Care Of Sweet Peas - How To Grow Sweet Peas

The sweet peas (Lathyrus odoratus) your grandmother grew truly deserved the name “sweet” because of their delightful fragrance. In recent years, breeders have put fragrance on the back burner, selectively breeding plants with outstanding flowers and a wide range of colors at the expense of fragrance. You can still find fragrant varieties, often labeled as “old fashioned” or “heirloom,” but modern varieties also have their charm. Taking care of sweet peas is easy. They prefer long, cool summers and don't last past spring in areas where summers are hot. Where winters are mild, try growing sweet peas over fall and winter.

How to Grow Sweet Peas

Sweet peas are easy to grow and reward gardeners with fragrant blooms and delicate climbing stems. They are surprisingly cold-tolerant and require only a little warmth to get started. With the right timing, soil preparation, and support, sweet peas will grow vigorously and flower generously.

When to Sow Sweet Peas

Sweet peas can be sown in late autumn or in early spring while there is still a chance of light frost. Autumn-sown plants are generally stronger and will flower several weeks earlier than those sown in spring.

During late autumn, sow seeds 1 inch (2.5 cm) deep in good-quality compost. If kept at around 16°C (60°F) for 7 to 14 days, shoots will begin to appear. Once seedlings emerge, move them outdoors to a bright, sheltered spot to harden off. Sweet peas are hardy and tolerate cool conditions well. Keeping them cool with plenty of light prevents leggy growth.

Sweet peas are attractive to mice, so protect newly sown seeds with a propagation lid or sheet of glass until shoots appear.

The seeds have a tough outer coating, but modern varieties no longer require chipping. Soaking seeds in warm water for 24 hours before sowing may help speed germination, though it is not essential.

Soil Requirements

Sweet peas grow best in fertile, moisture-retentive, well-draining soil. Before planting, incorporate plenty of organic matter, such as compost, into the soil to improve fertility and drainage. A 2-inch (5 cm) layer worked into the planting area in autumn will provide ideal growing conditions.

For container growing, use deep pots or specialized cell trays known as root trainers. Sweet peas develop long roots and benefit from deep containers that allow roots to grow downward without disturbance.

Light Requirements

Choose a sunny or lightly shaded site with good air circulation. Avoid planting sweet peas directly against walls where airflow is restricted. Plenty of light encourages strong growth and better flowering.

Step-by-Step: How to Sow Sweet Peas

1. Prepare the soil by incorporating compost or organic matter.
2. Sow seeds 1 inch (2.5 cm) deep.
3. Space climbing varieties 6 inches (15 cm) apart and bush types 12 inches (31 cm) apart.
4. Water gently and protect from rodents if necessary.
5. Germination typically occurs within 10 days but may take up to two weeks.
6. Once seedlings are growing strongly, pinch out the tips to encourage bushier growth and prevent legginess.

Supporting Sweet Peas

Sweet peas grow rapidly and most varieties will require support.

Climbing types need a trellis, wigwam, or A-frame of canes installed before planting to avoid disturbing roots later. Regularly tie stems to supports to keep growth upright and encourage straighter flower stems.

Bush varieties are more compact and generally support themselves, although they are still technically vines.

Exhibition growers often train sweet peas as a “cordon,” removing side shoots and tendrils to focus energy into fewer, larger blooms. When plants reach the top of their support, the stems can be untied and gently laid horizontally before training the tips upward again. Untrained plants will produce more flowers overall, though stems may be more curved.

Sweet Pea Care

Keep plants in a sheltered spot away from harsh winter weather. As temperatures rise in spring, growth will accelerate and plants will begin to branch. With proper support and care, sweet peas will reward you with abundant blooms throughout the season.

Pinch out the growing tips of the plants when they are about 6 inches (15 cm.) tall to stimulate lateral growth and bushiness. This is a good time to mulch the plants as well. Water the soil around the plants often enough to keep it moist, applying the water slowly and deeply.

Fertilize with half-strength liquid fertilizer twice during the growing season. Too much fertilizer encourages an abundance of foliage at the expense of sweet pea flowers.

You will need to pick the flowers regularly during the season. Aim to visit the plants at least once a week and cut every flower that is open. Removing old flowers means the developing buds will mature faster, and if old blooms are left on they will soon start to divert energy into producing seeds rather than flowers.

Caution: Sweet pea seeds resemble edible sweet peas, but they are toxic if eaten. If children are helping in the garden, make sure they don't put them in their mouths.

Sweet Pea Varieties

Sweet peas can be divided into two main groups - 'Grandifloras' and 'Spencer' types. For the expert, flowers of grandiflora types have fused keel petals, Spencer's do not. For everyone else, Grandifloras tend to have a stronger scent so are really useful if you want that quintessential sweet pea sent in the house. 'Hi scent' (pale pink) and 'Cupani' (purple and maroon) have some of the strongest scented flowers. If you want the really big, frilly flowers with long stems for exhibition, then you will be better off with a Spencer type. A couple of favorites are 'Chastworth' (pale blue) or 'White Frills' (white).

More recently, some sweet peas have been bred to combine the best of both of these types and are called 'Modern grandifloras'. One of the best all-rounders for both cut flower and scent is 'Albutt Blue' (pale blue). If your seeds have a solid color, they will produce a solid colored flower, if they are striped, the flower will be striped too. 

Common Sweet Pea Problems

Watch out for aphids Sweet peas are prone to attacks from aphids in the late spring whilst they are growing most rapidly. Keep a close eye on your plants for the first signs of any clusters of aphids at the tips. Aphids are easy to control with a suitable spray, such as a soap solution, or wiping them off with the fingers.

Prevent mildew by regularly watering One of the other problems that can cut short the display from sweet peas is powdery mildew. It appears as a fine powdery dust on all the leaves and stems and will quickly spread, causing plants to grow slower and possibly stop flowering. This is most likely to take hold during hot, dry spells in July and August. If the plants are kept well watered they are less likely to be affected, but always water at the base of the plants and do not spray the leaves, as this can increase its spread. 

Clean tools between plants to prevent transferring viruses Unfortunately sweet peas are prone to viruses. If plants become infected they will appear paler, less vigorous and any flowers may show unusual streaking and discoloration. Remove any plants that show signs of sickness to stop it spreading and clean any tools that may have sap on them with disinfectant.