Ask Mr. Smarty Plants

The two members of the genus Thuja that are native to North America are  Thuja occidentalis (arborvitae) which is native only to the eastern part of North America, and Thuja plicata (western red cedar), which is native to Washington according to this USDA Plant Profile. It sounds like maybe you visualized a beautiful garden, enclosed with evergreens and full of color, and stepped over some lines.

According to our Native Plant Database: "Western red cedar is a very useful, ornamental conifer. It responds nicely to pruning and is sometimes used as hedge material. It has a slow to medium growth rate, is susceptible to bagworm and heart rot, and is pH adaptable."  From that same page: "The evergreen’s typical height is 50-75 ft., but it can grow to 200 ft. The aromatic foliage is bright green and scale-like, forming horizontal sprays which bronzes to crimson-purple in winter. Large to very large tree with tapering trunk, buttressed at base, and with a narrow, conical crown of short, spreading branches drooping at ends; foliage is resinous and aromatic."

When you say "dozens" of arborvitae, how  big a property are we talking about? The expected size of the trees means they will be actively competing with each other, unless they are spread over quite a large area, and in the competition with the flowers, it's pretty easy to predict which one is going to lose. A tree on its way to being that big is going to be building a BIG root system, as it goes, not to mention casting a lot of shade on everything around it. A standard in estimating how large a root system will extend is that it can be two to four times larger in circumference than the circumference of the crown of the tree. Even though the crown in this case is described as "narrow," the bigger the tree, the wider "narrow" is going to be.

Again, we don't know the exact size or arrangement of your property, but we can probably safely say that no amount of chopping at the roots is going to make a safe place to grow flowers. You planted the trees for privacy, but you may have created the hedge around Sleeping Beauty's castle, through which nothing, including light and people, can pass.  We can't even recommend an action, but you are probably going to have to choose between the arborvitae and everything else. Thinned out to be farther apart, the trees will provide less privacy for right now, but will go on growing, probably better with some of the competition removed. 

We went looking on the Internet for some more information and found this article on Thuja plicata (western red cedar) from Las Pilitas Nursery, which is a favorite source for plants native to California. From this same article, we extracted this quote: "This tree has allelopathic properties." Allelopathy is a fairly recent hot topic in horticulture; it is the ability of a plant to emit substances that will inhibit the growth of plants competing with it for space and nutrients. Those trees have everything going for them: extensive roots, allelopathic qualities, height and size. We don't feel there is any other solution than a realistic rethinking of what you want to do with your garden space. 

Pictures from Google