Re: Decomposed granite as a mulch
- Subject: Re: Decomposed granite as a mulch
- From: T* a* M* R*
- Date: Wed, 13 Feb 2002 10:13:43 +1300
"M. & L. Doster" wrote:
>
> >For whatever reason, in FL the advice is against mulching citrus trees.
> >Look in commercial orange groves and you will see a clear areas under the
> >trees.
>
> Likewise, all the commercial orange groves I've seen in this area of
> California have bare soil.
>
> One reason is that bare soil minimizes cold damage during winter. The soil
> absorbs heat during the day and releases the heat at night (thereby warming
> the aboveground parts of the tree). A mulch or a cover crop (including
> weeds) interfere with this process (prevents the soil warming during the day
> and releasing heat at night). I've seen the results of a study where the
> temperature effect was measured; the bare soil resulted in the warmest air
> at night, then a mulch, and the worst was a cover crop.
Well
This certainly explains why the bare soil is preferred in areas where
the winter temperature is marginal. In NZ all of the commercial cirtus
crop and the majority of the home garden trees are grown in areas which
are virtually frost-free or the frosts are slight enough to be
disregarded.
The area where my garden is situated however is indeed a marginal one
and I
attempt ony the two most hardy of the locally-available varieties, the
NZ
grapefruit and the Meyer lemon. Even before I had the stone mulching,
thanks to a shelered site my mulched trees suffered no appreciable
winter damage by temps occasionally as low as -3° C, and usually carried
their crop through the cold weather to ripen in spring. However last
winter when we had a most unusual low of -6°C , though neither tree
showed any foliage problems with stone mulch, the fruit was damaged. On
the grapefruit only the less protected ones at the top of the tree were
affected, not dropping, but brown inside when picked. The lemons at a
very early stage of growth mostly turned black and fell of, but a couple
of larger ones near the ground survived, ripened and were usable (This
is a very tiny new tree no more than a metre tall or it might have had
more of the latter). Now both trees are back in bearing with plenty of
new crop.
I was most interested to see the suggestion by Krzysztof that the effect
of a stone mulch must be different from any organic cover. Stone would
certainly absorb heat during the day as well or even better than bare
ground (which after all must contain some organic matter to dilute its
effect). If uncovered, stones will then radiate their heat up to the
foliage
at night.
It seems to me that by applying organic matter to the soil UNDER a stone
layer the tree gets the best of all worlds -a cool root run near the
soil surface with good moisture retention and plenty of available
natural food plus a source of warmth on cold nights as well.
And if summer moisture is much of a problem it would surely be
effective
to create a basin around the stem to retain moisture and line this with
stones to shield the surface against evaporation during the heat of the
day. In fact the stones themselves can aid moisture-collection by
encouraging the condensation of dew on their surfaces during the cold
hours of the night, which will then trickle down the cracks to the soil
beneath.
Moira
--
Tony & Moira Ryan
Wainuiomata NZ,
where it's Summer in January and Winter in July.
