How to Grow Sweet Pea
'Black Knight' from Seed
A Victorian sweet pea from 1898 in deep maroon, still the standard for scent. Grow it for the fragrance, cut it short and put it where people sit.
'Black Knight' was introduced in 1898, and more than a century later it is still the variety other sweet peas are measured against for scent. The flowers are a deep maroon that looks almost black in shade and only shows its red in direct sun.
It is a Grandiflora, the older type of sweet pea. The flowers are smaller and plainer than modern Spencer varieties, and the stems are shorter, but the fragrance is far stronger. Three or four stems in a small vase will scent a room.
Quick Facts at a Glance
Plant Type
Hardy annual climber
Sowing Time
Oct to Nov · Jan to Mar, under cover
Flowering
June to August
Position
Full sun
Support
Canes, netting or obelisk
Difficulty
2 out of 5, easy
Understanding the Plant
'Black Knight' is a variety of Lathyrus odoratus, the sweet pea. It is a hardy annual climber that pulls itself up any support using tendrils. Grandiflora types like this one are close to the old wild sweet peas, which is where their strong scent comes from.
Modern Spencer sweet peas were bred for large, frilled flowers and long stems, and much of the fragrance was lost along the way. 'Black Knight' gives you the scent the Victorians grew sweet peas for, on flowers that are smaller but still very beautiful.
Pick constantly
Every seed pod that forms tells the plant its job is done. Leave pods on 'Black Knight' and it can stop flowering within a fortnight. Pick every few days, even if you give most of the flowers away.
When & How to Sow
Sweet peas can be sown in autumn or late winter. October and November sowings give stronger, earlier plants; January to March sowings are fine if you missed autumn. Sweet peas make long roots, so use deep pots or root trainers.
Protect seed from mice
Mice love sweet pea seed. Sow in a greenhouse, cold frame or porch that mice can't reach, or cover pots until the seedlings are up.
Sow 1 to 2cm deep in deep pots. Sow two seeds per root trainer or one to two per 9cm pot, in moist compost. Keep somewhere cool and bright. Seedlings usually appear within two to three weeks.
Grow on cool and pinch out. Keep plants in a cold frame or unheated greenhouse. When they have three or four pairs of leaves, pinch out the tip to encourage strong side shoots.
Plant out in spring. Harden off and plant out in March or April, 20 to 25cm apart, at the base of canes, netting or an obelisk about 1.8m tall.
Growing On Tips
Rich, deep soil
Dig plenty of compost or well-rotted manure into the planting spot. Sweet peas are hungry plants.
Tie in young shoots
Help young plants onto their supports with soft string until the tendrils take hold.
Water at the roots
Water deeply in dry spells. Plants that dry out drop their buds and stop flowering.
Cut every few days
Pick all open flowers and remove any pods. The more you cut, the longer it flowers.
Where to put them
Stems are short, so use small vases and place them by a chair, a bed or a desk where the scent is noticed.
Feed in flower
A high-potash liquid feed every week or two once flowering begins keeps plants going.
Common Problems & How to Fix Them
| Problem | Likely Cause | What to Do |
|---|---|---|
| Seeds disappear from pots | Mice | Sow where mice can't reach, or cover pots until seedlings appear. |
| Flowering stops early | Seed pods allowed to form | Pick every few days and remove any pods you find. |
| Buds drop before opening | Dry roots or sudden cold nights | Water deeply in dry weather and mulch around the base. |
| White powder on leaves | Powdery mildew, often late summer | Keep plants well watered at the roots and remove affected growth. Autumn-sown plants finish before it becomes serious. |
| Seedlings eaten | Slugs and snails | Protect young plants after planting out. |
When to Expect Flowers
Autumn-sown plants flower from early June, and late winter sowings from late June. With constant picking, flowering carries on through July and into August.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 🌱 Sow under cover | ||||||||||||
| 🪴 Plant out | ||||||||||||
| 🌸 Flowering |
Plant Specifications
The Victorian sweet pea grown for scent
Sow in autumn, plant out in spring against a sturdy support and pick constantly. A few stems of 'Black Knight' will fill a room with fragrance all summer.
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