by Iain Dawson (Horticultural Research Unit, ANBG)
Plant hardiness maps allow producers to label their plants as
being suitable for particular areas, and, in theory at least, this
results in happy customers who can confidently buy plants that will
survive in their locality. I have created 7 zones to fit our climatic range,
and used metric units.
As might be expected, the main factors determining average
minimum temperature are altitude, latitude and proximity to the coast.
Zone 1 covers the alpine areas of south eastern Australia.
Zone 2 the tablelands of south east Queensland, New South
Wales and Victoria, and the uplands of central Tasmania.
Zone 3 includes much of the southern half of the continent, except
for localities on or near the coast. Many of our weather stations
are on the coast or on off-shore islands (some of them are lighthouses)
and these are often a zone or two higher than adjacent mainland
stations because of the warming effects of the ocean in winter.
Zone 4, because of this warming effect, covers a broad area
from coastal Queensland across the continent to Shark Bay
and Geraldton in the west, also includes the Mornington
Peninsula, areas adjacent to Spencer Gulf and Adelaide, the south western coastal zone, Sydney and the
north coast of NSW, along with a number of localities dotted all around
the southern coast of the continent.
Zone 5 covers, some of the Queensland coast, Western
Australia north of Shark Bay and across the top end.
Zone 6 includes the Queensland coast north of Cairns, Cape
York Peninsula and the coast of the Northern Territory.
Zone 7 is mainly restricted to islands off the north coast.
There are many problems with maps of this type. For example,
the spread of weather stations is insufficient to give good resolution
of the zones and too many places with different climates are lumped
together.
Even worse are the problems of local factors such as aspect,
altitude, proximity to the sea and so forth. Everyone is aware that
different locations in the area are suitable for different plants but it is
hard to quantify these differences and even harder to draw a
meaningful map.
Plant hardiness refers to their
ability to survive the conditions of a particular location, including
tolerance of heat, soil moisture, humidity and so on. This map is based only on how well they survive low temperatures in winter. Even
that is a gross oversimplification. For example, are plants affected
more by a single extremely low temperature night, or is the number of
days of frost (the duration of winter) more important? In fact both
are important, but the statistic for the map only relates directly to
the former. Another limitation is that often plants will survive in an
area for some time, but every now and then there will be a
catastrophic cold snap that will kill them. Some risk evaluation - the probability
of getting a particularly severe low temperature - often would be
more useful for each locality rather than the average conditions.
Low temperature is not the only determinant of plant survival.
Other environmental factors such as high summer temperature, humidity,
soil temperature, etc. may be equally important. Also, many plants
will survive in a locality but won't flower if the daylength is
inappropriate or if they require vernalisation (a particular
duration of low temperature). The low temperature statistic is only
appropriate for woody perennial species, and even then its use is limited.
With annuals the time of planting can often be adjusted to allow
growth beyond their normal geographical range.
The map is thus only useful as
a very broad guide. It needs interpretation that takes into account factors other than low temperature that limit plant growth as well
as local knowledge.
As might be expected, the main factors
determining average minimum temperature are altitude,
latitude and proximity to the coast.
Even worse are the problems of local factors
such as aspect, altitude, proximity to the sea and
so forth.
An alternative system for
describing plant hardiness is to use indicator plants. Common plants
with known limits to their range are generally used for this purpose.
For example, many people will know whether lemons will grow in
their locality. If you then say Geraldton Wax will grow more or less
where lemons will grow you have defined the range of Geralton Wax
with some accuracy (whether or not it will flower is another
problem). Unfortunately no two plant species seem to have exactly the same
requirements and even within a species there are differences.
'Meyer' lemons, for example, are more cold tolerant than 'Eureka'.
You can really only define core areas and they are often very
arbitrary.
At some time in the future I
think we will probably get around the problems associated with
plant/climate maps with much more sophisticated database systems
that combine complex climate statistics and advanced plant growth
models. There are already software packages available to help you
select landscaping plants. Two that I have seen are the 'Grow
What Where' computer version, published by The Australian
Plant Study Group, and 'Plantguide' from Arbordata. However,
these have limited climatic inputs. You are asked, for example, to
select between fairly vague zones such as 'warm temperate' or 'eastern tablelands'. Climatic data bases that
allow you to assess the chances of particular climatic events taking
into account some local factors (eg. what are the chances of
getting more than five consecutive nights with temperatures lower than
-10°C on a south facing slope in Canberra?) combined with
data bases containing detailed knowledge of plant responses to their
environment (eg. how many nights of frost can Pandorea
jasminoides survive?) will go a long way towards answering the question
of what grows where.
This article was originally published in 'Australian
Horticulture' 90 (8) 37-39, 1991, when the author was employed
by CSIRO Plant Industry. |
