Horticulture Magazine

Lathyrus ‘Sweet Peas’

white and purple flowering lathyrus in the garden

Sweet peas are without a doubt one of the most popular annual flowers grown in the UK.

Beautiful, fragrant and useful, there are many reasons to grow them, or a perennial type, in your garden.

These climbing plants are rightfully popular for their delicate pea flowers in a range of hues, and for their pleasing fragrance.

Imagery and video featured in this article was commissioned by Horticulture.co.uk in collaboration with Organic Gardener Emily Cupit.

But as legumes, these plants can also be useful for their ability to co-operate with bacteria in their roots which are able to take atmospheric nitrogen and make it available in the soil.

Nitrogen is a key plant nutrient, so this is good news for plants placed in the spot after sweet peas, or, potentially, also for plants growing close by.

These flowering plants are also great for bees and other pollinators, and can help in attracting other beneficial insect life to your garden.

Overview

Botanical NameLathyrus
Common Name(s)Sweet Peas
Plant TypeAnnual / Perennial / Climber
Native AreaEurope – Cultivated
Hardiness RatingH3
FoliagePea-like, deciduous
FlowersNumerous hues
When To Sow / PlantMarch-April, October-November
Flowering MonthsJune – October
When To PruneFebruary
Sunlight

Preferred
Full Sun / Part Shade

Exposure
Exposed or Sheltered

Size

Height
1 – 2.5M

Spread
0.1 – 0.5M

Soil

Preferred
Loam, Sandy

Moisture
Well drained

pH
Any

There are two main types of sweet pea, the common annual type and perennials.

Annual sweet peas, Lathyrus odoratus, as gardeners know them today, were cross-bred and cultivated by Scottish plantsman Henry Eckford in the late 19th and early 20th Century. [source]

a bouquet of lathyrus flowers held in a hand with red, white and purple blooms

Bred from plants with purple flowers, in the wild, sweet peas are now available in a wide range of hues.

While these are the sweet peas best known to most gardeners, there are also perennial types, often known as ‘everlasting peas’.

These include cultivars of Lathyrus latifolius, L. grandiflorus, L. aureus, L. vernus, L. rotundifolius and L. nervosus, for example.

flowers and foliage of sweet peas against a blue sky background

Though these are not fragrant like the annual types, they will return year after year, bringing many benefits to your garden.

Annual Or Perennial?

The first decision you will have to make when growing sweet peas is whether the annual or perennial types are best for you.

Annual sweet peas are an excellent choice for novice gardeners, and for sowing with kids.

They can be a great choice for inclusion among the crops in an annual kitchen garden, for growing in pots or containers, or for a sunny mixed border.

Gwendoline Sweet peas with pink and white flowers grown in cordons
L. odoratus ‘Gwendoline’

There are over 50 cultivars of annual sweet pea which have been awarded with an RHS AGM.

Just a handful of the great options to consider include:

  • L. odoratus ‘Bobby’s Girl’ (pink)
  • L. odoratus ‘Charlie’s Angel’ (purple)
  • L. odoratus ‘Gwendoline’ (pink)
  • L. odoratus ‘King Edward VII’ (red)
  • L. ororatus ‘Mrs Bernard Jones’ (pink)

Perennial peas or everlasting peas do not need to be sown anew each year.

As perennials, they can remain in your garden over a number of years.

They can be a great choice for the sunny fringes of a forest garden or fruit tree guild, or for a herbaceous border.

Flowering Lathyrus latifolius 'Pink Pearl'
L. latifolius ‘Pink Pearl’

Some great perennial types to consider include:

  • L. grandiflorus (deep pink, AGM).
  • L. latifolius ‘White Pearl’ (white, AGM).
  • L. latifolius ‘Rosa Pearl’ (pale pink, AGM).
  • L. rotundifolius (deep pink/red, AGM).
  • L. vernus ‘Alboroseus’ (pink and white, AGM).

Sowing Guidelines

Sweet peas, both annual and perennial, are typically sown either in spring, between March and May, or in autumn, in October or November.

Sweet peas are particularly easy to grow from seed, even for beginners, so it is unlikely that you will need to buy plants.

However, if you wish to avoid this step you can also purchase plug plants in the spring.

sweet peas against a blue sky

Germination rates are usually excellent, though you can help ensure germination by carefully nicking each seed with a knife, avoiding the ‘eye’ area – this is not usually required.

Seeds can be sown in small, individual pots (3 seeds per 9cm pot), in soil blocks, or in seed trays – in good quality, seed-starting peat-free compost.

a mass of lathyrus growth in the garden with red and white flowers

Whenever they are sown, the seedlings are planted out after all risk of frost has passed, at around the end of May.

So if you sow them in autumn, you will need to make sure that you have a frost-free place to keep them over the winter months.

When & How To Plant Sweet Peas

lathyrus flowers in different colours with a large garden pictured in the background

Indoors sown sweet peas should be hardened off before you place them outside.

This simply means slowly acclimatising the young plants to outdoors growing conditions.

Where To Plant

Sweet peas can work well in a range of settings.

Annual types can work well, as mentioned above, in a kitchen garden as a companion plant for annual crops.

sweet peas in a large garden

Perennial types work well in borders or the fringes of other polyculture planting schemes.

Both types can also be grown in containers.

Spacing & Depth

Plugs or pots (with three seedlings in each) should be spaced around 20cm apart, through precise spacing is not hugely important and you do not need to worry about too much about separating the individual plants from one another.

Plant them to the same depth that they were in their previous container.

Caring For Lathyrus

Most sweet peas are not only incredibly easy to grow, but also very easy and straightforward to care for over time.

Supports

Most sweet peas require some form of support, whether that is other plants, a trellis or other support structure.

Make sure you choose a support tall enough to support the particular variety or varieties that you wish to grow.

Lathyrus odoratus growing on a hazel wigwam

But remember that you might be able to use natural or reclaimed materials and do not need to buy anything new.

Most sweet peas are self-clinging climbers, which have tendrils which latch onto supports.

However, there are some sweet peas that will need to be tied into support structures.

Some perennial types have a different growth habit and don’t need support at all.

Preferred Sunlight

All sweet peas can cope with a little light shade, but most will do best in an open position in full sun.

Soil Requirements
magnified view of pink sweet pea flowers

Sweet peas need a moisture retentive yet free draining soil or potting mix.

They will thrive in loam or sand but can cope with a range of soil types and are unfussy about pH.

They will not particularly like a heavy soil, and can rot in very heavy and very wet conditions, and a very dry and low-nutrient soil can also increase the chances of issues like powdery mildew setting in.

But in most situations, sweet peas will remain largely hassle-free.

Watering

Water well upon planting.

Continue to water regularly when growing in pots or containers, through additional watering will only be required during very dry periods when sweet peas are growing in the ground.

Temperature & Humidity
another magnified view of sweet pea flowers in pink

Annual sweet peas are typically H3 hardy, meaning that they are hardy only in mild and coastal areas of the UK (down to -5 to 1°C.).

They will however, thrive in the temperatures afforded in most UK gardens during the summer months.

Fertilising

Especially with annual sweet peas, it is a good idea to feed plants with an organic, potassium-rich plant feed once a week or so after the flowers begin to form, to promote strong and healthy flowering.

This is especially beneficial when plants are growing in less fertile, lighter soils, or in pots.

Perennial types can benefit from a mulch (such as comfrey leaves) around the base, though take care not to mound mulch around the stems.

Dividing

Larger, mature perennial peas can often be divided in spring or autumn, and the divisions can be carefully moved to new locations in your garden.

Pinching Out
portrait view of lathyrus plants growing together

When annual sweet pea plants sown in spring are around 10cm tall, you should pinch off the growing tips to encourage bushier growth.

Autumn sown plants will usually branch naturally on their own.

Pruning

If you want the best blooms with long, straight stems, growing annual sweet peas as cordons will offer the best results.

Select the strongest shoot and train it up a cane or tall stake, and pinch out all the other side shoots and tendrils when they appear.

hand using black and yellow secateurs to take cuttings from a lathyrus plant

Perennial peas can be cut back to the ground in the autumn.

But for the sake of garden wildlife, it is best to wait until February before perennials are cut back.

The dead foliage and stems leave shelter for a range of creatures.

Picking Flowers

female gardener taking cuttings from a lathyrus plant with a timber fence in the background

Sweet peas make great cut flowers.

Flowers should be picked frequently, as this will encourage more to form and prolong the flowering season.

Common Pests & Diseases

Sweet peas are not generally very difficult to grow successfully, but they can be very prone to slug damage, especially when young.

Aphids can also be a problem for young plants.

Powdery mildew can set in, especially if plants are under-watered.

Viruses can also attack sweet peas, though this is not particularly common.

Saving Lathyrus Seeds

sweet pea flowers in pink, red and white against a blue, cloudy sky

If you are growing annual sweet peas, saving the seeds is a good way to make sure you do not have to buy more seeds to enjoy these plants in your garden again the following year.

Simply leave seed pods on the plants until they get dry and papery, then collect them on a dry day in autumn, shell them from the pods, and store them in a paper bag in a dry spot until you are ready to use them.

FAQ’s

Do I need a trellis when growing sweet peas?

Most sweet peas will require some form of support, except for certain perennial types with a different growth habit.

When do sweet peas flower?

Between June and October when cut flowers are harvested regularly.

Do sweet peas come back every year?
bouquet of sweet pea flowers held in a hand

Perennial types will come back each year, annual types need to be sown anew each year.

Can sweet peas be grown in containers?

Yes, as long as they are provided with the right support, and positioned in the right spot, sweet peas can be grown in containers.

How many times a year should you pinch out sweet pea flowers?

Pinch out sweet pea flowers regularly to enjoy repeat flowering.

What should I do with sweet peas when they’ve finished flowering?

The annual type should be lifted and composted as soon as you have saved seeds for next year.

Perennial types should be left in place for wildlife over winter and cut back in around February the following year to make way for new growth.

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