Nearly all of the data was collected in Oregon’s Mid-Willamette Valley, in greater Salem area, including city of Keizer, a Salem suburb.  Salem is some 45 miles (72.4 kilometers) to 50 miles (80.5 kilometers) south of Portland.  The Willamette Valley is situated between the Cascade Mountains on the east and Coast Range of mountains to the west, from the Columbia River on the north and to south of Eugene, Oregon, about 120 miles south of Portland on the south.

The Willamette River flows through city of Salem.  It is western boundary of city of Keizer, Oregon.


Bush Pasture Park (a park and botanical garden), Chemeketa Community College, Deepwood Estates (includes a garden), State Capitol grounds (a state park), and Willamette University in Salem, among them, have most of the plants that provided data.  In appropriate seasons, additional taxa were checked at various locations within cities of Salem and Keizer, a suburb contiguous with Salem’s northern border.  A few taxa were included at Oregon State University’s North Willamette Research and Extension Center at Aurora, Oregon, approximately 30 miles (48 kilometers) north northeast of Salem.

Coordinates for the above areas are:

Salem – 44.9308° N latitude, 123.0289° W longitude;
Keizer – 45.0006° N latitude, 123.0219° W longitude (the city borders Salem on the north and is bisected by the 45th parallel North latitude within the city limits);
Bush Pasture Park – 44.9317° N latitude, 123.0381° W longitude;
Chemeketa Community College – 44.9781° N latitude, 122.9769° W longitude;
Deepwood Estates – 44.9294° N latitude, 123.0317° W longitude;
State Capitol – 44.9387° N latitude, 123.0301° W longitude;
Willamette University – 44.9358° N latitude, 123.0314° W longitude;
North Willamette Research and Extension Center – 45.2803 N latitude, 122.7509 W longitude.

Bush Pasture Park, Deepwood Estates, State Capitol, and Willamette University are near Salem’s downtown business district, within a few blocks to south, southeast, and east.

Chemeketa Community College is in northeast Salem, about 4 miles from the downtown business area.

Salem and Keizer, and Bush Pasture Park and Deepwood Estates, are contiguous locations, Keizer being the northern boundary of Salem and Deepwood the eastern boundary of the Park.

Keizer, a northern suburb of Salem, straddles the 45th parallel latitude, equidistant north to south, north pole to equator.  The major data collection sites are from 2 to 5 miles south of the 45th parallel.

Approximate elevations, above sea level, of the respective locations where data was obtained, are within Salem city limits, a range of 141 feet (43 meters) to 470 feet (143.3 meters); Bush Pasture Park, 162 feet (49.4 meters) to 178 feet (54.3 meters); Chemeketa Community College, 189 feet (57.6 meters); Deepwood Estates, 158 feet (48.2 meters); Keizer, 134 feet (40.8 meters); State Capitol grounds, 171 feet (52.1 meters); Willamette University, 164 feet (50 meters); North Willamette Research and Extension Center, 99 feet (30.2 meters).


The above sites have a modified marine climate.  Oregon state climatologist George Taylor defined Oregon’s Willamette Valley climate:

The climate of the Valley is relatively mild throughout the year, characterized by cool, wet winters and warm, dry summers.  The climatic conditions closely resemble the Mediterranean climates which occur in California, although Oregon’s winters are somewhat wetter and cooler. Growing seasons in the Willamette Valley are long, and moisture is abundant during most of the year (although summer irrigation is common).

Like the remainder of western Oregon, the Valley has a predominant winter rainfall climate. Typical distribution of precipitation includes about 50 percent of the annual total from December through February, lesser amounts in the spring and fall, and very little during summer.  Rainfall tends to vary inversely with temperatures — the cooler months are the wettest, the warm summer months the driest.

The Mid-Willamette Valley area is in U. S. Department of Agriculture (USDA) climate zone 8b.  Annual average extreme minimal temperatures range from 15 to 20° F (-9.4 to -6.7° C).  All-time low temperature for Salem was in December 1972 -12° F (-24° C).

Sunset Western Garden Book, published by Sunset Publishing Corporation, Menlo Park, California, has compiled climate zones for Western North America that are more definitive and inclusive than the USDA zones, and do not correspond with USDA zones.  Sunset zones range from A1, A2, and A3 for Alaska, 1 through 24, north to south, in Western North America, to H1 and H2 for Hawaii.  The Mid-Willamette Valley is in Sunset’s Zone 6.

Sunset Western Garden Book describes its Zone 6 thus: (update)

Warmer summers and cooler winters distinguish Zone 6 from coastal Zone 5.  Tucked between the Coast Range and the Cascades, Zone 6 includes the Willamette Valley in Oregon, the Columbia River Valley between Vancouver and Longview, and Cowlitz drainage from Longview to Toledo (Washington).

The Coast Range buffers the impact of Pacific storms, but Zone 6 is still a maritime climate, with a long growing season (from 155 days at Cottage Grove (Oregon) to 280 days in Portland (Oregon) neighborhoods) and 40 to 55 inches of annual precipitation most places.  The continental influence is felt two to four times each winter when chilly interior air flows west through the Columbia Gorge and produces wind and freezing rain clear to the Portland airport.  In spite of this, Portland is among the mildest parts of Zone 6 – a great place to experiment with borderline plants like eucalyptus, acacias, and oleanders. Summer temperatures in Zone 6 average 10 to 15°F (5 to 8°C) higher than those along the coast, while Winters are cold enough to trigger good fruit set.  Ten-year extremes average 0 to 10°F (-18 to -12°C).  Warm Summers and chilly winters make the Willamette Valley one of the West’s best-known growing areas for berries, Hazelnuts, roses, flowering fruit trees, and broadleaf evergreens. The Willamette Valley’s hills and small mountain ranges create many microclimates.  South- and west- Facing slopes are warm enough to produce world-class Pinot Noir grapes, while north- and east-facing slopes are perfect for shade tolerant plants like rhododendrons, fatsias, and camellias.  These hills have perfect air drainage, so winters get less frost than the valley floor.

The table below displays average monthly climate indicators for Salem, Oregon.  It is applicable for all the sites above where phenology information has been collected.


Salem Temperature Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Avg. Temperature (OF) 39.6 42.8 45.7 49.1 54.6 61.4 66.3 66.7 61.6 52.7 45.0 40.0 52.1
Avg. Max Temperature (OF) 46.4 51.5 55.7 60.4 67.0 74.5 81.6 82.0 76.0 64.2 52.4 46.4 63.2
Avg. Min Temperature (OF) 32.7 34.1 35.6 37.7 42.2 48.3 50.9 51.4 47.1 41.1 37.5 33.6 41.0
Days with Max Temp of 90 F or Higher 0.0 0.0 0.0 0.0 < 0.5 2.0 6.0 6.0 2.0 < 0.5 0.0 0.0 16.0
Days with Min Temp Below Freezing 13.0 11.0 9.0 5.0 1.0 < 0.5 0.0 0.0 < 0.5 3.0 7.0 13.0 62.0
Salem Heating and Cooling Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Heating Degree Days 787 622 598 477 322 141 41.0 52.0 131 381 600 775 4927
Cooling Degree Days 0.0 0.0 0.0 0.0 0.0 33.0 81.0 104 29.0 0.0 0.0 0.0 247
Salem Precipitation Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Precipitation (inches) 5.9 4.5 4.2 2.4 1.9 1.3 0.6 0.8 1.6 3.0 6.3 6.8 39.2
Days with Precipitation 0.01 inch or More 18.0 16.0 17.0 14.0 11.0 8.0 3.0 4.0 7.0 12.0 18.0 19.0 147
Monthly Snowfall (inches) 2.9 1.3 0.5 0.0 < 0.05 0.0 0.0 0.0 < 0.05 < 0.05 0.3 1.6 6.6
Other Salem Weather Indicators Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Average Wind Speed 8.0 7.7 7.8 7.2 6.7 6.6 6.7 6.3 6.1 6.0 7.5 7.9 7.0
Clear Days 3.0 3.0 4.0 4.0 6.0 7.0 15.0 14.0 11.0 6.0 3.0 2.0 77.0
Partly Cloudy Days 5.0 4.0 6.0 7.0 8.0 8.0 8.0 9.0 8.0 8.0 5.0 4.0 80.0
Cloudy Days 24.0 21.0 21.0 19.0 18.0 15.0 8.0 9.0 10.0 17.0 22.0 25.0 208
Avg. Relative Humidity 61.0 81.0 77.5 73.5 72.0 69.5 67.0 63.0 63.5 67.5 74.5 82.5 83.5

The above information shows December and November as wettest months, on average, following by January, February, and March.  July and August, on average, are the driest months, each with less than an inch of rain.

All plants observed are growing outdoors with no special winter protection.


Growing Season

In United States and Canada growing season usually means the days between last frost, in spring in most states, and first frost in fall.  Warmer states’ growing season extends into the winter months.  Southwest Arizona, coastal California, Florida, Louisiana, and south Texas of the 48 contiguous states tend to have the longest growing season in their warmest parts.  Average length of growing season in the 48 contiguous states has increased by about 2 weeks since beginning of 20th century.

Adapted plants go through all development stages typical of the species or cultivar.  In shorter growing seasons the stages will be more compressed to complete all of them.  In longer growing seasons the development stages will be extended for longer periods.

It is sometimes suggested that for tender plants add two weeks to the average date in the spring to protect against the possibility of late season frost. And, in the fall, subtract two weeks from the average date to be on guard against an early frost.

Average growing season range at Salem, Oregon is 150-180 days, with minimum of 143 and maximum of 204 days.  Following are growing season length for selected cities in United States and Canada for comparison.


Soils in lower elevations of Willamette Valley were laid down by Missoula floods.  These floods were caused by a natural breaching of ice dams in northern Idaho 19,000 to 15,500 years ago during the end of Ice Age.  The ice dams formed huge lakes, Lake Missoula, extending all the way into southwestern Montana, encompassing 500 cubic miles.  When the dams broke great amounts of water rushed through northern Idaho, eastern Washington, eastern Oregon, through the Columbia River Gorge, into Willamette and other valleys, and to the Pacific Ocean.  Lands in path of the rushing water were scoured, depositing the soil in various valleys along the way to the sea.  The Willamette Valley was transformed into a 100 miles long, 60 miles wide, and 300 feet deep lake for relatively short times. As the Valley slowly drained, deposits that eventually became the Amity, Willamette, Woodburn, and other soil series were left behind.  Today more than 300 feet of sediment lie in the floor of the Valley.

Soils at higher elevations in the Willamette Valley are of volcanic origin, resulting from extended volcanic activity many years ago during the Pleistocene and more recent Holocene epochs.  These include the Jory and Nekia soil series within Salem city limits.

Soils at the sites within the Salem city limits – Bush Pasture Park, Deepwood Estates, Chemeketa Community College, State Capitol, and Willamette University – are mostly well drained Willamette and moderately well- drained Woodburn, soil series, both classified as silt loams.

Soils in the Keizer area are predominantly well-drained Cloquato and Willamette, and moderately well- drained Woodburn, soil series, all silt loams.

The contiguous Bush Pasture Park and Deepwood Estates share Clackamas gravelly loam, a soil that is not well drained, with mostly native plant species and relatively few introduced species.  They also share the well-drained alluvium Salem gravelly silt loam.  Bush Pasture Park additionally has a significant acreage of the well-drained and fertile Willamette silt loam, one of best soils in Willamette Valley.

Chemeketa Community College campus soils are of somewhat poorly drained Amity, the poorly drained Concord and Dayton, and moderately well drained and fertile Woodburn series.  All are classified as silt loams.  Major difference is the presence of impervious subsoils under the poorly drained soils.

Soils on State Capitol grounds and Willamette University are the moderately well drained and fertile Woodburn silt loams.

Well-drained Latourell and moderately well-drained Quatama, both loams, and Willamette silt loam are the major soils series at North Willamette Research and Extension Center.

Many of the soils have been amended with organic materials and fertilizers, often considerably altering the native soils, to improve plant growth.  Infrastructure construction has much altered the native soils in places in most of the sites.