soil nutrients and moisture. Dry shade is always a challenge for landscapers.

Although pine needles will not cause problems with acidity in the long term, they do break down relatively slowly as the needles are covered with a waxy layer that resists bacteria and fungi, and they have a high carbon content relative to nitrogen.

By the time the needles are brown and dry, any potentially growth inhibiting substances such as terpenes have dissipated in the air. Terpenes are volatile organic compounds found in many plants. Dry brown pine needles are actually harvested and baled commercially in North America and are a popular garden mulch.

Wood chips also have a high carbon and low nitrogen content. In order for soil organisms such as fungi and bacteria to break down wood chips and needles, they must use nitrogen from the soil, which is needed for plant growth. This is referred to as nitrogen drawdown, but it can be overcome by applying a fertiliser containing nitrogen before mulching. Composted pine bark is widely used in the nursery industry as a component of potting media.

The best practical option is to compost the fresh wood chip and green pine needle mix in a large heap for a few months with added nitrogen before using as mulch. Although some plants that don't like to be mulched, such as bearded iris, whose rhizomes actually need to bake, mulching is generally very beneficial, reducing weeds and watering, as well as improving the soil. Remember not to pile any mulch against trunks and stems.