Peas (Pisum sativum) belong to the Leguminosae family,
which includes beans and is characterised by having seeds in pods
and root nodules containing bacteria that fix nitrogen. The nitrogen
fixation process, where atmospheric nitrogen is made available to
the plant, is particularly important as it can be utilised to increase
soil fertility during crop rotations. Peas are annual plants that
are sown in the autumn or early spring and require a cool temperate
climate. The edible portion is the seed and, for some types of peas,
also the seedpod.
There are two types of pea generally grown by home gardeners. These
are the garden pea, where the peas are removed from the pod before
being eaten, and edible pod peas (sugar snap and sugar pod or snow
peas) where the whole pod including immature peas is eaten. Sugar
pod peas have been specifically bred to have a tender, low-fibre
pod. They are also known as mange tout, which is a French name that
translates as "eat all". Another type of pea, known as
the field pea, is mainly grown commercially. These are harvested
when the pea is dry and there are many varieties with different
colours, shapes and end-uses.
The history of peas
Peas probably originated from Ethiopia, Turkey and the eastern
Mediterranean. There is evidence that they were cultivated as early
as 6000 BC and they were a component of Greek and Roman diets. By
1000 AD, peas had become an important crop in northern Europe, particularly
in Britain, but they were probably harvested as dry peas. In the
late 1700s a pea-breeding programme resulted in two varieties that
were eaten green.
Peas have been grown in New Zealand since the arrival of the European
settlers. Trials of varieties imported from the UK and USA were
carried out in the 1920s and breeding programmes to develop varieties
suitable for New Zealand conditions were initiated at that time.
Until the 1950s commercially produced garden peas were generally
canned, but now canning accounts for less than 1% of the processed
pea crop. At present the majority of processed garden pea crops
are frozen, with a large proportion of these being used in frozen
mixed vegetable packages. However, dehydration is now a significant
method of processing and preservation, accounting for 6–10%
of the harvested crop. Currently about 40,000 tonnes of peas are
grown for processing each year on approximately 10, 000 ha. The
main areas of commercial production are Canterbury, Marlborough,
Manawatu and Hawke’s Bay. Frozen peas are now New Zealand’s
fifth most valuable vegetable export, and peas are exported to 35
countries.
Peas are a very successful crop to process since they retain an
exceptionally high proportion of the vitamins and other nutrients
found in fresh peas. In particular, the nutritive value of frozen
peas is almost as good as fresh peas.
Sowing and harvesting
Peas are cool season plants, which means that their main growth
and flowering period is during spring when temperatures range from
10-16°C. Seed can be planted in autumn (May) but this may have
little advantage over early spring (August and September) sowings,
particularly if the soil tends to be waterlogged during winter.
To achieve the good drainage that autumn-sown peas require, it may
be necessary to plant into raised beds. Pea planting can continue
throughout spring and early summer (October and November) but yields
decrease with later plantings, as the plants flower earlier due
to the warmer conditions. In warm areas peas can be grown all year
round.
Soil requirements
Achieving optimal soil conditions prior to planting is very important
for obtaining good pea yields. Soil compaction will lead to death
of pea plants, so it is essential that the soil be well aerated
by digging before planting. Any compost, manure or fertiliser should
be thoroughly mixed throughout the soil as young pea roots can be
damaged by direct contact with these materials.
Soil pH should be around 6.5, which means that lime may be beneficial,
especially if the peas are following a potato crop. Because peas
can obtain nitrogen through their root nodules, they are often planted
into nitrogen-depleted soils, such as after potatoes, and nitrogen
fertiliser is unnecessary. Peas can also follow root or leaf crops,
but should not be planted after other legumes like beans.
Sowing and staking
Pea seed should be sown less than 50 mm deep in single or double
rows. The between-plant spacing is 100 mm for single rows and 140
to 200 mm for double rows. Between-row spacings range from 600–1000
mm, depending on the cultivar’s vine length. The type of cultivar
used will also determine what type of rows to sow and the need for
staking.
Dwarf pea cultivars do not need staking, but growing them over
a stick lattice will enhance yield and lengthen their productive
life. For the taller varieties, place stakes at the end of the rows
and run strings between the stakes on the outside of the rows. This
is when double or even triple rows can be an advantage as the peas
support themselves on the inside of the rows and staking is only
necessary on the outside of the pea block. Sugar pod peas can grow
up to 2 m high and are often best grown against some vertical support
such as a netting fence or trellis.
Weed control
During the early growing period it will be necessary to remove
weeds, as competition will reduce yields. However, take care with
hoeing to avoid damaging the delicate pea roots. Irrigation may
be important in dry areas as peas consume a lot of water. Yield
and quality will be reduced if peas become water stressed, particularly
during the two critical periods of flowering and pod-filling (when
the young pea seeds are expanding).
Harvesting procedure will depend on the type of peas being grown.
Garden peas should be picked when the pods are swollen and smooth
and the peas inside are round, but before the pods develop a wrinkled/rough
surface. When harvest time is approaching check the pea crop regularly
by picking several pods and opening them. Quality will drop rapidly
when the peas mature since they become hard and starchy. The pea
plants go through 2-3 rounds of flowering, so the first pods to
mature will be near the base of the plant. The harvested peas should
be used as soon as possible after picking, since fresh peas will
quickly lose flavour. Always store them in the refrigerator and
use in less than a week. Leave the peas in the pod and shell them
immediately before the peas are cooked for eating or blanched for
freezing. Don’t overcook fresh peas or they will go mushy!
Sugar pod peas can be harvested when the pod is quite flat and
snaps easily in half. Some varieties can still be harvested when
the peas make small bumps in the pods, but as the pod matures it
becomes more fibrous and less palatable. Generally the pod can be
eaten whole, but with some varieties it is necessary to remove the
strings along the seams of the pod. Sugar pod peas mature quickly
and should be picked 5–7 days after flowering. It is usually
necessary to pick sugar peas every second day to ensure their quality
remains high.
Picking the peas also encourages the plant to continue flowering
and extends the harvesting season. If overgrown pods are found on
the plant, pick them immediately. It may be possible to use the
developing pea as a garden pea, but the pod should be discarded,
as it will be too fibrous to eat. Sugar pod peas can be stored in
the refrigerator for up to 2 weeks, since they do not lose quality
as quickly as garden peas.
Sugar pod peas can be stir-fried or steamed. If they are overcooked
they will lose crispness and can burn easily because of the high
sugar content. Sugar pod peas can also be blanched for freezing.
Sugar snap peas are harvested when the pods and peas are swollen
but still sweet and juicy. The whole pod with peas enclosed is harvested
and eaten raw or lightly cooked. They are well suited to stir-fry
dishes. New varieties are stringless.
Garden pea cultivars can be categorised by the height of the plant
and the time of maturity. Thus, there are dwarf and tall cultivars
and early, mid and late season cultivars. A brief description of
the most common cultivars used by home gardeners is given below.
Pea breeding programmes in New Zealand by government-funded and
private breeding organisations have resulted in the release of a
number of pea cultivars, but most of these are intended for commercial
rather than home growers.
Garden peas
William Massey is the classic garden pea variety
used in New Zealand. This is an early season, dwarf cultivar that
matures in 70–80 days. It was one of the pea cultivars originally
brought into New Zealand in the 1920s as Kelvedon Wonder. During
the 1950s, resistance to the pea wilt disease was introduced to
William Massey in New Zealand breeding programmes.
Little Marvel is a dwarf variety with similar
maturity to William Massey. It is susceptible to pea wilt disease
but can perform better than William Massey under unfavourable seasonal
conditions.
Greenfeast (synonymous with Lincoln) is a mid-season
(80-85 days), dwarf variety that, like William Massey, was introduced
to New Zealand early in the 1900s and subsequently selected for
pea wilt resistance. Immunity to bean yellow mosaic virus (also
called pea mosaic) was introduced to this cultivar in the 1960s.
It is a very hardy pea and is widely grown although tends to have
uneven sized peas and can have uneven maturity. The peas are paler
than most other garden peas, as the seed has a yellow cotyledon;
most garden peas have a green cotyledon.
Onward is also a mid-season, dwarf variety and
has resistance to pea wilt disease. Another popular home variety,
Onward requires better growing conditions and is about a week later
maturing than Greenfeast.
Giant Alderman is a late maturing (up to 100 days),
tall cultivar. It is lower yielding than most other cultivars but
does have the advantage of allowing fresh peas to be harvested over
a longer period. Unlike most garden peas, it grows well in the northern
regions of New Zealand.
Sugar snap is one of the most popular edible pod
pea varieties. It is early maturing, grows up to 1.5 m high and
requires staking. Dwarf and climbing varieties of sugar snaps are
available, with the dwarf variety maturing 2 weeks earlier than
the climbing one.
Snow Queen is resistant to powdery mildew and
is available as both dwarf and climbing varieties. It is a very
sweet sugar pea.
Garden peas are one of the most common vegetables in the New Zealand
diet. Peas are mainly carbohydrate, but they are also a good source
of protein, and they have 3% fibre and less than 1% fat. Peas also
contain some important vitamins and minerals. For example, fresh
garden peas contain 16 mg vitamin C/100 g and 70 mg folic acid/100
g, which are both more than one-third of the Australian recommended
daily intake, and 1.7 mg iron/100 g, which is more than one-fifth
of the recommended daily intake for adult males. Peas are a good
source of vitamin E, and also contain carotenoids, the precursors
to vitamin A, and a number of minerals, including potassium and
magnesium.
Garden peas are usually cooked by boiling. Unfortunately, during
boiling some of the nutrients from peas, particularly vitamin C,
can leach out of the peas into the water. To ensure valuable nutrients
are not lost do not boil peas for too long and then use the boiling
water to make sauces or gravy.
Sugar pod and sugar snap peas are often used in stir-fry dishes.
This method of cooking is an excellent way to ensure that the vitamin
C is retained in the peas. Steaming is another method of cooking
sugar peas without losing vitamin C. Of course, the best way to
utilise the nutrients from edible pod peas is to eat them raw in
salads, freshly picked from the garden.
Peas, like most vegetables, are susceptible to a number of diseases.
These can affect the germinating seedling, the roots and the growing
tops, as well as the pods and the peas inside. Fortunately, few
pests affect peas. The most important pest is aphids, which transmit
viruses to the pea plants. To minimise the effects of pests and
diseases on peas in the home garden, the principles of Integrated
Garden Management (IGM) should be used.
Integrated Garden Management
for peas
IGM techniques initially rely on preventing pests and diseases
entering the crop. This is not always possible so the crop should
be watched closely during growth for the first signs of pests or
disease. Once the pest or disease has been identified, an appropriate
control technique should be selected. This may involve more than
one method and will not necessarily involve the application of chemicals.
Chemical control can be very effective for large areas of crop or
when a pest or disease attack is particularly severe. However, in
home gardens, the application of chemicals, especially insecticides,
can upset the ecological balance within the garden and lead to further
outbreaks of other pests and diseases. Some IGM techniques useful
for growing pea crops are discussed below. These include crop rotation,
resistant cultivars, seed treatment, weed control, fertiliser, irrigation,
biological control and chemicals.
Crop rotation For hundreds of years, farmers have known that it is best
to avoid growing crops in the same position every year. Whilst early
farmers may not have understood why, we now know that this is due
to the build-up of diseases in the soil. For peas, root rots, septoria,
sclerotinia, downy mildew and ascochyta blight, are some diseases
that may increase in severity in subsequent crops grown in the same
position.
There are other benefits of crop rotation and one of these is the
nitrogen-fixing ability of legume crops, such as peas. Thus, in
a crop rotation peas can follow plants like potatoes that have extracted
much of the available nitrogen from the soil. There will be no disadvantage
to the pea crop, and the following crop will also gain some benefit
from the nitrogen that is fixed by the bacteria growing on the roots
of the pea plants and left in the soil when the pea roots decompose.
Resistant cultivars
Pea breeding programmes have been particularly effective at producing
pea cultivars resistant to diseases. For example, pea wilt was once
a serious disease of pea crops but now most commonly available pea
cultivars are resistant to the disease. Recent breeding work has
resulted in the release of powdery mildew resistant garden peas
(Trounce) and sugar peas (Snow Queen).
Seed treatment and soil conditions
Peas are usually sown into cold, wet soil, and while they are tolerant
of cold temperatures, growth of the seedlings is often slow. This
means they are prone to attack by fungi that live in the soil. To
ensure the seedlings have the best chance of getting established
and resisting the fungi, pea seed is often treated with fungicide.
This protects the germinating seed and subsequent seedling until
the plant is large enough to withstand the effects of these organisms
or growing conditions improve. Many of the pea seeds bought in packets
from garden centres have already been treated with fungicide.
Ensuring seed is planted into well drained, aerated soil that has
not been compacted is another IGM technique to help prevent diseases
developing in peas. These soil conditions will assist the pea seedling
to get established quickly and withstand the effects of any diseases
that are present. The effects of sowing peas in compacted soil are
not just seen in pea seedlings. Much research has shown that peas
growing in compacted soil are more likely to become infected by
root rots and the infections will be more severe than those peas
growing in well-aerated soils. Good levels of organic matter also
help soil aeration and pea root development.
Always plant fresh pea seed (check the “Use-by” date
on the package), since the germination of pea seeds decreases when
the seeds have been stored for several years, unless the seed has
been stored under special conditions (cold and dry).
Weed control
Peas can be quite badly affected by competition from weeds, particularly
in the early stage of crop development. Whilst weed competition
can have a big impact on yield, pea plants that are weak are also
more susceptible to pests and diseases.
Another aspect of weed control is the transfer of diseases to peas
from other plants, especially legumes. This is particularly important
with virus diseases that are transmitted by aphids. Ensure that
clover or lucerne growing near pea crops are kept free of aphids
to prevent the transmission of diseases like alfalfa mosaic virus,
top yellows, pea mosaic virus (=bean yellow mosaic virus) and pea
seedborne mosaic virus.
Fertilisers and irrigation
Peas that are growing rapidly, with sufficient nutrients and water,
will be less susceptible to diseases than peas that are stressed
by under or over-supply of these factors essential for growth. Some
root rots are more likely to occur when peas are grown in low soil
fertility conditions, while aphanomyces root rot is frequently found
in peas that are growing in wet soils. Whilst peas will not flourish
in very poor, impoverished soils, in general they do not need the
high levels of fertility that many other vegetables do.
Biological control
Biological control is when one organism that does not affect the
crop plant is used to control another organism that does have adverse
effects on the crop. A good example with peas is incorporating brassica
leaves into the soil to help control or reduce the severity (soil
index level) of the fungus Aphanomyces euteiches, which
causes aphanomyces root rot.
Chemicals
If all else fails it may be necessary to use chemical sprays or
dusts. 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.
The main pests and diseases affecting peas in New Zealand and some
suggestions for their control are outlined below.
Fungal diseases
Seed and seedling diseases. Some common soil fungi,
such as Pythium and Fusarium species as well as
Rhizoctonia solani, can cause seeds to rot before germinating
or seedlings to die soon after germination. This is particularly
a problem in wet soils. To prevent or minimise the effects of these
diseases, high quality seed should be sown into well-prepared and
well-drained seedbeds. Fungicide treatment of the pea seeds can
help prevent these diseases.
Powdery mildew (Erysiphe pisi). The symptoms
of this disease are small, white lesions usually on the upper surface
of the leaves. The disease can spread very rapidly, turning all
surfaces of the plant a grey-green colour. Powdery mildew is particularly
prevalent in warm, dry seasons and is more of a problem in late-sown
crops. Pea cultivars resistant to the disease are now available.
If a susceptible cultivar is used and the season is likely to cause
powdery mildew outbreaks, fungicides or sulphur (an organic alternative
to chemical fungicides) can be sprayed onto the growing pea crop.
Downy mildew (Peronospora viciae). This
is a completely different disease from powdery mildew, and tends
to occur in cool, wet seasons and in early-sown crops. Symptoms
are variable but may include yellow to brown blotches on the upper
leaf surface with fluffy, white to grey patches on the underside
of leaves. Pods may also be affected, with white, cottony growth
inside the pods and aborted peas. Fungicides can help to control
the disease. Other methods of control include prevention of initial
infection by burning or burying pea residues and crop rotation.
Ascochyta blight. This disease is caused by a
complex of three closely related fungi and occurs during cool, wet
seasons. Circular (leaves) or elongated (stems) brown lesions are
found on the plants, and sometimes small black dots can be seen
in the centre of these lesions. The fungi can infect and be transmitted
by the seed and can survive in plant debris. It can also survive
in the soil for some years. Seed fungicide treatments can reduce
infection, and crop resides should be burned or buried. Long crop
rotations will help to reduce the disease.
Root rots. Various fungi, including Fusarium
solani f. sp. pisi (and other Fusarium species), Phoma
medicaginus var. pinodella, Rhizoctonia solani,
Tricocladium basicola and some Pythium species, can cause root
rots. The symptoms are reddish-brown streaks on the roots, which
coalesce into dark lesions encircling the roots. The roots may become
blackened and weak. Plants may collapse during periods of moisture
stress, such as in hot, dry weather. Fungicide seed treatments may
give some control of root rots, while there are some pea cultivars
that are less susceptible than others. Other IGM techniques for
control of this disease are long crop rotations ensuring the peas
are not stressed by inadequate water or nutrients and growing the
peas in soil that is not compacted.
Aphanomyces root rot (Aphanomyces euteiches).
Unlike the previous root rots, aphanomyces root rot is more severe
in cool, wet conditions, being particularly prevalent in waterlogged
soils. The root lesions are honey coloured and plants become stunted
and yellow. The disease is also found on other legume crops, so
long rotations are needed to avoid transmission of the disease in
the soil. Brassica leaves contain compounds that may reduce the
disease in the soil, so incorporate these leaves into the soil before
growing a pea crop.
Pea wilt (Fusarium oxysporium pisi Race 1).
This was a serious disease from the late 1930s and 1940s until the
1960s. However, selection and back-crossing resulted in transfer
of resistance to this disease into most common garden pea varieties
and it is now no longer regarded as a serious problem.
Some of the viruses that affect peas include alfalfa mosaic virus,
cucumber mosaic virus, bean yellow mosaic virus (also known as pea
mosaic virus), pea seed-borne mosaic virus and top yellows virus.
Once a virus has infected a pea plant there is nothing that can
be done to remove the virus from the plant. Thus prevention is the
main way of ensuring the crop does not become infected. Viruses
are mainly transmitted from an infected plant by aphids but can
also come from planting infected seed. Many of the pea viruses are
also found on other legumes, such as white clover and lucerne, and
some weeds. It is helpful to keep these plants away from areas where
peas are being grown. There are some pea cultivars that are resistant
to some viruses, such as bean yellow mosaic virus and top yellows
virus, so it is advisable to use these cultivars to prevent infection.
Ensure that the pea seed being sown is free of pea seed-borne mosaic
virus.
Bacterial blight (Pseudomonas syringae pv. pisi).
This disease may occur in cool, wet weather or particularly after
hail, frost or physical damage to the plants. Dark green or brown
water-soaked lesions appear on the upper surface of leaves. Pods
may also be affected. Avoid handling the crop, e.g. weeding, during
wet weather as this may damage the plants and allow infection to
occur. The bacteria are carried on the seed or pea residues but
will not survive in the soil once the pea tissue has decomposed.
Sow seed that has been certified free of the disease.
Pests
Few insect pests affect peas, the main problem being aphids that
transmit the viruses described above. Two of the most common aphids
that transmit pea viruses are the pea aphid (Acyrthosiphon pisum)
and the peach potato virus (Myzus persicae). There are
insecticides that will control aphids, but it is probably more important
to control weeds or other legumes that may be infected with viruses.
Sparrows and other birds (e.g. magpies) may cause problems in pea
crops by pulling out the peas just as they're germinating. To deter
the birds, stretch cotton or string across the seedbed (2-5 cm above
the soil) using small sticks poked into the ground to anchor the
cotton. The birds will get their legs tangled up in it and are frightened
away.
Peas are a very rewarding crop to grow. There is nothing quite
like fresh peas picked from the garden and eaten raw. It is important
to monitor the pea crop regularly, especially near harvest, to ensure
that the ripe pods are picked as they mature and are not allowed
to become over-mature.
