Sweet peas are old fashioned annuals with ruffly, fragrant flowers in rich shades of blues, purples, reds, pinks, and whites. They’re essential in cut flower and cottage gardens and for the best show of flowers, you’ll want to start the seeds at the right time. This article provides all the information you need to know about starting sweet peas seeds indoors as well as direct sowing in the garden. If you’re wondering when to plant sweet peas, keep reading.
What are sweet peas?
Sweet peas (Lathyrus odoratus) are one of the most popular types of cut flowers and grown for their colorful and often richly scented blooms. Most sweet peas are vining annual plants that grow 6 to 8 feet tall and need the support of a trellis or other structure. That said, there are also compact sweet peas, like the dwarf variety ‘Knee Hi’, that have bushy growth. These are perfect for pots and hanging baskets. Antique varieties produce 3 to 5 pea-like flowers per stem, while choice varieties like ‘Spencer’, ‘Cutherberson’, and ‘Mammoth’ have been bred for long stems and extra-large blooms, with 5 to 6 flowers per stem.
Please note that unlike garden peas, sweet peas are not edible and all parts of the plant are poisonous.
2 options for when to plant sweet peas
Knowing when to plant sweet peas is the best way to encourage the healthiest and most productive plants. They’re tolerant of cool weather and unbothered by light frost. Your climate is the main factor for when to plant sweet peas and there are two options:
- Option 1 – Autumn: In zones 8 and above, sweet pea seeds should be planted outdoors in autumn. They’re typically sown in October or November, about the same time spring-flowering bulbs are planted. You may not see a lot of plant growth in autumn, but the seeds are busy developing robust root systems and quickly sprout up when the weather warms in spring. Some gardeners in mild regions plant a second sowing in early spring to ensure the longest season of sweet pea flowers.
- Option 2 – Early spring: In cooler climates, zone 7 and lower, sweet peas are planted late winter through early spring. Seeds can be sown directly in the garden or started indoors. I start my sweet pea seeds indoors as transplanting seedlings results in more vigorous plants than those that are direct sown. Below you’ll learn more about when and how to start sweet peas seeds indoors as well as get tips on direct sowing in garden beds.
When to plant sweet peas indoors
While you can direct sow sweet pea seeds, giving them a head start indoors under grow lights or in a sunny window gives the plants the strongest start. You’ll need to first figure out when to plant sweet peas indoors. To do this you need to know your last expected frost date. Sweet pea seedlings should be moved to the garden 2 to 3 weeks before the last frost date. So if my last average frost date is May 20th, I’ll be transplanting my sweet pea seedlings outdoors around May 1st.
Ok, now I know when to transplant the seedlings in my garden, but when do the seeds need to be started inside? Next, we need to look at how many weeks of growth sweet peas need indoors before they’ve moved outside. They’re fairly quick to grow and should be sown indoors 4 to 6 weeks before you intend to transplant them to the garden. This means I need to count backwards 4 to 6 weeks from May 1st to determinate the indoor planting date. A quick glance at the calendar tells me I need to start my sweet pea seeds under my grow lights sometime between mid-March to early April.
How to start sweet peas indoors
Now that we’ve figured out the timing, it’s important to know how to plant the seeds. Let’s look at the supplies you’ll need for starting sweet peas indoors.
- 4 inch pots or cell packs placed in seeding trays
- Seed starting growing mix
- Plant labels and a waterproof marker
- Grow lights or a sunny windowsill
- Watering can
When you’re ready to plant, fill pots or cell packs with the pre-moistened growing medium. Sow the seeds 1/4 to 1/3 inch deep. Don’t bury the seeds too deeply or they may never germinate. Once planted, water the pots and move them under a grow light or place them in a sunny window. When the first seeds sprout, turn on the grow light, leaving it on for 16 hours a day.
How to transplant sweet pea seedlings
About a week before you intend to transplant sweet pea seedings into the garden, start the hardening off process. Transplanting is best done 2 to 3 weeks before the last frost date. You can harden off seedlings on a deck, patio, or anywhere there is shade to help them acclimatize to outdoor growing conditions. I harden sweet peas off in my unheated greenhouse or a cold frame using row cover or shade cloth to create shade. Gradually introduce more light over the 5 to 7 days it takes to harden off the plants.
Now that the seedlings are hardened off, it’s time to transplant them into a prepared garden bed. The site should offer full sun, unless you’re in a warm climate where afternoon temperatures soar. In that case, plant in a spot with afternoon shade. I live in a northern climate and want my plants to have maximum light. Therefore I plant in full sun. Sweet peas need rich, fertile soil, so amend with compost or rotted manure before planting. I like growing sweet peas in raised beds as they prefer well-drained soil. Aim for a soil pH in the 6.0 to 7.5 range.
I transplant the seedlings 5 to 6 inches apart at the base of a trellis or other support. I plant a double row, spacing the rows 5 to 6 inches apart. If you don’t have a lot of garden space, you can plant sweet peas in pots, window boxes, or planters. Space seedlings 5 inches apart and provide support to vining varieties growing in pots. An obelisk or container trellis is ideal.
How and when to plant sweet peas by direct seeding
As noted above, you don’t have to start sweet pea seeds indoors. In mild climates the seeds are direct sown in autumn, while in colder regions they’re direct sown in late winter or early spring, about 6 weeks before the last frost date. Sweet peas are tolerant of light frost.
Direct sow sweet pea seeds in a prepared garden bed planting them 1/4 to 1/3 inch deep and 5 to 6 inches apart. I use a garden dibber to make shallow holes. I always sow sweet peas in double rows, spacing the rows 5 to 6 inches apart. Once planted, water the bed and keep the soil consistently moist until the seeds sprout and are growing well.
Do you need to soak sweet pea seeds?
One question is whether you need to soak sweet pea seeds before you plant them. Soaking softens the hard seed coat to promote good germination. You don’t need to soak sweet pea seeds, but I typically do as it’s a very easy step to help ensure high germination rates. To soak, place seeds in a bowl and cover with at least an inch of lukewarm water. Leave them to soak for about 12 hours. I soak sweet pea seeds overnight, planting them the next morning.
Another option is to scarify the seeds by rubbing them between two sheets of sandpaper. To do this, empty a packet of seeds onto a sheet of sandpaper and place another sheet of sandpaper on top – be sure the rough sides of the papers are facing in. Rub the seeds between the sandpaper for 10 to 15 seconds to scratch up the surface. This will help the newly planted seeds absorb water for germination.
Want to learn more about the timing of when to plant sweet pea seeds and how to do it? Watch this video:
How long does it take sweet pea seeds to sprout?
Germination time depends on several factors including soil temperature, sowing depth, and even variety. I’ve found certain sweet pea varieties sprout quicker than others. Generally, you can expect sweet peas to emerge in 14-21 days if the temperature is between 55 to 65F (13-18C). If you’re starting seeds in a warmer spot the seeds will sprout quicker.
Caring for sweet peas
Sweet peas are relatively low-care plants, but I do pinch the seedlings to promote branching and I keep an eye on soil moisture. Here’s what you need to know about growing sweet peas.
- Support – Sweet pea vines climb using tendrils and they’ll happily scale many types of structures including trellises, fences, garden mesh, netting, or arbors. It’s best to set up the trellis or netting before planting so you don’t damage young seedlings.
- Pinch – Pinching sweet pea seedlings results in well-branched plants and the heaviest flower production. I pinch when the plants are 6 to 8 inches tall by removing the central growing tip with my fingers. I pinch back to just above a healthy set of leaves, leaving two to three sets of leaves to develop into vigorous side shoots.
- Water – Sweet peas need consistent moisture; never let them dry out as this impacts plant health and flower bud production. I water deeply several times a week if the weather is hot and there has been no rain. To make irrigation quick and easy, lay a soaker hose along the root zone of the plants. I also mulch the soil with straw or shredded leaves to conserve moisture.
- Feed – The final tip for growing sweet peas is to provide plenty of nutrients. I begin by amending the soil with compost or aged manure (learn more about soil amendments here) and then fertilize with a liquid organic flower fertilizer every 3 to 4 weeks. Follow the package directions.
For more information on when to plant vegetables and flowers, be sure to check out these detailed articles:
- When to plant zinnias indoors or outdoors
- When to plant sunflower seeds
- When to plant pumpkins
- When to plant daffodil bulbs
- Growing snap peas from seed
Were you wondering when to plant sweet peas? If so, I hope I’ve answer your question.