Here is advice from the City of Seattle for disposing of your holiday tree.
Seattle residents may compost holiday trees and holiday greens free of charge between Dec. 26 and Jan. 13.
At Recycling & Disposal Stations: Trees should not exceed eight feet in length and must be free of decoration so they can be turned into compost for local parks and gardens. Trunks should not exceed four inches in diameter.
At the Curb: Food and yard waste subscribers can put trees and greens out on their regular collection day. Cut trees into sections, six feet long or shorter, with branches trimmed to less than four feet to fit into the collection trucks. Bundle each section with sisal string or twine (not plastic). Decorated, flocked and plastic trees are not recyclable, and will be charged as extra garbage.
At Apartments: Check with your manager. Multi-family buildings can put out one tree next to each food and yard waste cart per collection day at no extra charge.