Few people make
a full-time living from their home garden, so it follows that most
gardeners have other reasons for following this pastime. For some
it is a hobby, many want to create a peaceful living environment
while others want to save money and produce nutritious food. Despite
these vastly differing objectives everyone wants healthy plants
in their garden. This is where integrated garden management (IGM)
Ever since humans changed from being hunter-gatherers to actively
farming plants and animals for food and other necessities, people
have looked for ways to increase yields, improve quality or to reduce
the effects of pests and diseases. Over the past several hundred
years this has become an occupation for some people – the
scientists who spend their lives studying certain plants or animals
and looking at ways to get better returns from farming.
Towards the end of the 20th century, scientists in New Zealand
began to pool their knowledge and develop packages for managing
certain crops. This arose out of the need to decrease reliance on
chemical control of pests and diseases and became known as integrated
pest management (IPM). A technical definition of IPM is “the
control of pests by employing all methods consistent with economic,
ecological and toxicological requirements while giving priority
to natural limiting factors and economic thresholds”.
The IPM packages developed by scientists for farmers can also be
utilised by home gardeners. The concept behind IGM is to use a range
of approaches to ensure that plants remain healthy and the effects
of pests and diseases are limited. IGM principles are often even
more relevant for home gardeners than commercial growers, since
people are often very concerned about maintaining a healthy environment
around their home and also have a greater tolerance to damage caused
by pests and diseases than many of our export markets!
IGM involves a range of techniques, not all of which are relevant
for every plant being grown in the home garden. In fact, some techniques
beneficial to some plants and in some situations may be harmful
to other plants and in other conditions. Like most things in life,
it is all a question of balance! Some of the most common IGM techniques
are discussed below and more information is available in the sections
on specific plants.
The first step in IGM is always to prevent a pest or disease from
getting into the garden. Some of the techniques listed below will
help with prevention, like disposing of diseased plants by burying
or burning so that subsequent crops are not infected. Always buy
healthy plants and certified seeds to ensure that new diseases or
insects are not brought into the garden.
Observe your plants closely. Watch for distorted growth, wilting
or yellowing. These are all signs that the plant is not healthy.
Is this caused by a disease, an insect pest or perhaps something
wrong with the soil? Look up books, use the internet or ask an expert
at a garden centre to help with the diagnosis. Often if a problem
is identified early enough then a solution can be found.
Many pests and diseases are specific to one crop. By changing the
position in the garden that the crop is grown in every year these
diseases can be limited, as the new crop is not reinfected from
the soil or previous crop residues. For example, nematodes and powdery
scab that infect potatoes may survive in the soil or on tubers remaining
from the previous crop. In the ornamental garden it is often advisable
to plant a different species when replacing a plant that has died.
Improved soil fertility can be another benefit of crop rotation
in the vegetable garden. Leguminous plants, such as peas and beans,
have root nodules containing bacteria that fix nitrogen. The nitrogen
fixation process makes atmospheric nitrogen available to the growing
plant and may also increase the level of soil nitrogen available
to subsequent crops.
One of the best methods of limiting the effects of pests and diseases
is to grow resistant plants. In this situation the resistant plants
are not infected or they can tolerate the pest or disease without
major losses in yield or quality. Much of the effort in plant breeding
programmes is directed towards selecting for pest and disease resistant
plants. Where possible choose cultivars that are resistant to pests
and diseases. For example, the sugar pea Snow Queen is resistant
to powdery mildew, while the floribunda rose Iceberg is known for
having disease-free foliage.
In the vegetable garden planting and harvesting dates can be adjusted
to avoid the times when pests and diseases are at their worst. For
example, delaying planting of potatoes until soil temperatures have
risen will help prevent powdery scab damage, while it is advisable
to harvest carrots before the period of peak flight activity (April-early
May) of carrot rust fly.
It is a good idea to buy seed that has been certified free of diseases.
Some home gardeners prefer to harvest their own seed but make sure
that the parent plant is healthy. When buying plants for the garden
look at them carefully to make sure you are not bringing new pests
or diseases into your garden!
Before sowing seeds or a potted plant, make sure the ground is
well prepared. This includes providing good drainage, supplying
appropriate fertiliser (which may include adjusting the pH) and
ensuring the soil is well aerated and not compacted. Research has
demonstrated that peas growing in compacted soil are more likely
to become infected by root rots, while camellias require neutral
to acid soil.
It is well known that healthy plants are more resistant to pests
and diseases than plants that are growing poorly. The garden needs
regular maintenance and application of the IGM principles. This
need not take a lot of time – a nightly stroll round the garden
can reveal an area that has not been watered or some tomatoes than
fertiliser. Diseased leaves of roses can be picked off and disposed
of by burial or burning.
Plants may be weakened and become more susceptible to pests and
diseases if they have to compete with weeds for moisture and nutrients.
In addition some weeds harbour pests or diseases that are transmitted
to ornamental or crop plants. For example, hemlock can harbour leaf
blights or spots that can infect carrots. So remember that weeding
isn’t just to make the garden look good!
Plants that don’t have sufficient fertiliser or water won’t
produce good yields and they are more likely to be affected by pests
and diseases than healthy plants. However, ensure that the correct
fertiliser is being applied and don’t over-water. For example,
Aphanomyces root rot is frequently found in peas growing in wet
soils, while excessive nitrogen fertiliser on potatoes can lead
to increased levels of bacterial soft rot.
Biological control is when an organism is used to minimise the
effects of another organism that is a pest or disease to the crop
or ornamental plant. It is important that the beneficial organism
does not damage the target or surrounding plants. Insects, micro-organisms
and plants can be used for biological control. A common example
of biological control is ladybirds eating aphids. The beneficial
organism can be artificially reared and released or the home gardener
can create conditions in the garden that will encourage the presence
of the beneficial organisms. This may include minimising the use
of damaging sprays or companion planting, which is described below.
A practice much loved by organic gardeners, the science behind
companion planting is only just beginning to be revealed. The theory
is that companion plants produce compounds that attract beneficial
organisms or repel organisms that are pests or diseases. For example,
nasturtiums produce a mustard oil that aphids dislike. Another good
companion plant is buckwheat, which produces nectar to feed parasitic
wasps. Although the wasps sound rather nasty, the are quite small
and only attack insects such as leafroller caterpillars.
If all else fails it may be necessary to use chemical sprays or
dusts. Some chemicals are more selective than others, which means
that only the target pests or diseases are affected. These chemicals
should always be used rather than the “broad spectrum”
chemicals that may affect beneficial organisms as well as the targets.
Always read the directions carefully and follow them. Increasing
the rate of chemical is not a substitute for correct application.
If repeated applications are required, try and use a chemical from
a different chemical group to avoid the build-up of resistance within
the pest or disease population. Note that some chemical sprays and
dusts, e.g. sulphur and Derris Dust®, are certified for organic