The seeds of Sweet peas vary in color from pale buff to black. To help the seed of Sweet peas to germinate, nick the darker seeds with a sharp penknife, removing a small piece of the seed coat opposite to the eye. You can soak Sweet peas seeds to obtain a faster germination, but this may be a problem with rotting.
Sow seeds in seed trays, root trainers, pots or in special Sweet pea tubes. Use a loamless potting compost in which you add 20 percent grit or standard seed compost. Dark seed usually germinate well in fairly damp compost that is only just moist. Cover the containers with glass and keep them at around 15 Celsius degrees (59 F). When the seedlings will appear you can move them into a cold frame.
Prick out the seedlings from their container and pot them individually when the plants are about 3.5 cm high. Fill 6 cm pots or Sweet peas tubes with a similar compost that you used for the sowing process. Pinch out the root tips and then repot. Autumn-sown Sweet peas should be pinched out only if they have not produced sideshoots by mid-winter. Stop spring-sown seedlings at the second pair of leaves.
To overwinter the seedlings keep the cold frame open as much as weather allow you during slight frosts to harden off the plants. If the temperature goes below -2 Celsius degree (28 F) close and insulate the frame. In heavy rain prop open the lights on the frame for ventilation. Give a dilute liquid fertilizer in late winter.
Plant out autumn-sown Sweet peas in mid-spring in a sunny, open site with well-drained, humus-rich soil. Add well-rotted manure to the lower spit. Set plants at 23 cm apart, water during dry spells and when flowers appear. Apply liquid fertilizer two or three times at fortnight intervals from mid-summer onwards. Cut the dead flowers to encourage continuous flowering.