How to grow lathyrus

Choosing sweet peas and perennial peas

When choosing between sweet peas and

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

Fragrance

Most sweet peas (Lathyrus odoratus) are highly scented, so choose these if you love a fragrant garden and sweet-smelling blooms to pick for indoors.

Perennial Lathyus are generally unscented but Lord Anson’s blue pea (Lathyrus nervosus) is an exception.

How long the plant lasts

Sweet peas (Lathyrus odoratus) only live for a year, dying after setting seed. But don’t let this put you off as they are super easy to grow from seed. Perennial species such as Lathyrus latifolius come back year after year, but mostly lack fragrance and there are fewer to choose from.

Colour

The widest choice of flower colours comes from the many selections of annual sweet pea (Lathyrus odoratus), including bicoloured forms. However, some unusual colours such as yellow and orange can be found in the perennial types.

Habit

Most Lathyrus are climbing plants, so support these with canes, netting or let them scramble through a shrub or over a bank. Clumping forms such as spring-flowering Lathyrus vernus and dwarf bushy sweet peas, including ‘Snoopea’ and ‘Bijou Mixed’, need no support are great for ground cover and containers.

Buying sweet peas and perennial peas

The biggest selection of sweet pea seeds can be found from online seed supplier, though many garden centres also sell a few in packets. Some nurseries and garden centres offer pots of germinated sweet pea

A seedling is a young plant grown from seed.

Seeds of perennial Lathyrus, such as Lathyrus latifolius, are available from a handful of seed specialists but many gardeners choose to buy them as plants. Our Find a Plant tool is a good way to find the one you want.

When to plant sweet peas and perennial peas

• Sow annual sweet peas in either October-November or late January-April and plant them out in April
• Plant perennial Lathyrus in spring or autumn

Where to plant sweet peas and perennial peas

Find a sunny spot with some space for the plant to either climb or bush out (depending on its habit). A little light dappled shade is also okay, most notably for Lathyrus grandiflorus.

How to plant sweet peas and perennial peas

• Harden off seedlings sown indoors for 10-14 days before planting out to prevent cold shock
• Prepare the soil, ideally at least four weeks before planting, by mixing in soil improver such as garden compost or manure
• Sprinkle some general purpose fertiliser such as Growmore, Vitax Q4 or blood, fish and bone into the intended planting area, following the recommended dose on the pack
• Water your plants before planting so that the roots have been fully wetted
• Ease seedlings out of pots or modules, taking care not to crush or snap the stems and remove pots from larger plants
• Space plants 20-30cm (8in-1ft) apart
• Position climbing plants close (within 5-7.5cm or 2-3in) to their intended support, with two plants at the base of each cane if growing up a wigwam
• Water again immediately after planting to settle the soil around the roots

See "Propagating" section for sowing sweet peas direct into the ground.

Tie new shoots of climbing plants to canes, pea sticks, netting, trellis or shoots of shrubs they will be growing up, using garden twine or sweet pea support rings. Tendrils in contact with supports will naturally begin to grip on but keep tying in new shoots if they are wayward or in danger of breaking.

Plants won’t flower so well if they dry out, so check those in the ground every 3-4 days in dry weather, and those in pots daily. Pour on enough water to wet the full depth of roots each time you water.

On light or infertile soils, apply a high potassium liquid fertiliser, such as tomato feed, once or twice a week. This helps maintain flowering, especially with annual sweet peas (Lathyrus odoratus).

When flowering begins, pick or remove dead flower heads as often as possible. This is particularly important for annual sweet peas (Lathyrus odoratus), which will stop flowering if seedpods are allowed to develop.

The National Sweet Pea Society has more tips on success with sweet peas.

See "Pruning and Training" section for care of ‘cordon’ sweet peas.

Pinching out

For garden growing, pinch out shoot tips of young spring-sown sweet peas when plants reach about 7.5-10cm (3-4in). This encourages strong side-branching. Autumn-sown seedlings often branch naturally.

Cordons

If you fancy entering your sweet peas into a local show, or simply want to maximise the size of blooms for cutting, select the strongest shoot and keep pinching out all the tendrils and any other sideshoots as they develop. This method makes the plant concentrate its energies on the blooms. These single-stemmed plants are referred to as ‘cordons’.

They need regular tying in to a tall stake or bamboo cane, one stem per cane. You can use string looped around the cane and stem in a figure of eight to hold the shoot in place as it grows. To make it easier, use loops of wire, called sweet pea rings, that go around stem and cane and are lightly pressed until they grip.

Cordons can also grow much taller than the usual expected height of 2m (6½ft).

Cutting back

Cut perennial Lathyrus back to ground level in autumn or, preferably, leave the cutting back until February so the dead stems provide cover for wildlife in winter. Simply pull out and compost annual sweet peas once flowering has finished at the end of summer. In hot, dry seasons this might be a bit sooner.

Sowing sweet peas

Methods

Sow sweet peas in pots of seed compost in autumn or spring then plant them out in April, or sow them directly into the soil in April. Pot-sowing is generally more reliable than direct sowing, though it is more time-consuming. If you sow in autumn to increase chances of early flowers, keep seedlings in a coldframe or cold greenhouse during the winter months to prevent the seedlings becoming leggy. Keep the lid of the coldframe raised unless heavy frost is expected. Spring-sown sweet peas flower in 12-14 weeks.

Preparing the seed

Sweet peas often have a hard seed coat making them more difficult to germinate. To soften the seed coat, place your seeds on a layer of moist vermiculite or kitchen tissue in an airtight container in a warm room, and sow them as soon as they swell or begin to sprout. Some gardeners prefer instead to soak the seeds overnight but this can lead to them rotting if you don’t sow them immediately afterwards. For stubborn seeds that haven't swollen, remove a small chip of the hard seed coat opposite the 'eye' (small, round scar) using a sharp penknife to help moisture enter and aid germination.

Sowing in pots

• Sow seed individually in rootrainers or 9cm (3in) pots filled with seed compost. Alternatively plant five to seven seeds to a 13cm (5in) pot, spacing the seeds 2-3cm (¾-1¼in) apart
• Cover the seeds with 1cm (½in) of compost or vermiculite
• Water in and keep at about 15°C (59°F)
• Covering the pots with clear polythene or glass to lessen the risk of them drying out is optional, but remember to remove the covering after germination
• If you’ve sown them several to a pot, carefully transplant single seedlings into 9cm (3in) pots once they have reached about 3.5 cm (1½in)
• When the seedlings are 7.5-10cm (3-4in), use your thumb and finger to pinch out (remove) the top of the shoot just above a healthy leaf. This will encourage side shoots - these will alway be stronger than the central stem you have pinched out

Sowing in the ground

• Clear the soil of any weeds and dig in a soil improver such as garden compost, manure or composted bark at least four weeks prior to sowing
• Only sow your seeds when the soil shows signs of warming, usually in April
• Insert into the soil any temporary supports such as pea sticks or bamboo canes to form a wigwam 
• For rows of sweet peas, take out a shallow drill of soil 2.5cm (1in) deep and place a seed every 5-7.5cm (2-3in)
• To allow space for the plants to thrive, thin seedlings eventually to 20-30cm (8in-1ft) apart. It may seem a waste to compost the surplus seedlings but unless handled very carefully their roots are usually too damaged when pulled out to recover
• If sowing around a wigwam of canes, push two seeds into the soil to a depth of 2.5cm (1in) at the base of each cane

Perennial peas

Sow seeds in spring as for annual sweet peas.

Divide larger plants in spring or autumn. Wait until after flowering if dividing Lathyrus vernus in spring.

• A grey leaf covering is caused by powdery mildew, made worse by drought or insufficient watering
• Aphids will suck sap, particularly around the shoot and flower tips
• Plant viruses are known to attack sweet peas, but this isn't that common
• Protect young plants from slugs and snails
• Drought and temperature stress causes scorched foliage and bud drop. Protect young plants if significant temperature drop is forecast, and always harden off indoor raised plants before planting out. There's not much you can do in hot weather (over 30°C),other than keeping plants well watered, but flowering usually improves again after the hot spell. Water during dry weather and avoid getting the foliage and blossom wet
• Dense clusters of distorted leafy shoots, often close to ground level, are leafy gall
• Seedlings may grow weak and leggy, which is caused by insufficient light and excess warmth. If this occurs, move seedlings to a cooler and brighter spot