Peas
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.
Interesting fact
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.
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.
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.
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.
Popular pea cultivars in New Zealand
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.
Trounce is resistant to powdery mildew.
Edible pod peas
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.
Nutritional Value
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.
Pests and Diseases
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.
Pests and diseases of peas
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 reddishbrown 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.
Diseases caused by viruses
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 diseases
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.
A final word
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.