Sweet peas (Lathyrus odoratus) are originally from Italy and Sicily. They were discovered around 1699 by a monk named Father Cupani. These are vining annuals that come in colors such as white, yellow, pink, red, crimson, mauve, and purple. The plants climb by means of tendrils and normally must be staked to grow as they are supposed to. The plants flower from late spring to autumn if deadheaded regularly. Otherwise, they go to seed. Modern varieties do not have much fragrance, but heirloom varieties smell wonderful.
Sweet peas have a reputation of being somewhat difficult to grow. However, if given the right conditions, they are relatively easy to grow.
First, they only grow in rich, well drained soil in full sun to part shade. The soil should be tilled to a depth of six inches, then have three inches of compost added and tilled again until the compost is thoroughly mixed in the soil. When the sweet peas come up, they should be mulched in compost, as well. Even then, they must be fertilized every two weeks in the summer when they are growing. Use a water soluble, well balanced fertilizer such as a 10-10-10 and pour it on the soil around the sweet peas.
Sweet peas should be sown in the fall around late September and early October in areas with mild winters and hot summers. Plant them one inch deep. Space them at least three inches apart to allow for air circulation between them. Germination takes ten days to two weeks. When seedlings are several inches tall, thin them to six to eight inches apart to allow air to circulate around them. Poor air circulation promotes mildew among sweet peas.
In areas with cold winters, sweet peas should be started indoors and planted outside after all chance of frost is passed. They will bloom later than plants started from seed in the flower bed. Sow each seed in a three to four inch pot so you do not have to disturb the root system when transplanting it.
Birds love sweet pea seedlings. Protect them with upside down strawberry containers to keep the birds out. Once they have outgrown these containers, they are probably safe from the birds. They are, however, still vulnerable to slugs and snails. Use a iron based slug killer such as Slug-go or Escar-go to kill the slugs. These poisons are not as poisonous to pets or children as the old copper based ones.
Sweet peas should be kept evenly moist throughout their life. They really do not like changes in the moisture of their soil, so make sure it is always moist and never soggy.
Sweet peas have trouble from aphids and thrips when blooming. They are sensitive plants and do not respond well to insecticidal soaps. Use a strong blast of water to spray the aphids off of your plants. Blue sticky traps, when hung among the plants, usually take care of the thrips.