Sweet peas are cool-weather annuals that handle some frost (NC State), so they go in early in spring, well before your last frost. Illinois Extension says to sow from mid-March to mid-April; GardenPal turns that into an estimate of about 6 to 4 weeks before your average last frost. Early sowing matters because NC State notes sweet peas struggle in hot, humid summers.
Sweet peas (Lathyrus odoratus) are grown for their fragrant flowers, not for eating. NC State describes them as a climbing or bushy annual from the Mediterranean that reaches up to 8 feet on a support, or about 3 feet as a bush; Illinois Extension lists a height of about 6 feet. They want the same cool start as garden peas, and the same kind of trellis, but they are a different plant.
Planting dates by last frost
The table counts from your average last spring frost. Illinois Extension's mid-March to mid-April window lands roughly 4 to 6 weeks ahead of a typical central-Illinois last frost, so GardenPal uses that offset for every region; it is our estimate, not an extension figure. If you start seed indoors, sow into individual small pots, because University of Missouri Extension lists sweet peas among the flowers that don't transplant well.
| Average last spring frost | Sow seed outdoors (GardenPal estimate) | Start indoors in small pots (GardenPal estimate) | Plant hardened seedlings out (GardenPal estimate) |
|---|---|---|---|
| Mar 1 | Jan 18 – Feb 1 | Dec 21 – Jan 4 | Feb 1 – Feb 15 |
| Mar 15 | Feb 1 – Feb 15 | Jan 4 – Jan 18 | Feb 15 – Mar 1 |
| Apr 1 | Feb 18 – Mar 4 | Jan 21 – Feb 4 | Mar 4 – Mar 18 |
| Apr 15 | Mar 4 – Mar 18 | Feb 4 – Feb 18 | Mar 18 – Apr 1 |
| May 1 | Mar 20 – Apr 3 | Feb 20 – Mar 6 | Apr 3 – Apr 17 |
| May 15 | Apr 3 – Apr 17 | Mar 6 – Mar 20 | Apr 17 – May 1 |
| Jun 1 | Apr 20 – May 4 | Mar 23 – Apr 6 | May 4 – May 18 |
Why the early start matters
NC State warns that sweet peas can be hard to grow where summers are hot and humid, and that flowering drops off once seed pods are allowed to form. The earlier the vines get going, the more weeks of cool weather they have to flower before heat shuts them down. NC State's cultivar list includes heat-tolerant types such as 'Old Spice', which are worth choosing in warm regions.
Site, soil and support
| Step | What extension sources say |
|---|---|
| Sun | Full sun; tolerates some shade (NC State) |
| Soil | Rich, fertile, well-drained soil with organic matter added (NC State) |
| Water | Keep evenly moist and mulch (NC State); keep moist while growing (Illinois Extension) |
| Support | Climbing types need a trellis or other structure; bush types can grow as mounds (NC State) |
| Rotation | Avoid planting sweet peas in the same spot in consecutive years (NC State) |
Cut often for more flowers
Both sources say the same thing: pick. Illinois Extension says to keep cutting to promote flowering, and NC State says to deadhead spent flowers because developing pods reduce blooming. A vase of stems every few days is the easiest way to keep the vines producing.
Pests
NC State lists no serious insect or disease problems but says to watch for slugs, snails and aphids, which are most active in the cool, damp weather sweet peas like.
Frequently asked questions
When should I plant sweet peas?
In early spring. Illinois Extension says to sow from mid-March to mid-April; GardenPal estimates that as about 6 to 4 weeks before your average last frost.
Can sweet peas handle frost?
NC State says they tolerate some frost, which is why they can be sown before the last frost date.
Should I start sweet peas indoors?
You can, but sow into individual small pots. University of Missouri Extension lists sweet peas as hard to transplant and best started at about 55°F.
Why did my sweet peas stop flowering?
Usually heat or seed pods. NC State says flowering decreases when pods develop and that sweet peas struggle in hot, humid summers; Illinois Extension says to keep cutting.
Are sweet peas edible?
No. NC State lists sweet peas as poisonous and advises not planting them near edible peas.
How tall do sweet peas grow?
Up to 8 feet on a support, or about 3 feet as a bush (NC State); Illinois Extension lists about 6 feet.
Enter your frost dates to see sweet peas' early-spring neighbors, such as garden peas, spinach and lettuce, on one calendar.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- NC State Extension Gardener Plant Toolbox, "Lathyrus odoratus" (sweet pea)
- University of Illinois Extension, "Sweet Pea" (flowers)
- University of Missouri Extension, "Starting Plants Indoors From Seed" (G6570)
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.