How-to Guide
How to Grow Sweet Peas
The short answer
Sow annual sweet peas either in October-November or from late January to April, and plant them out in April; spring sowings flower in about 12 to 14 weeks (RHS). In North American gardens, Missouri Extension's version is simpler: sow outdoors as soon as the ground is workable in very early spring — sweet peas like cool starts.
Give them full sun, fertile well-drained soil, and a real climbing support from day one. Pinch shoot tips when seedlings are 3-4 inches tall to force branching, water every 3-4 days in dry spells, and pick or deadhead relentlessly — every seed pod you allow is flowers you lose.
When to Sow
The RHS growing guide gives two windows for annual sweet peas: autumn (October-November, overwintered as young plants) or late winter into spring (late January-April), with plants set out in April. Spring-sown sweet peas flower in roughly 12-14 weeks.
Missouri Extension recommends sowing outdoors as soon as the ground is workable in very early spring — sweet peas germinate and root best in cool soil, and summer heat ends their season.
✔ Sweet pea seed coats are hard. The RHS suggests softening them with moisture or lightly chipping the coat before sowing to speed germination.
Site, Soil, and Support
Full sun (light dappled shade is tolerated), fertile soil enriched with organic matter, and a 5-8 cm mulch layer to hold moisture (RHS).
Support is not optional: most sweet peas climb to about 2 m (6.5 ft). Plant within 2-3 inches (5-7.5 cm) of canes or netting so tendrils catch on immediately — two plants per cane on a wigwam is the RHS standard.
Pinching and Training
When seedlings reach 3-4 inches (7.5-10 cm), pinch out the growing tip. The plant responds with strong side shoots and many more flowering stems (RHS).
Water and Feeding
Water every 3 to 4 days during dry weather — drought is the main trigger for powdery mildew and bud drop (RHS). The RHS also recommends a balanced liquid feed every two weeks while in growth on richly flowering plants.
Cut Flowers = More Flowers
Sweet peas stop blooming the moment they set seed. Deadhead regularly, or better, cut stems for the vase every few days — the harder you pick, the longer the season runs (RHS).
Common Problems
- Powdery mildew — grey coating on leaves, worse in drought-stressed plants; keep watering steady.
- Bud drop — buds fall before opening, caused by drought and temperature swings.
- Aphids clustering on shoot tips and flower stems.
- Leggy, weak seedlings — a light problem; give young plants maximum brightness (RHS).
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Royal Horticultural Society — How to grow lathyrus (sweet peas) — Sowing windows (Oct-Nov or late Jan-Apr, plant out April, 12-14 weeks to flower), pinching at 7.5-10 cm, support placement 5-7.5 cm, watering every 3-4 days, deadheading, and problem list (powdery mildew, bud drop, aphids)
- University of Missouri Extension — Planting wonderful sweet peas — Best to plant seeds outdoors as soon as the ground is workable in very early spring