Sweet Pea

About Sweet Pea

Sweet peas are climbing annuals grown for their intensely fragrant, butterfly-shaped flowers in shades of white, pink, red, purple, and bicolour. A cottage garden favourite in the UK since the 17th century, they are unmatched for cutting — a handful of stems will scent a room for days. Most varieties climb to 1.5–2 m and need support from canes, a wigwam, or a trellis. The secret to a long display is deadheading without exception: allow even a few pods to ripen and flowering stops almost overnight. Heritage varieties such as 'Matucana' and 'Painted Lady' carry the strongest perfume; modern Spencer varieties produce the largest, most ruffled blooms.

UK fact: Sweet peas have been grown in British gardens since seeds were sent from Sicily to England in 1699 by the Sicilian monk Francisco Cupani. The annual National Sweet Pea Society Show is one of the longest-running specialist flower shows in the UK, and competitive growing of long-stemmed exhibition sweet peas remains a uniquely British pursuit.

Watering Sweet Pea

Water deeply twice a week during dry weather once established in the ground; consistent moisture prevents plants going to seed prematurely, which ends flowering.

Newly sown seedlings need careful watering — soggy compost at the base causes damping off. Once plants are established in the ground, water deeply twice a week during dry spells rather than giving daily light sprinklings. Sweet peas have deep roots that prefer a thorough soak every few days. Consistent moisture is important during flowering: if the soil dries out, plants quickly begin to set seed and stop producing new buds. A mulch of compost or bark around the base helps retain moisture and keeps the roots cool.

Sunlight requirements

Sweet peas need full sun and good air circulation — at least six hours of direct sun daily in a south- or west-facing position for the longest stems and strongest fragrance.

Sweet peas need a position in full sun with at least six hours of direct sunlight per day. In shade, plants become weak and prone to powdery mildew, producing fewer, poorly scented flowers. A south- or west-facing aspect is ideal. Good air circulation is equally important — avoid planting in airless, sheltered corners where humidity builds up, as this dramatically increases the risk of mildew.

Soil and feeding

Rich, deeply cultivated, moisture-retentive but well-drained soil gives the best results — the traditional method is a deep compost-enriched trench prepared in autumn.

Sweet peas are heavy feeders that repay good soil preparation. The traditional approach is to dig a trench 30 cm deep in autumn or winter, line it with well-rotted compost or manure, and backfill. They prefer a neutral to slightly alkaline pH of 6.5–7.5. On lighter soils, incorporate plenty of organic matter at planting time. Container growing is possible in large pots (at least 30 cm deep) using a rich compost with extra drainage, but plants will never be as vigorous as those grown in open ground.

Once buds appear, feed fortnightly with a high-potash liquid fertiliser; avoid high-nitrogen feeds which promote leafy growth at the expense of flowers.

Before planting out, work a balanced granular fertiliser into the soil. Once flower buds form, switch to a fortnightly liquid feed of a high-potash fertiliser — tomato feed is ideal and inexpensive. High-nitrogen feeds encourage lush leafy growth at the expense of blooms, so avoid them entirely once plants are in the ground. Container-grown plants need feeding more frequently, approximately weekly from the onset of flowering.

Pruning Sweet Pea

Deadhead every day or two without exception — a single ripening pod signals the plant to stop flowering. Pinch out growing tips at 10 cm to encourage a bushy, multi-stemmed habit.

The single most important task with sweet peas is daily deadheading. Check plants every day or two during peak flowering (June–August) and remove any spent flowers and developing seed pods the moment they appear. If even a handful of pods are allowed to ripen, the plant interprets this as reproductive success and dramatically reduces flower production within days. When seedlings reach about 10 cm tall, pinch out the growing tip to encourage side-shoots — this produces a bushier plant with more flowering stems rather than a single, weaker leader.

Propagating Sweet Pea

Raise from seed — sow in modules or root trainers in October–November for autumn plants overwintered in a cold frame, or January–February for spring plants; soak seeds overnight before sowing.

Sweet peas are always raised from seed. There are two main sowing windows in the UK. Autumn sowing (October–November) produces stronger plants with better root systems and earlier flowers, but seedlings need overwintering in a cold frame or frost-free greenhouse. Spring sowing (January–February) is simpler and suitable for unheated homes. Sow in deep modules or root trainers — sweet peas resent root disturbance and tall, narrow containers promote the deep root run they prefer. Sow two seeds per cell, 2.5 cm deep. Nick the hard seed coat with a knife or soak seeds overnight in lukewarm water to improve germination. Germinate at 10–15°C; high temperatures reduce germination rates. Pot on carefully when roots fill the module, handling by the root ball, not the stem.

1. 1Prepare seeds

   Nick the hard seed coat with a sharp knife or soak seeds overnight in lukewarm water. This softens the coat and significantly improves germination speed and success.

2. 2Sow in deep modules

   Fill root trainers or deep modules with seed compost. Sow two seeds per cell at a depth of 2.5 cm. Sweet peas dislike root disturbance, so avoid shallow trays.

3. 3Germinate at cool temperatures

   Place in a cold frame or on a cool windowsill at 10–15°C. Do not use a heated propagator — warmth above 18°C reduces germination. Seedlings appear in 7–14 days.

4. 4Pinch out growing tips

   When seedlings reach 10 cm tall, pinch out the growing tip above the second pair of leaves. This encourages multiple side-shoots, producing a bushier plant with more flowering stems.

5. 5Harden off before planting out

   From mid-March onwards, gradually acclimatise seedlings to outdoor conditions over 10–14 days. Start by placing them outdoors in a sheltered spot during the day and bringing them in at night.

6. 6Plant out and provide support

   Plant out from late March (south) or mid-April (north) once hardened off. Set plants 20–25 cm apart at the base of canes, a wigwam, or trellis. Water in well.

Common problems

Powdery mildew is the most common problem, producing a white powdery coating on leaves and stems. It is encouraged by dry roots combined with humid air around the foliage — ensure consistent watering and good air circulation. There is no cure once established; remove affected leaves promptly and treat with a sulphur-based fungicide. Aphids (blackfly in particular) cluster on growing tips and flower stems; pinch off heavy infestations by hand or spray with insecticidal soap. Slugs and snails attack young seedlings, especially at planting-out time — use slug pellets or grit around the base. Birds can pull up newly sown seeds; cover seed beds with fleece or netting until germination. If plants flower well but then stop abruptly despite deadheading, check carefully for any small pods hidden in the foliage.

The Loam app can help identify what's wrong with your plant — photograph the affected leaves and it will suggest the most likely cause.

Seasonal care calendar

Monthly care guide for Sweet Pea in the UK, based on average conditions.

Frequently asked questions

When should I sow sweet peas in the UK?

There are two windows: October–November for autumn-sown plants (overwintered in a cold frame, earlier flowers), or January–February for spring-sown plants (easier, works on a cool windowsill). Avoid sowing later than mid-February for spring sowings — late plants rarely have time to produce a full season of blooms before the heat of July slows them.

Why have my sweet peas stopped flowering?

The most common cause is allowing seed pods to form. Even a few ripening pods tell the plant its job is done, and it stops producing new flowers almost immediately. Check every stem carefully for pods, remove any you find, and deadhead religiously going forward. Other causes include prolonged hot weather, inconsistent watering, or plants having run out of the growing season (August onwards).

Do sweet peas need a frame or support?

Yes — climbing varieties (the most common type) grow to 1.5–2 m and need firm support from the start. Bamboo canes in a wigwam or row, a trellis, or wire mesh against a fence all work well. Bush or dwarf sweet peas (such as 'Patio Mixed') are self-supporting and suitable for pots and window boxes, though they tend to be less fragrant than climbing types.

Are sweet peas toxic to dogs and cats?

Yes — the seeds of ornamental sweet peas (Lathyrus odoratus) contain compounds that are toxic to dogs, cats, and humans if eaten in quantity. They are not edible and should not be confused with garden peas (Pisum sativum). Occasional nibbling of leaves is unlikely to cause serious harm, but ingestion of seeds can cause weakness, lethargy, and neurological symptoms. Keep children and pets away from seed pods.

Can I grow sweet peas in pots?

Yes, but use a large container — at least 30 cm deep and 30 cm wide per plant, as sweet peas have deep root systems. Use a rich, moisture-retentive compost and provide a tall support (wigwam or canes). Pot-grown plants need watering daily in hot weather and feeding weekly once flowering begins. They will perform less vigorously than ground-grown plants but still produce good cut flowers.

Related plants

• Lavender
• Cosmos
• Sunflower