Sweet peas are an old-fashioned favorite that make delightful cut garden flowers. They also look great scrambling up obelisks and trained up trellis or supports, as well as filling out borders with their heady scent.
To get the best results, start your sweet pea seeds indoors in a greenhouse or a sunny windowsill. Then transplant them outside when they sprout.
Sow Indoors
Sweet peas are not an easy plant to grow. They are slow to germinate, require a specific growing environment and are susceptible to fungus and other pathogens.
A good place to start your seeds is indoors about eight weeks before the last frost date in your area. This will ensure the soil is warm enough to germinate, but not too hot or cold.
For colder areas, you may wish to soak the seed for 24 hours before sowing. You can also nick the outer shell with a sharp knife or nail clippers, so that it opens up more and germinates quicker.
Once the seedlings have sprouted, you can move them outside into a cold frame or sheltered area of your garden to get maximum sunlight. Once they have grown four leaves, pinch out the top two leaves to encourage side shoots which will help to bush out the plant and produce more flowers.
Sow Outdoors
Sweet peas need full sun and free-draining soil. Select an open spot and prepare the bed by adding aged manure or other finished compost to the soil. Work it in and rake to make it level.
When planting, it’s important to bury your seeds 1 inch deep. Typically, you’ll sow them two inches apart. This will provide a full bed of plants without gaps.
As with all garden plants, you need to protect your newly sown seeds from slugs, snails and birds. Beer traps, copper bands and the biological control nemaslug are all effective deterrents.
Annual sweet peas are climbers, so they need a support such as trellis, netting, a teepee made of canes or a border obelisk to encourage growth and flowering. Once the seedlings have established, pinch off their central growing tips just above a leaf joint. This will promote bushy growth.
For a good crop, fertilize regularly. Mix in plenty of aged manure and compost, and use high potash feeds.
Fertilize
Sweet peas need a lot of food to thrive. Fertilize them with a well-rotted manure or garden compost 6 weeks before sowing to increase the amount of nutrients in the soil and encourage healthy roots.
The best time to fertilize is in early spring, when the plants are just beginning to grow. Apply a general feed every few weeks or sprinkle comfrey pellets.
Once seedlings have a few pairs of true (full) leaves, pinch out the top sets to encourage bushier growth and help keep them from becoming leggy. This will also stop the seedlings from producing seed pods, which can deter flowers.
Sweet peas require vertical support and can often climb over 1.8m (6ft). Trellis or strings are ideal, but wire netting with a grid that’s at least 2” wide works as well. After the vines grow a few inches, they’ll use their tendrils to grasp the trellis. Watch them daily to make sure they’re not wandering off or getting entwined on the trellis.
Water
Sweet peas need consistent moisture in order to thrive. This means setting up drip irrigation or soaker hoses as soon as possible, and watering deeply 2 to 3 times per week.
Alternatively, dig up and prepare a bed in autumn or spring where you want to plant your sweet peas. This will warm the soil and place rich nutrients deep below the roots to help them survive summer heat.
When planting, bury the seeds at a depth of one inch. Then space them 3-5 inches apart, and cover them without creating a mound.
If you are growing climbers, such as those on trellis, hardware cloth or netting, make sure the supports you use are strong enough to support the weight of the plants. Typically, these will need to be at least 1.8 metres (6ft) high in order to flower well.
Finally, if you have trouble keeping your sweet pea vines straight, you may need to snip the tendrils regularly. This will encourage the plant to produce stronger stems and flowers that will last longer.