At a glance
Sweet peas are one of the most characteristically British of all garden flowers and among the most rewarding annual climbers for a UK garden. The combination of extraordinary fragrance, a huge range of colours from purest white through every shade of pink, purple and red, and a long cutting season from June to September makes them exceptional. A well-grown sweet pea plant produces dozens of stems per week and the more you cut, the more it flowers – making them uniquely generous as a cut flower.
They are also one of the few summer flowers where an autumn sowing significantly outperforms a spring one. Sweet peas sown in October and overwintered as small plants in a cold greenhouse or cold frame produce flowers three to four weeks earlier than those sown in February, with stronger root systems and more vigorous growth from the start. For gardeners with access to any frost-free but cold growing space, autumn sowing is the method to use. For a similarly high-performing scented climber that blooms at the other end of the season, dahlias planted alongside a sweet pea support in May take over as the sweet peas fade in late summer.
Best varieties for UK gardens
For maximum scent, choose old-fashioned and heritage varieties like Matucana rather than modern Spencer types bred primarily for flower size and stem length. Modern Spencer varieties are superior for cutting – longer stems, more flowers per stem, better vase life – but sacrifice some of the intense fragrance of older varieties. Growing both types gives the best of both worlds.
When and how to sow
Sweet peas can be sown in October or November for overwintering, or from late January through March for a spring start. October sowings produce the strongest, earliest plants. Sow in deep modules or root trainers – sweet peas have long taproots that resent disturbance, so the deeper the container, the better. Fill with seed compost and sow one seed per module, 2cm deep.
Some growers nick the seed coat with a knife or soak seeds overnight before sowing to improve germination – both methods work and can marginally speed germination, but are not strictly necessary with fresh, good-quality seed. Germination takes 7-14 days at around 15°C. Move seedlings to the brightest available position as soon as they emerge.
Growing on through winter
Autumn-sown sweet peas need to be kept in a cold but frost-free environment through winter – a cold greenhouse, cold frame or unheated porch all work. The aim is to keep them growing slowly, not to push them into rapid growth. If plants are kept too warm they become tall and leggy before they can go outside; kept too cold they are damaged or killed. A temperature range of 2-10°C is ideal.
When seedlings reach 10cm, pinch out the growing tip to encourage two strong side shoots rather than a single weak main stem. This single action is the most important thing you can do at this stage – pinched plants are bushy and vigorous; unpinched plants produce a single etiolated stem that never performs as well.
Planting out and support
Plant out once all frost risk has passed – late April for autumn-sown plants that have been hardened off, late May for spring-sown plants. Sweet peas climb by tendrils and need something to grip – netting, trellis, or a row of canes linked by horizontal strings all work well. For a traditional wigwam of canes, use 8-10 canes 2.4m long pushed firmly into the ground and tied at the top. Provide one plant per cane, planting at the base of each.
Space plants 20-25cm apart along a row if growing on netting or trellis rather than a wigwam. Plant deeply – up to the lowest leaves – and water in well. Sweet peas fix nitrogen in their roots and improve the soil as they grow, making them excellent predecessors for hungry crops in a rotation.
Feeding and care
Feed with a high-potassium liquid feed every ten to fourteen days once the first flowers appear – tomato feed is ideal. Sweet peas grown in the ground need less feeding than those in containers, which need consistent feeding throughout the season. Water during dry spells – moisture stress causes premature seed pod formation and early cessation of flowering. Tie in wayward stems regularly as the plants grow – sweet peas climb best when guided rather than left to scramble.
Picking – the essential rule
Pick sweet peas every two to three days without fail. This is not optional – it is the single most important management task. Any flower that is left to form a seed pod sends a signal to the plant that its reproductive purpose has been achieved, causing it to reduce flower production dramatically. A regularly picked sweet pea plant in good conditions produces stems continuously for three to four months. One that is left unpicked for a week produces a cluster of pods, then stops flowering within days.
Pick when the lowest flower on the stem is just opening and the upper buds are still closed. This gives the longest possible vase life – fully open flowers picked in hot weather last only two to three days, while stems picked in bud last a week or more. Cut stems as long as possible, going back to a leaf node to encourage strong replacement growth.
Sweet pea seeds and all parts of the plant are toxic if ingested. The seeds in particular contain toxic amino acids that cause a condition called lathyrism if eaten in quantity. Keep sweet pea seeds away from children and do not confuse them with edible peas – the plants look superficially similar when young but sweet pea pods should never be eaten. Sweet peas are grown purely as ornamentals.
Common problems
Powdery mildew appears on sweet peas in dry conditions from July onwards, particularly on plants where air circulation is poor in a dense wigwam. Water at the roots rather than overhead and improve airflow by removing the lowest leaves as the season progresses. Aphids congregate on growing tips and flower buds – check weekly and deal with colonies promptly before they distort the new growth. Slugs and snails damage young plants at planting out time; protect with organic pellets for the first two weeks.
Bud drop – where buds form but fall off before opening – is usually caused by irregular watering, insufficient light or sudden temperature change. It is most common in the weeks after planting when roots are still establishing. Consistent watering and avoiding sudden moves between very different temperature environments prevents most cases.
