Lathyrus odoratus, or the sweet pea plant, certainly lives up to its name. These plants are known for their strong, sweet smell, and produce vibrantly colored flowers ranging from white to fluorescent pink. Lathyrus seeds grow best in U.S. Department of Agriculture hardiness zones 6 through 9, with an average annual minimum temperature above 10 degrees Fahrenheit below zero. These plants can suffer from many fungus and mildew infections, so you must plant them carefully in well-draining soil.
Difficulty: Moderately Easy
Till the soil to a depth of 12 to 15 inches in an area of your garden that receives six to eight hours of direct sunlight daily. Begin tilling as soon as the final frost has passed so that you can plant your Lathyrus seeds in early spring.
Soak your Lathyrus seeds in warm water for 24 hours prior to planting. This immersion will help speed up germination time in the young seeds.
Dig a 10-inch-deep, 3- to 4-inch-wide trench in your prepared ground. The length of your trench will vary depending on how many seeds you want to sow. Seeds must be placed 2 to 4 inches apart when planting.
Line the bottom 5 inches of the trench with manure or compost. This natural soil amendment will increase drainage and add nutrients to the soil as it decomposes.
Fill the remainder of the trench with regular potting soil mixed with balanced fertilizer at a ratio of four parts soil to one part fertilizer. Plant each seed 1 inch deep in the soil, spaced 2 to 4 inches apart. Water the soil thoroughly so that it is moist all the way through but not soaked. You should see the seeds begin to grow within two weeks.
Pinch back the growth when the plants reach 4 inches tall. Remove the tip of the stem just above the highest leaf. This will encourage fuller growth.
Mulch around the sweet peas with a 2- to 3-inch layer of organic mulch. Make sure the mulch doesn’t touch the stem of the plant, as this can cause burning. The mulch will keep the ground cool and moist.
Water the sweet peas whenever the top 2 inches of soil feel dry to the touch. Feed the plants with a balanced liquid fertilizer once per month during the growing season, generally early spring through early fall. Stop fertilizing before the first frost to allow the plants to go dormant.
Remove the spent heads of the sweet peas before they drop off of the plant. This will prevent seeds from spreading and taking over your garden.
You can also start Lathyrus seeds indoors in pots for early germination and bloom. Start the seeds three to four weeks before the final frost in your area and transplant the young plants outside after the final threat of frost has passed.