It is no coincidence that the deep dark green color of the foliage of Monterey Cypress is so similar to that of Monterey pine. Both inhabit coastal regions that are so often foggy that they need to absorb as much sunlight as possible. On harshly exposed hillsides where there is plenty of sunlight and not much competition with other trees, blue oak and digger pine, which is also known as gray pine, have grayish foliage. In the Rocky Mountains where the air is dry and clear, blue spruce and Rocky Mountain juniper have bluish foliage that is resilient to intense exposure.
In home gardens, a variety of foliar color can be quite appealing. The light blue of a well bred blue spruce is even more striking against a backdrop of dark green Monterey pines. Even in sheltered gardens where they might be partially shaded some of the time, blue spruce, as well as most blue varieties of juniper, century plant and echeveria. are as blue as they would be in the wild.
Some echeverias, salvias, lavenders and westringias with grayish foliage are more discriminating. They need to be very exposed to maintain their foliar color. They will be more green with partial shade. Regular watering also compromises the color of century plant and some salvias.
Bronze, reddish, purplish, yellowish and variegated foliage may not seem like it would be so dependent on exposure because such coloration is generally the result of genetic variation, such as mutation. Some variegations are caused by viral infections. Yet, exposure often enhances color.
Japanese maple is an understory tree that prefers to be at least partially sheltered by taller trees; but those with purplish foliage are merely bronzed green if too shaded. Golden honeylocust can be a vibrant yellow that contrasts nicely in front of a row of deep green redwoods, or it can be a sickly yellowish green if so close to the redwoods that it is partially shaded. The trick is knowing what sort of exposure each one wants.
highlight: century plant
The horridly sharp and stiff terminal spines, and the nastily stout marginal teeth of the big leaves of century plant, Agave Americana, are very intimidating. Such striking but dangerous foliage should be appreciated only from a safe distance. Individual leaves can get about six feet long, and are arranged in rosettes that can get more than twice as broad, so need plenty of space. Many century plants have metallic gray foliage. Others are variegated with various white or yellow stripes.
Despite the name, healthy century plants rarely wait half as long to bloom. Most bloom within only a quarter of a century. Many that get watered can bloom much sooner.
The imposingly tall and branched flower stalk that eventually emerges from the center of each rosette can be as striking as the foliage is. Vigorous stalks can get taller than twenty feet! The yellow flowers are too high to be seen closely, and are protected by even more nastiness.
Each rosette dies and eventually needs to be removed as young but wickedly barbed pups (basal offshoots) begin to develop around it after bloom. Cutting down such tall and spiny stalks surrounded by such objectionable foliage is quite a chore. Pups can easily develop into a wicked thicket if not thinned!