Agriculture Reference
In-Depth Information
biodiversity
, that is, a reduction in the number
of habitats and species present in the wild, less
genetic variation in the wild and fewer cultivated
species (p. 45). Threats to biodiversity include habitat
destruction, pollution, introduced species, global
warming and overexploitation of natural resources.
When focusing on environmental sustainability, there
are many issues that have relevance for gardeners and
horticulturists. Some of these are:
X
peat
X
invasive non-native species
X
'greenhouse gas' emissions
X
waste
X
removal of rare species from the wild.
Peat
is used in composts (Figure 1.9), and sometimes
as a soil conditioner too. It has unique characteristics
which make it ideal as a growing medium (see
Chapter 15). Peat is extracted from peatlands mainly in
the north temperate zone in areas of high rainfall and
low temperature and is formed of the incompletely
decomposed remains of sedges, mosses, grasses and
reeds. Peatlands are important for four main reasons:
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They form a unique natural habitat supporting
many rare species.
X
They are an important carbon sink, locking up
about a third of the world's soil carbon.
X
They are an archive containing archaeological and
geochemical historical information going back
hundreds of years.
X
They play an important role in the water cycle
contributing to fl ood prevention, water quality and
quantity of freshwater.
Peatlands form very slowly at a rate of only 1 mm
a year. A 10 m deep layer of peat has taken around
10,000 years to form, so when peat is extracted,
the damage is irreparable. Outside the European
Union the importance of peatlands is recognized
but legislation to protect them varies. Within the
EU, most peatlands are now so rare that they are
being designated as Special Areas of Conservation
and member states are now required to protect
them rather than permit them to be mined and then
restored. To tackle the problem, targets have been
set to phase out peat in bagged composts by 2020
for amateur gardening and by 2030 for professional
growers. The main difficulty with reducing peat usage
is finding suitable alternatives which are themselves
sustainable, consistent, the right cost and deliver
similar results to peat-based composts. Acceptable
peat alternatives for seed sowing and some groups of
plants such as acid-loving plants have been difficult to
develop. Some of these alternatives are discussed in
Chapter 15.
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1
Figure 1.9
Bags of compost at a garden centre
containing peat
Invasive non-native species
include many weeds,
pests and diseases which have arrived in this country
from elsewhere. Their impact is second only to habitat
destruction in reducing biodiversity. In the heyday
of plant collecting (see Chapter 2), little thought
was given to the effects on native wildlife of foreign
plants. While most were well behaved, some garden
escapees are now causing increasing problems for
native plants and habitats and also pose a challenge
to forestry, tourism, agriculture and construction.
Since they originated in countries far removed from
Britain and Ireland, they were not accompanied by
their natural diseases and predators that might have
kept them in check. Three examples of invasive plant
species are:
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Fallopia japonica
(
Japanese knotweed
) was
introduced in the nineteenth century by E.A.
Bowles as an attractive ornamental (Figure 1.10a).
It can grow at an alarming metre or more a month,
penetrating tarmac and concrete and is highly
resistant to weedkillers. The annual cost to the
construction industry for removal is signifi cant
(it cost £70 million to remove it from the London
Olympic site).
X
Rhododendron ponticum
is a familiar weed
in woodlands with attractive purple fl owers in
spring (Figure 1.10b). It is one of nature's most
successful plants, spreading readily by seed
and rooted branches, and its dense evergreen
canopy cuts out the light and produces poisons
and a deep leaf litter which suppress the growth
of everything beneath it including woodland tree
seedlings. It was introduced as an ornamental
garden shrub in 1763 and also used as a
