Integrated Garden Management (IGM)
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) comes in.
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.
Prevention
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.
Identification
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.
Crop rotation
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.
Resistant cultivars
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.
Planting and harvesting date
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.
Clean seed and healthy plants
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!
Soil preparation
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.
Garden management and hygiene
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.
Weed control
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!
Fertilisers and irrigation
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
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.
Companion planting
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.
Chemical control
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 use.