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Colorado Blue Spruce

With such densely foliated low limbs reaching the ground, Colorado blue spruce functions as much as large shrubbery as a small tree.

Colorado and Utah share the same stately state tree; the Colorado blue spruce, Picea pungens, which is native to the Rocky Mountains between Arizona, New Mexico and southern Idaho. Colorado blue spruce are stoutly conical trees that grow slowly to eventually get more than seventy five feet tall. Where they need to compete with other trees in forests, the biggest are nearly twice as tall with trunks nearly five feet wide. However, because they grow so slowly, and do not get much bigger then they need to, well exposed trees stay proportionate to home gardens for many decades. Many shrubby compact cultivars actually stay less than ten feet tall. Foliage can be grayish green to silvery pale blue. The stiff and sharply pointed needles are about three quarters to an inch and a quarter long, and densely set on relatively rigid stems.

The elegant silvery blue foliage of the Colorado blue spruce is striking either in front of or behind darker evergreen foliage, like that of redwoods or junipers. These stout and densely foliated trees make any garden look a bit more woodsy.

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Cork Oak

The bark actually looks like cork.

This is a tree that takes some time to impress. Bloom is uninteresting. Foliage is no more distinctive than that of coast live oak. Instead, the most spectacular characteristic of cork oak, Quercus suber, is the boldly striated and uniquely spongy texture of its mature bark. Such bark takes a few years to develop, but gets so thick that it seems significantly older.

As its name implies, cork oak had historically been the exclusive source of bark for corks and cork products. As modern and more practical materials diminished demand for such bark, cork oak became more popular as an evergreen shade tree. It is quite happy within the arid chaparral climates of California. In fact, it behaves much like native oak species.

Mature cork oak trees generally stay less than forty feet tall, even if their trunks are wider than three feet with their unusually thick bark. Without excessive irrigation, their roots are notably complaisant. Low branches are more visibly sculptural than high branches. With pruning for adequate clearance though, trees with high branches are striking street trees. Foliar and floral debris is quite messy during spring bloom.

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Holly Leaf Cherry

Holly leaf cherry fruit looks better than it is. The flavor is good; but there is very little pulp around the big seeds.

Since it is native to coastal chaparral between Monterey and the middle of the Baja California Peninsula, holly leaf cherry, Prunus ilicifolia, is right at home in local gardens of the central and south coasts. Although it can be a nice refined hedge if not shorn too frequently, it is better where it has space to grow wild into a mounding shrub, and can eventually grow into an undemanding informal screen as high and broad as fifteen feet. Width is easier to limit with selective pruning than with shearing. Besides, unshorn plants bloom with clusters of minute white flowers in spring, and produce interesting deep red or purplish red cherries in autumn. Unfortunately, the cherries have very big pits with only minimal sweet pulp. The small glossy leaves have somewhat bristly teeth almost like those of some hollies.

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Silk Tree

Silk tree leaves are bipinnately compound.

Resiliency is typically an attribute. It is how silk tree, Albizia julibrissin, adapts to various urban landscapes. Unfortunately, it is also how it naturalized within a few ecosystems of North America. It grows easily from seed, whether or not it is appropriate to where it does so. Many naturalized specimens somehow find good situations in which to grow though.

With good exposure, most mature silk trees develop rather low but broad canopies. They have potential to grow taller than forty feet, but if not competing with taller trees, may stay half as tall. Their arching limbs flare elegantly outward in low mounding form. Their finely textured foliage provides appealingly uniform shade that is neither too dark nor too light.

The lacy and bipinnately compound leaves of silk tree are between half a foot and a foot long. Each leaf divides into as many as a dozen pairs of pinnae (leaflets). These pinnae divide into about twice as many pairs of pinnulae (leaflets of leaflets). Such minute foliar components disintegrate during autumn defoliation, and can disappear into groundcover.

The pink and fluffy summer bloom can actually be messier than the deciduous foliage. It does not disintegrate as it falls, so may accumulate on top of vegetation below. Cultivars generally bloom with richer pink color, although at least one blooms with white. ‘Summer Chocolate’ exhibits richly bronzed foliage that contrasts strikingly with pastel pink bloom.

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Black Oak

Not many native deciduous trees turn orange or brownish red in autumn like California black oak can.

Though native mostly to the Sierra Nevada and Coast Ranges between San Francisco and Oregon, California black oak, Quercus kelloggii, also inhabits isolated colonies in lower mountains as far north as the southern half of the Oregon Coast, and as far south as San Diego County. It actually occupies more area than any other hardwood tree in California. Wild trees competing in forests can get more than a hundred feet tall and live for five centuries. Well exposed urban trees may take their first century to eventually get about half as tall with trunks as wide as four feet.

Even while young, California black oak is a distinguished tree, with a broadly rounded canopy and elegantly arching limbs. The smooth silvery bark of young trees eventually becomes dark and uniformly checked with maturity. The distinctive deeply lobed leaves are about five inches long, and turn gold, soft orange or even brownish red before falling in autumn.

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London Plane

London plane might resemble Norway maple.

The parents of this unintentional hybrid are supposedly American sycamore and Oriental sycamore. No one really knows. London plane, Platanus X acerifolia, appeared within a private collection in London at the middle of the Seventeenth Century. It became popular there two centuries later, through the Victorian Era, because of its resilience to pollution.

Mature London plane trees might be taller than a hundred feet. Few here are old enough to be much taller than sixty feet though. Regardless, defoliation of such grand deciduous trees might be overwhelming through autumn. Foliar tomentum (fuzz) can be surprisingly irritating while raking too much foliage. Autumn color is unimpressively brownish yellow. The specific epithet ‘acerifolia’ translates to ‘maple foliage’, because the foliage resembles that of Norway maple.

London plane remains among the most common of street trees. It really is commendably adaptable to inhospitable urban situations. However, it is not perfect. Its roots eventually displace pavement. The foliar canopies can eventually grow disproportionately broad for compact urban parkstrips. For some people, the foliar tomentum can be a major allergen.

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Chestnut

These chestnuts that may later be ‘roasting on an open fire’ are now falling from the trees that produced them over summer.

Of all the nut trees that are actually quite easy to grow, the chestnut, Castanea (various specie and hybrids), has somehow become the most obscure. It probably lost popularity while native forests in eastern North America were being annihilated by rampant disease (which never became such a threat in the west), but may be unpopular simply because it can get so big. Mature trees are regularly more than seventy feet tall and nearly as broad.

Chestnut trees are productive for those who like the nuts, but simply very messy for those who do not. The smooth meaty nuts are contained within offensively spiny husks known as ‘burrs’. A few varieties of chestnuts fall freely from the burrs. Most need to be separated from their burrs even after they fall to the ground. The evenly serrate leaves will soon be turning amber gold or brown for autumn.

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Sweetgum

Sweetgum is impressively colorful for autumn.

This might be the most spectacular and most reliable of autumn foliage available locally. Sweetgum, Liquidambar styraciflua, begins to develop a brilliant blend of yellow, orange and red in response to the earliest mild chill of autumn. It defoliates slowly to retain much of its colorful foliage through the earliest rain and wind of winter, and perhaps even later.

Sweetgum leaves are palmate, and about four inches wide, with five pointed lobes. One very rare cultivar has hierarchically lobed leaves, with lobes on lobes. Another has blunt lobes. Some cultivars as well as individual trees favor particular foliar colors for autumn. ‘Burgundy’ exhibits more dark red color than typical, and retains foliage later than typical.

Mature trees can grow fifty feet tall, but are not very broad. They can get taller and lankier to compete with other tall trees. Their upright form conforms to grove arrangement within large landscapes and parks. Unfortunately, their aggressive roots can displace concrete. Their branches can be structurally deficient. Their spiky and hard fruit can be obnoxious.

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Chilean Nightshade

Chilean nightshade is less aggressive than related potato vine.

Yellow centered blue flowers like those of the familiar potato bush, on vines almost like those of the comparably familiar potato vine would be like the not so common Chilean nightshade, Solanum crispum. However, Chilean nightshade needs to be trained onto support in order to climb, and only reaches fifteen feet or so. The cultivar (cultivated variety) ‘Album’ is like a shrubbier redundancy to potato vine, since it has white flowers. ‘Glasnevin’ is the most popular cultivar because it flowers freely and is hardier to frost.

The nearly inch wide and slightly fragrant flowers bloom in small clusters from May or June until about now. Small but sometimes prominent green berries that turn yellowish orange and then dark purple are toxic. Partial shade inhibits bloom and vigor.

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Spuria Iris

Spuria iris should be less rare.

Bearded iris are famously diversely colorful. Not much lacks from their floral color range. Spuria iris, Iris spuria, are quite different. Their floral color ranges only from purplish blue to bright white, all with prominent yellow throats. The least rare of this rare species is the subspecies carthaliniae, almost all of which blooms white. Seed is generally true to type.

Seed might be abundant without timely deadheading. However, propagation is easier by division of the copiously branching rhizomes. Such rhizomes are fibrous and tough, with comparably tough and wiry roots. They migrate to develop broad colonies, which should appreciate thinning every few years. They rarely get too crowded to bloom nicely though.

Spuria iris blooms for almost two weeks during late spring or early summer. Two or three flowers bloom in succession on stems that are nearly as high as their deciduous foliage. Leaves are elegantly narrow and upright like those of cattail, but get only about three feet tall. Carthaliniae subspecies defoliate later than others, which defoliate through summer, then foliate for autumn.