grounds-care professionals prefer particular mulches, but what’s best
for a site actually depends on the needs of the situation.
Although mulch producers often make claims about the durability,
composition or colour, little research exists to confirm such claims.
A team of Florida researchers recently evaluated the
characteristics of six organic mulches to define their qualities and
usefulness for certain situations.
researchers obtained commercially available local mulches from
Pine needles (Pine Straw)
researchers tested them for chemical, allelopathic and decomposition
properties. The concluded the following:
The most nutrient- laden mulches were the utility trimmings and pine
straw. Both of these mulches contained less than 1% nitrogen.
The nitrogen content in the utility mulch was due to the presence of
green leaves in the material.
The presence of nutrients in the pine straw supports the research done
by several Universities on the Fertilizing of Pine Plantations.
Research has found that repeated annual rakings deplete nutrient
reserves at the pinetree plantations.
It is estimated that the raking and removal of the pine needles,
removes 20 lbs of Nitrogen and 2 lbs of Phosphorus per acre. It is
recommended that Plantations apply 175 to 200 lbs Nitrogen, 50 lbs
Phosphorus and 50 lbs Potassium per acre on areas raked annually.
The most decomposition resistant materials were pine straw,
bark, cypress and melaleuca. These materials contained higher levels of
lignin than the utility trimmings and eucalyptus.
Lower decomposition rates result in longer intervals before mulches need
to be reapplied.
All of the mulches tested were initially allelopathic due to the
presence of certain aromatic compounds.
The researchers measured the allelopathy in terms of germination
The most allelopathic mulches were the pine straw and the utility mulch
which both suppreseed seed germination after 1 year. The majority of the
mulches lost their allelopathic qualities within a few months.
All mulches lowered the pH of the underlying soil.
In the evaluation, the initial soil pH was 5.0.
After 1 year, eucalyptus lowered pH to 4.8, utility trimmings and
melaleuca lowered it to 4.7, cypress and pine bark lowered to 4.6, while
the pine straw dropped the pH to 4.4.
The researchers considered soil acidification a drawback, but this trait
can be beneficial depending on initial soil pH and whether the existing
plants prefer acid soils.
All mulches lose their initial colour.
The rate of colour loss is dependent upon the conditions. The
graying of the mulch is mostly on the surface where the sun has
impacted. If the mulch is raked some of their initial colour will
If colour is a concern, best solution is to replenish the mulch with a
thin layer of new mulch.
There is no
perfect mulch for all situations. An
understanding of the differences between materials should help the
grounds managers choose the most ideal mulch for a given site.