July Tree of the Month: Santa Cruz Island Ironwood

By David Gress

Santa Cruz Island Ironwood is a rare tree species that once grew in vast areas of California and the American Southwest. However, its range was drastically reduced over the course of the last six million years by geologic and climatic changes that resulted in the uplift of our coastal mountains and the drying of the interior lands to the east – changes to which it could not adapt.  Consequently, it now is endemic to a remarkably small area – just three of the Channel Islands off our coast: Santa Cruz Island, Santa Rosa Island, and San Clemente Island.

It has also declined in genetic variance.  There is now only one species (Lyonothamnus floribundus) – and of that species only two subspecies exist, the Santa Cruz Island Ironwood (ssp. aspleniifolius) and the Catalina Ironwood (ssp. floribundus).

Even though it was introduced as early as 1900 into Santa Barbara by the pioneering horticulturist Dr. Francesco Franceschi, who recommended it as an ornamental tree for its spectacular flora display and graceful foliage, Santa Cruz Island Ironwood has been infrequently planted in our local landscapes.

Nonetheless, this lovely native tree has been appreciated and recognized by our community over the years.  In 1935, it was designated by the Board of Supervisors as the official tree of Santa Barbara County.  The Santa Barbara Botanic Garden features it in its logo – and chose its name for its biannual magazine, Ironwood.

In its island habitat, Santa Cruz Island Ironwood is often found in either shrub form (with short multiple stems) or in tree form (with a low crown dwarfed by windy conditions).  In landscape cultivation, it is a medium- to fast-growing tree, depending on growing conditions.  It can reach a height of 25 to 50 feet tall and a spread of 15 to 20 feet wide.  It usually has a straight single trunk but frequently develops basil shoots that can become multiple trunks.

Its canopy is dense when young, becoming more open with age.  The foliage consists of fern-like pinnately-compound leaves (4- to 8-inches long), each with 2 to 9 leaflets (2- to 4½-inches long and ½-inch wide).  The deep green leaflets are lobed into many segments that resemble those on the spleenwort fern (Asplenium).  These fern-like leaves give the tree one of its most distinctive ornamental qualities – and provide the derivation for both its subspecies name and its other common name, “Fern-leaf Ironwood”.

The stunning flower clusters make this tree a real standout in the spring.  The individual flowers are white and small (¼-inch wide) but emerge at the ends of branches in flat-topped panicles that contain hundreds of flowers.  When in full bloom, they give the tree a luminous glow.  Each flower is “perfect”, meaning it contains both female parts and male parts.  The nectar is eagerly enjoyed by native and domesticated bees and by other pollinators.   

Pollinated flowers produce fruits that consist of two small follicles (¼-inch long) that are brown and woody with a persistent calyx (flower base).  Each fruit follicle contains four tiny seeds that are disbursed in the autumn when the follicles ripen, dry, and open, allowing the seeds to fall. The spent fruit clusters, often considered unattractive, persist for some time – or are easily pruned off.

Another distinguishing feature is the gray-brown outer bark – this peels off in long thin strips to reveal the crimson red color of the inner bark.

Santa Cruz Island Ironwood can be somewhat difficult to grow, due to its sensitivity to heavy, poorly drained soils.  It develops best in locations with sandy loam soil and the influence of coastal moisture.  It can do well in full sun to partial shade.  When established, it is quite drought- tolerant but appreciates intermittent deep watering during very dry periods.  It is cold-hardy to 15 degrees F.  It can be grown as a single specimen – but looks better in a grouping or grove, because of its open growth habit at maturity.

Propagation was once considered difficult from seeds collected on the islands, since the trees there are confined to clonal or genetically identical groves.  Therefore, the first nursery stocks were propagated from stump sprouts.  Now that seeds are collected from cross-pollinated trees grown on the mainland, almost all Ironwoods are propagated using seeds. 

The common name most frequently used for this tree, “Ironwood”, refers to its wood, which is remarkably hard.  It has a fine grain and a distinctive red color with a yellow tint; consequently, it is highly prized by wood turners and sculptors for use in wood crafts and artistic objects. Its limited availability reduces its larger scale commercial use – and increases its price.

As noted above, Santa Cruz Island Ironwood’s additional common name, “Fern-leaf Ironwood”, refers to its delicate fern-like foliage. The other subspecies, “Catalina Ironwood”, which is endemic to Santa Catalina Island, bears simple, entire (non-segmented), smooth-edged leaves – this feature distinguishes it from its subspecies relative.                                                                                              

The Santa Cruz Island Ironwood’s botanical name is Lyonothamnus floribundus ssp. aspleniifolius.  The first part of the genus name, Lyonothamnus, honors William Scrugham Lyon (1851-1916), a Los Angeles area nurseryman who collected Ironwood specimens on Santa Catalina Island in 1884 and sent them to the famous American botanist Asa Gray who then named the species.  The second part of the genus name, thamnus, comes from the Greek word, “thamnos”, meaning “shrub”.  The specific epithet, “floribundus”, is Latin and means “freely flowering”.  The subspecies name, aspleniifolius, means “leaves like the Asplenium fern”.

Ironwood’s primary value is for use as an ornamental tree – it makes a lovely addition to any local private garden or larger landscape.  Being a tree native to our area, it is certainly well-adapted to our climate.  It makes an outstanding low-maintenance tree, if given the appropriate soil conditions.  It is perfect to include in a garden of California native plants designed for drought tolerance and wildlife habitat.  It really should be planted more extensively in our area.

Santa Cruz Island Ironwoods can be seen in several locations about town: at the Santa Barbara Botanic Garden (at the entrance and in front of the library); on the north side of Shoreline Drive (just below Shoreline Park); in the 500 block of W. Carrillo Street (north side); and, on Highway 101 (at the Patterson Avenue northbound offramp and on the west side of the Ward Memorial (Hwy 217) offramp).    

Tree-of-the-Month articles are sponsored by Santa Barbara Beautiful, whose many missions include the increase of public awareness and appreciation of Santa Barbara’s many outstanding trees and, in a long-time partnership with the City Parks & Recreation Department, the funding and planting of trees along the City’s streets.

Those who wish to honor a special someone can do so with an attractive commemorative marker that will be installed at the base of an existing street tree in the City of Santa Barbara.  Because Santa Barbara Beautiful has participated in the planting to date of over 13,000 street trees, there are plenty of trees from which to choose!  Application forms are available on the Santa Barbara Beautiful website, www.sbbeautiful.org.

Article and Photos by David Gress