The post you linked to that had the recipe for gritty mix with vermiculite was for someone growing in the hot windy desert. I'd skip the vermiculite and play with the screened Tureface:grit ratio to up water retention.

Freezing

Commonly,
each species of plant has a general range of cold-hardiness. Within
species and cultivar, cold-hardiness is genetically determined. That
is to say that a plant that is propagated from cuttings or tissue
culture will have the same ability to resist cold as the parent
plant. Plants cannot "develop" a greater degree of
cold-hardiness by repeated or prolonged exposure to cold, even after
100 years (trees).

If
we pick any plant at random, it may or may not be able to withstand
freezing temperatures. The determining factor is the plants ability
to prevent freezing of bound water. Bound water is the water inside
of cells.

There
are actually three kinds of water to consider when we discuss
"freezing". The water held in soil - When this water
freezes, and it can freeze the soil mass solid, it doesn't
necessarily kill the plant or tissues. Then there is free or unbound
water, also called inter-cellular water. This is water that is found
in plant tissues, but is outside of living cells cells. This water
can also freeze solid and not kill the plant. The final type of water
is bound water or intra-cellular water. If temperatures drop low
enough to freeze this water, the cell/tissue/plant dies. This is the
freeze damage that kills plants.

Fortunately,
nature has an antifreeze. Even though temperatures drop well below
freezing, all plants don't die. In hardy plants, physiological
changes occur as temperatures drop. The plant moves solutes (sugars,
salts, starches) into cells and moves water out of cells to
inter-cellular spaces in tissues. These solutes act as antifreeze,
allowing water in cells to remain liquid to sometimes extremely low
temperatures. The above is a description of super-cooling in plants.
Some plants even take advantage of another process to withstand very
low temps called intra-cellular dehydration.

The
roots of your trees can stay frozen for extended periods or go
through multiple freeze/thaw cycles w/o damage, so long as the
temperature does not fall below that required to freeze
intra-cellular water. If roots remain frozen, but temperatures remain
above killing lows, dessication is the primary concern. If the tree
is able to take up water, but temperatures are too low for the tree
to grow and make food, stored energy becomes the critical issue.
Dormant and quiescent trees are still using energy from their
reserves (like a drain on a battery). If those reserves are depleted
before the tree can produce photosynthesizing mass, the organism
dies.

There
are a number of factors that have some affect on the cold-hardiness
of individual plants, some of which are length of exposure to
seasonal cold, water availability (drought stressed plants are more
cold tolerant), how recently planted/repotted, etc

No
one can give a definitive answer that even comes close to accurately
assessing the temperature at which bound water will freeze that
covers the whole species. Unbound water is of little concern &
will usually freeze somewhere around 28*.

Some
material will be able to withstand little cold & roots could
freeze/die at (actual) root temperatures as warm as 25-27*. Other
plants may tolerate much colder actual root temperatures - as low as
10*. There's just no way of knowing unless you have a feeling for how
cold-tolerant the genetic material the plant was derived from might
be, and finding out is expensive (from the plant's perspective). ;o)
Another example of this genetic variance is that trees found growing
and fruiting well closer to the equator need no chill time, while
other trees, derived of genetic stock from a more northerly
provenance may need a period of chill to grow with optimum vitality
in the subsequent growth period/cycle.

It's
wise to remember that root death isn't instantaneous at one
particular temperature. Roots succumb to cold over a range of chill
with cultural conditions affecting the process. The finest roots will
die first, and the slightly thicker and more lignified roots will
follow, with the last of the roots to succumb being the more
perennial and thickest roots.

Since
any root death is a setback from an energy allocation perspective,
and root regeneration takes valuable time, it's probably best to keep
actual root temperatures in the 25-40* range as long as we can when
the tree is resting, even though the organism as a whole could
tolerate much lower temperatures. Even well established trees become
very much like cuttings if all but the roots essential to keep the
tree viable are lost to cold. Regeneration of roots is an expensive
energy outlay and causes the trees to leaf out later than they
normally would and shortens the natural growth period and reduces the
potential increase in biomass for the next growth cycle and perhaps
beyond.

And

We
know that when trees are exposed to freezing, the finest hair roots -
the ones that do the lions share of the work are the first to die. In
many woody plants, these roots begin to die as soil temperatures drop
below 30-32*. As temperatures drop further, larger and larger roots
succumb to killing low temperatures. The point is - that many trees
that SURVIVE are left with only the largest roots to support them
because much of the rootage has frozen. These trees are slow to
respond in the spring because they need to utilize stored energy to
regenerate lost rootage before they can move sufficient water and the
nutrients dissolved in water to support either growth or the flush of
foliage that makes the food that allows the tree to grow (this, in
the case of deciduous material).

So,
while trees might survive exposure to borderline killing low
temperatures unprotected, we KNOW it is better for the tree,
especially from an energy management perspective, if we give them
protection that ensures actual root temperatures don't drop low
enough to kill even the finest roots. For most temperate trees, that
means we should strive to keep low root temperatures in the upper 20s
at their lowest, and below 42* to keep them from growing until spring
when we can get them into good light w/o worrying about frost/freeze.

Al