In general, violets are one of the favorite flowers in America; certainly, the houseplant African violet is the most popular with indoor gardeners. And there is of course the widely cultivated, hybridized Pansy, in all their big, bold colors and large faces. And the somewhat smaller Violas, like the old-fashioned Johnny-Jump-Ups and the intense, sweetly fragrant Viola odorata.
Of these latter, perennial type violas, there are more than 300 species native to North America. And of that number, about thirty are native to the Pacific Northwest! So widespread and similar in appearance, they can sometimes be difficult to identify in the wild. Everywhere from bogs and wetlands at low elevations, to moist-to-dry woodlands and low to mid-elevations, to dry meadows and rocky outcroppings at higher elevations – there are native violets to be found at every location and in every situation.
So likewise, there are there native violas for every garden. Cheery and virtually pest and disease-free, they are an agreeable groundcover, gradually spreading into a colorful carpet throughout the spring months.
Violas can be divided into "woodland" and "non-woodland" varieties; the former are native to the mostly moist, shady settings of low to mid-elevation forests, and the others at home in more open shade/sun of meadows and the drier, less fertile settings native east of the Cascades and the upper elevations, suitable for home rock-gardens. All are springtime bloomers.
In our region, there are three woodland varieties that are most commonly found :

Viola glabella – Stream Violet
This is one of the most commonly widespread violets in Western Oregon. Sweet yellow flowers rising slightly above bright green heart-shaped leaves, brightening up woodland settings; spreading by underground rhizomes, V. glabella will eventually carpet the ground with its cheery presence. Given enough moisture, it can become somewhat invasive; drier conditions can keep it more in check. But why not let it have its way throughout a woodland setting? Slugs and snails find the leaves of this violet to their liking, but other than that all-too-common pest, these violets are pretty immune to trouble in the garden.

Viola sempervirons – Evergreen Violet
Another woodland violet for moist conditions, V. sempervirons can be found in mossy, mostly-shady areas from low to mid-elevations, so it is happiest in similarly moist, shady to partly sunny spots in the garden. It is more compact and diminutive than V. glabella, but with similar yellow flowers. It is a suitable choice for planting between stepping stones.
Viola adunca – Early Blue Violet
Low-growing, with more triangular leaves and bright blue to blue-violet flowers, V. adunca prefers shade to dappled sun and moist soil, but is adaptable to a much wider variety of settings, thriving in even full sun if given sufficient moisture. Its unique characteristic is that the foliage is a major food source for the endangered Oregon Silver Spotted Butterfly, so if you find this violet's foliage is being eaten, look for a caterpillar first – you may be providing important habitat for an endangered Oregon native!
There are numerous other native violets that you should be aware of and watch for, if you want to have a wider variety in your landscape. Here are some of them:

V. beckwithii - Central and SE Oregon native; upper petals usually deep maroon, lower petals white to lavender with darker streaking.

V. douglasii – similar to V. howellii, growing in sunny meadows and grassy slopes that are wet in spring, dry in summer; yellow flowers with purple streaks on lower petals and brushed on the underside of upper petals.

V.nuttallii - flowers yellow and brownish purple with brownish purple veining.

V. palustrus – flowers are pale whitish lavender with darker veining. Uncommon, growing in marsh and swampy areas.

V. trinervata – considered one of the more garden-worthy of the dry-land species; roundish lower petals are pale lavender with yellow, upper petals a darker violet. Grows in seasonally moist, summer dry areas.
Peggy Acott
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Viola adunca
Dog Violet
Family: Violaceae
Genus: Viola
Common name: Violet
Native Range: Up to thirty species are native throughout the Pacific Northwest, from sea-level to sub-alpine, both sides of the Cascades, in moist woods, grasslands, and dry, rocky areas.
Characteristics: Primarily clumping, basal tufts of low-growing plants, often spreading by system of underground rhizomes. Leaves are generally small and heart-shaped (though drier-country violets tend to have thinner, more distinctly-cut leaves), with singular, showy flowers of yellow, bluish purple or white (depending on species) with five distinct petals, some bearded, some striped.
Seed pods generally in a tripod that tend to explode when ripe upon contact, spreading seeds; some have a sticky substance attractive to ants, who then take them and bury them, further helping the violets' natural spreading capabilities.
Culture: Most species are shade-lovers, but some are happy in part to even full sun. The woodland species like it on the moist side, but there are those that happily thrive in dry shade to sun!
Pests/Diseases: Slugs and snails can be a problem for the woodlanders; other than that, native violets seem to be pretty much pest/disease-free.