CORVALLIS - Eggplants are tropical in origin and therefore sensitive to cool temperatures. Members of the tomato family (Solanaceae), eggplants will not grow much or set fruit unless night temperatures are above 50 degrees and soil temperatures are above 60 degrees.
In most parts of Oregon this means that eggplants should be planted as "starts," young plants with six to seven leaves. The best time to plant young eggplant starts out in the garden in Oregon is mid-May (warmer parts of the state) to mid-June (cooler parts of the state), according to Jim Myers, Oregon State University professor of vegetable breeding, and Deborah Kean, researcher at the OSU Vegetable Research Farm in Corvallis.
Don't be seduced by the early warm spring temperatures this year into transplanting your eggplant starts out too early, warned Kean.
"In years like this one, the temptation is to set the plants out in early May," said Kean. "The last two years we have had a frost around May 7 and eggplants and tomatoes have been killed." Frosts east of the Cascades come even later.
In western Oregon, a good rule is not to set out starts of tender annuals until after May 15. In the colder regions of eastern and central Oregon, it is best to wait until your soil has reached at least 60 degrees.
Depending on the variety and location of your garden, eggplants take about nine to 11 weeks of warmer weather to produce harvestable fruits. If you live in either a coastal area or the mountain and plateaus of central and eastern Oregon, you must help young eggplants along with a cloche, spun fiber row cover (example: Remay), or some other heat enhancing method, where temperatures drop below 50 degrees at night.
You can make a cloche or hot cap using a gallon plastic milk jug with the bottom cut out. Cover them until the first hot stretch of summer.
Home gardeners all over the state can also help eggplants reach maturity sooner by warming the soil at planting time. Try mulching eggplant starts with black plastic or by covering them with spun fabric row cover. Plant eggplants about two feet from each other in a fertile soil. Four to six plants should produce enough eggplants for a family of four.
A common mistake is to harvest eggplants too late. Eggplant should be harvested after they reach the proper size and color for their type, but before their fruit becomes seedy.
"The luster of the skin is a good indicator of readiness for harvest," said Myers. "If they are glossy, the fruit should be good. If the skin looks dull, then it is over the hill,” said Myers. If left on the plant too long, they will become tough and off flavored.
The main disease problem with growing eggplants in Oregon is verticillium, according to Myers.
"There's not much we can do about it, though," he said. "It causes leaf browning and necrosis (tissue death) that is evident at the end of the growing season. All varieties are susceptible, some more than others. The list below includes those that have better tolerance for verticillium."
The OSU Extension Service recommends the following varieties of eggplants as growing well in most areas of Oregon, except at the coast or the higher elevations of the state, where a protective cloche is needed: Dusky (65 to 70 days), Epic, Bambino (miniature, great for shish kabobs), Bride (elongated, white with a purple blush), Calliope (striped), Cloud Nine (white), Megal (elongated), Orient Express (elongated).