When to Sow Sweet Peas in the UK

Sweet peas are unusual among UK garden flowers in having two genuinely valid sowing windows, six months apart — and which one suits you depends on what you have to offer them over winter.

Autumn sowing (October–November)

Sow into deep pots or root trainers (sweet peas dislike shallow, disturbed roots) and overwinter the seedlings in a cold frame or unheated greenhouse — cool but frost-free is the target. Autumn-sown plants build a stronger root system over winter and typically flower two to three weeks earlier, with sturdier stems, than spring-sown plants.

The trade-off is space and vigilance: you need somewhere frost-free but cold to keep them from late autumn until planting out in spring, and mice reliably find and eat overwintering sweet pea seed, so protect pots with a wire cage or similar if you go this route.

Spring sowing (February–April)

Simpler and lower-risk: sow indoors on a windowsill or in a heated propagator from February, or direct outside from April once the soil has warmed. No overwintering to manage, but flowering starts later and the season is a little shorter as a result.

Improving germination

Sweet pea seed has a hard coat that can make germination slow and patchy. Either soak seed for 12–24 hours before sowing, or chip the seed coat with a sharp knife or nail file (opposite the "eye," the small pale mark) to let water in faster. Don't do both — over-softened seed rots rather than germinating.

Pinching out

Once seedlings have 2–3 pairs of true leaves, pinch out the growing tip. This feels counterproductive but is the single biggest driver of a bushy plant with lots of flowering side-shoots, rather than one leggy stem that flowers thinly.

Quick reference

• Autumn sow: October–November, cold frame or unheated greenhouse, flowers earlier
• Spring sow: February–April, simpler, no overwintering
• Improve germination: soak or chip the seed coat, not both
• Pinch out at 2–3 pairs of true leaves for a bushier plant

Compare sweet pea seed and supports across UK retailers on GrowMuir.