Producing sweetly fragrant blossoms in any range of colors except green, yellow or black, sweet peas (Lathyrus odoratus) are annual climbing plants with origins in Italy. "Old-fashioned' sweet peas are particularly fragrant in flower while modern hybrids provide a wider array of flower sizes and colors and variegation. These plants appreciate cool weather and will grow 5 to 8 feet tall when allowed to grow up a string, wire fence, wood trellis or upright twig braces. Sweet peas also make attractive cut flowers for indoor arrangements or vase bouquets.
Choose a garden location that has a crumbly, fertile soil that has good drainage but is not dry. It should also be in a relatively sunny location, receiving between 6 and 10 hours of direct sunlight daily.
Cultivate this planting area with a garden shovel, turning the soil over to create a crumbled, loose soil profile to a depth of 6 to 12 inches. Incorporate organic matter such as compost or well-cured manure into the soil. This rich soil mixture will sustain the sweet pea plants nicely in the growing season.
Install some form of support structure for the sweet pea plants to climb. Use a prefabricated wire fence section, nylon mesh netting or wood trellis and place it in the planting bed. Alternatively, a tepee structure of bamboo stakes or dead tree branches, at least 5 feet long, can be pushed into the soil and act as the support structures for the sweet peas.
Nick the hard seed coats of the sweet pea seeds with a knife. Make a tiny chip mark on the opposite side of the "eye" or dark spot on each seed. This wound makes it easier for the plant embryo inside the seed to germinate.
Plant the seeds in the soil bed next to the support structures. Make a hole with your fingertip about 1/2 inch deep and drop one seed in the hole and refill it with soil. Tamp the soil down gently with your fingers. Place seeds 6 inches apart in a row or other pattern that works in unison with your support structure.
Water the planted seeds with a watering can with a sprinkler attachment. Monitor the planting area, watering as needed if natural rainfall is lacking, so that the soil is moist but never dry or soggy.
Fertilize the growing sweet pea plants with a liquid, well-balanced fertilizer (10-10-10) according to dosage directions on the product label. Apply this fertilizer as part of a watering every two weeks during the growing season.
Train stems of the sweet peas to be upright and carefully twine them into the support structure as they grow. After rainfall, check the plants as the rains may cause stem tips to flop off the supports.
Pinch off old (dead) withering flowers from the sweet pea plants with your fingertips. Called "deadheading," removing old flowers prevents seed formation and causes the plant to spend its energy producing many more flowers.
Pull out sick or dead sweet pea plants once flowering ends and temperatures in summer consistently remain above 80 degrees F. Excessive heat will cause plants to turn yellow and flowering to cease. In fall, a frost will also kill back stems, and eventually the entire plant, when temperatures dip below freezing.