It's one of the main criticisms of the current legalization process. Open-growing needs to be passed in addition to decriminalization and legalization. At the moment, the pharmaceutical industry is doing its best to keep that from happening.
Here's the current statuses:
Washington:
http://www.liq.wa.gov/marijuana/faqs_i-502
http://www.criminaldefenselawyer.com/resources/can-i-grow-marijuana-my-own-personal-use-washington.h
Can I grow my own marijuana now? Can I sell my homegrown marijuana?
Home grown marijuana for recreational use, as well as sale, remains illegal. Recreational use marijuana must be purchased from a state-licensed retailer.
Alaska:
http://norml.org/laws/item/alaska-penalties
Under 6 plants, yes. 6-25 plants is a bit of a grey area at the moment:
http://www.theweedblog.com/is-growing-marijuana-legal-in-alaska/
Oregon:
http://www.oregon.gov/olcc/marijuana/Pages/Frequently-Asked-Questions.aspx
Q: Can I grow marijuana at home?
A: Yes, with limits. The act allows home grow of up to four (4) plants and eight (8) oz. per residence beginning July 1, 2015. Homegrow plants and product must be kept out of public view.
Colorado:
https://www.colorado.gov/pacific/marijuanainfodenver/marijuana-retailers-home-growers
Can I legally grow marijuana at home?
Answer - Yes. Adult residents can grow up to 6 marijuana plants per person, with more than 3 in the mature/flowering stage at any time. No more than 12 total plants are allowed per residence regardless of the number of adults living there. Marijuana plants must be kept in an enclosed, locked area.