From what I understand the animal life in Seattle is severely mutated due to the environment (or rather, the pollution thereof), which leads me to wonder, does this apply to nature/vegetation as well? Are the plants in Shadowrun 5e/sr5's Seattle also mostly mutated and abnormal?
Let's see what the books have to say. Let me start...
Runner Havens (SR4, Fanpro print 2006)
Seattle's climate is classed as being in the climate of temperate rainforest and gets 200 centimeters of precipitation a year. It also lies under a near-constant smog, at least when there is no steep breeze from the sound. 1
Council Island is heavily forested and undeveloped. 2 Fort Lewis is likewise made up of "thousands of acres of undeveloped forest and hills" 3
Puyallup was covered in ash and lava and houses "boiling geysers in the Hell’s Kitchen lava fields". It also suffers under a "constant ash fall from Mount Rainier" and "more acid rain than any other district in Seattle." 4
Redmond's southeast is the site of the Trojan-Satsop meltdown of 2013. As a result, Beaver Lake and the surrounding area nicknamed Glow City are radioactively contaminated, though not as heavily as the SOX 5 (aka Luxembourg, Saarland, Alsace). This area is what OP identified, with the devil rat packs. However, this is also the area where groups of hippies have built the bio-domes that grow almost* organic food, filter usable water from the wastes and produce their own electricity. These are the "vegetation-covered bio-domes of the Plastic Jungles" 6
Renton, aside from suburban areas, has "large golf courses, private parks, and country clubs", with the associated plants: trimmed hedges, topiaries, carefully tended grass, and trees. 7
Snohomish is dominated by farmland and owned to a huge degree by different "agribusiness and aquaculture crops." As far as the eye can see, you'll see "high-yield genetically modified crops and herds of bio-enhanced livestock, while the Snohomish River and Sound are lined with aquafarms harvesting all manner of interesting seafoods." 8
Seattle Sprawl: Emerald Shadows (SR5)
The book repeats the basic climate, but adds that while they have more rain days than NYC, they have less precipitation overall. The Mountains to the east make Seattle a natural smog-trap. Rain happens generally in the morning and evening, alternating or joining with fog. Temperature is rarely above the "mid-thirties" and doesn't go below freezing temperature too often, the worst would be about 10 degrees below 0. This implies that the writer used degrees Celsius. In Fahrenheit that's 86 °F and 14 °F respectively. 9
The town is - in the all surrounding SSC (Salish-Shidhe Council) - surrounded by food production areas, mainly "a wide variety of fruit and grains", implying sprawling fields and orchards. While some agriculture happens inside the town, it is better described by "its greenery and temperate weather" 10
The rest of the town is depicted quite similar to the Runner Havens descriptions, at times, putting stress on where things are manmade. Or destroyed. Like Redmond:
Nothing thrives here. It’s infertile. Unproductive. Toxic. 11
OP is mistaken: Seattle's climate and vegetation actually has not changed a lot compared to 1990s Seattle, and not all of Seattle is an irradiated wasteland. In fact, that is only true for a small area of the Redmond Barrens called Glow City. The most drastically altered area is Puyallup, which gained areas where you can encounter ash flats, lava fields, and geysers.
Glow City itself has a few snippets that can be found in sourcebooks from older edition books, but little to nothing about the vegetation there 12.
However, there is lot of paranormal fauna in Seattle: Corporations will secure their buildings with Awakened Ivy, Guardian Vines, GloMoss, and Haven Lily as parts of the active and passive defense against Magical intrusions 13.
- Runner Havens (2006) p.63
- Runner Havens (2006) p.102
- Runner Havens (2006) p.104
- Runner Havens (2006) p.105
- The SOX were first mentioned in Target: Wastelands (ca. 2001) pp.26, have a german/frensh exclusive sourcebook, 10 mentions in the Six Wolr Almanac
- Runner Havens (2006) p.106
- Runner Havens (2006) p.106
- Runner Havens (2006) p.107
- Seattle Sprawl: Emerald Shadows pp.10
- Seattle Sprawl: Emerald Shadows p.11
- Seattle Sprawl: Emerald Shadows p.57
- New Seattle (1999) p.65, 118 ; Target: Wastelands: (ca. 2001) p.12 ; Seattle 2072 (2009) p.117
- Street Magic (2006) p.126
Let me add something about the weather specifically all from real-life Seattle, because these descriptions do not really do it justice.
First, no one in Seattle carries an umbrella.
Second: there's rain, and there's rain. Someone not from Seattle likely pictures actual rain with drops and everything. But it's not like that. It's a mist that is constantly instantly evaporating. Not a mist that hangs, not most of the time. Basically, a mist you can clearly see through that isn't misty.
I am from Florida with relatives that live in Seattle. One evening there was a thunderstorm while I was visiting. I thought hey, this is Seattle, apparently, it rains here all the time. Well, it doesn't. Not like that. The next day it was all the locals in every shop and restaurant could talk about. Remarks on "did you hear that thunder" "oooh I saw the lightning" and how it was really coming down in fat drops. A rainstorm like this would not be noteworthy in Florida at all, and it certainly would not be a topic of excited conversation.
As I visited more often and for longer what I found was that Seattle was...wetly dry. Basically, there is a constant mist, clouds, and spatterings of what I as a Flordian would hardly call rain. In Florida we would say "looks like it hasn't decided." The only people wearing raincoats are tourists.
When I say wetly dry it's because while there is a mist, somehow it isn't humid. In Florida things take forever to dry. In Seattle you'll get misted, but in a minute you will be dry again.
Is it cloudy? Yes. Is there fog--yes, but generally burns off midday. Sometimes, there will be a day or two when the fog just comes and SITS on the city. It does some odd things to sound. While Florida very occasionally gets fog and smoke fires, it's just not like the fog in Seattle.
That doesn't mean there aren't sunny days, and the people there don't find whatever excuse for a beach they might have and lay upon it. But gloom and clouds/rain/sleet snow is happening about 150 days out of 365. In 2020 for the first 80 days of the year it was all clouds.
Summer is mild. Lots of apartments have no AC. Some have no installed heaters and mobile heaters are popular. During the winter months the streets are SLICK because the precipitation freezes before it evaporates and they have some hilly streets. Snow can happen and does, but mostly it's just grey skies and cold.
EDIT: The weather in 2021 is highly unusual...family up there took a thermometer reading at 108. This is sending people into a panic because this a place where no aircon has been needed. If you want to use the highly unusual weather as a template for what things might be like in a Shadow Run universe, you certainly can. The real-life Seattle is, of course, just a jumping-off point.