Sweet peas

Sweet peas (Lathyrus odoratus) are probably the most popular annual flower, being especially prized for their colour and scent. Smaller cultivars are available for hanging baskets and containers.

Sweet peas growing informally up canes. Credit: RHS Herbarium.

Quick facts

Common name Sweet pea
Botanical name Lathyrus odoratus
Group Annual climber (also dwarf bush cultivars)
Flowering time Late spring to summer
Height and spread 45cm-2.5m (18in-8ft) by 30cm (1ft)
Aspect Full sun and well-drained but moisture retentive soil
Hardiness Hardy
Difficulty Easy

Sowing seed

Sweet peas are easy to grow from seed, although you can buy plants in garden centres in spring. But there is a far wider range of colour and scent if you grow from seed.

Sowing time

Indoor sowing: Sow indoors from October to November, overwintering young plants in a cold frame or unheated greenhouse. You may get earlier blooms this way. Otherwise, sow from January to April.

Outdoor sowing: Sow directly into the ground in March or April, although the results are generally less satisfactory than indoor sowing.

Sowing method

Sweet peas often have a hard seed coat making them more difficult to germinate. To soften the seed coat, place seeds on a layer of moist vermiculite or kitchen tissue in an airtight container in a warm room, and sow as soon as they swell or begin to sprout. Don't soak the seeds. For stubborn seeds that haven't swollen, chip the hard seed coat opposite the 'eye' (small, round scar) using a sharp penknife to help moisture entry and germination.

  • Sow seed individually in root trainers 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
  • Water in, cover the pots with clear polythene or glass, and keep at about 15°C (59°F). After germination, remove the covering
  • If not sown individually, plant out single seedlings into 9cm (3in) pots once they have reached about 3.5 cm (1½in)
  • If sown in autumn, transfer the young plants to a cold frame to prevent the seedlings becoming leggy. Over winter, keep the frame opened as much as possible, but protect from heavier frosts
  • If sown in spring, harden off young plants before planting out
  • Plant out from April onwards, spacing plants 20-30cm (8in-1ft) apart

Pinching out

Opinions vary, but when growing for the garden it is generally considered beneficial to pinch out tips when plants reach about 10cm (4in). This encourages strong side-branching.

    Cultivation notes

    Grow sweet peas in fertile, well-drained, humus-rich soil, in full sun or very light dappled shade. For best results, incorporate organic matter such as garden compost or well-rotted manure at least four weeks before planting, and apply a general fertiliser, such as Vitax Q4 or Growmore, at the manufacturers' recommended dose. After planting, water the plants well during dry spells. On poorer soils, a bi-weekly, high potassium liquid fertiliser such as tomato feed may be beneficial.

    Pruning and training

    Supports: Sweet peas are usually allowed to scramble up pea sticks, canes wigwams or trellis. Alternatively, use post and netting supports. Use dwarf bush-type sweet peas for pots, hanging baskets or as ground cover.

    For a long and regular supply of blooms: cut flowers frequently, before they produce seed pods.

    Cordon training

    This method is used by professional growers to produce top quality blooms. Cordons are trained as single-stemmed plants to individual canes, with sideshoots and tendrils removed so that all the plant's energies are diverted into flower production. See the National Sweet Pea Society website for more information on this method.

    Cultivar Selection

    There are many cultivars available, from high-quality exhibition blooms to those prized for their colour. Here are three that are particularly strongly scented:

    Lathyrus odoratus ‘White Supreme’: White flowers, highly scented
    L. odoratus ‘Charlie's Angel’: Large clear pale blue flowers, scented, recommended for exhibition
    L. odoratus ‘Gwendoline’: Large flowers with frosted pink effect, strongly scented

    For more choices, check for RHS AGM Plants or use the RHS Find a Plant for over 50 cultivars with images and details.

    Problems

    Sweet peas can suffer from a wide range of problems, though few are very serious:

    • A grey leaf covering is caused by powdery mildew
    • 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
    • Do 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. 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

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