Washington or high elevations in eastern Washington because fruit needs to reach maturity quickly in cooler summer temperatures. Varieties that have longer growing seasons are better suited to the warmer summer temperatures of eastern Washington.
Tomatoes can be very temperamental, and if not cared for correctly, you can end up with few fruit or mostly green tomatoes. Water is key to a healthy plant. Expect to water about 1” per week during peak tomato growth. Using a mulch will help reduce water loss. Overwatering can cause increased leaf growth at the expense of tomato fruit. Too much water also encourages disease problems.
For more information, please see http://cru.cahe.wsu.edu/CEPublications/FS145E/FS145E.pdf
Submitted by: Gary Fredricks, May 26, 2015