Perennial

• The perennial sweet pea (Lathyrus latifolius) reaches a mature size of 4 to 7 feet in height. Unlike other sweet pea varieties, perennial sweet pea attains these heights when it receives plenty of sun along with moist, well-drained soil.
  
  

Annuals

• Many sweet pea annuals (Lathyrus odoratus) reach 4 to 6 feet in height during the growing season. To reach its full height, the plant requires cool temperatures in moist, well-drained soil and full to partial sunny areas of the garden.
  
  

Exceptions

• Some trailing sweet pea varieties grow even bigger when supported by a vine, fence or trellis. The plants can reach up to 9 feet in good climate conditions. Smaller annual sweet pea bushes such as Snoopea only reach up to 2 feet in height while Explorer grows to just 14 inches tall.