Small trees, single to sometimes multistemmed, 5–10(–15) m tall. Bark red-brown and gray, peeling in strips. Leaves evergreen, leathery; petiole 12–35 mm, often winged; blade simple (subsp. floribundus) to lobed (subsp. aspleniifolius), entire to toothed; elliptic to lanceolate or oblong, 6–18 cm. Inflorescence a terminal, 50–800-flowered, compound corymb or panicle. Flowers white; hypanthium 3–4(–5) mm tomentose; sepals 5, 1.5–2 mm, persistent; petals 5, obovate to orbiculate; stamens 15+, 2 mm, filaments unequal. Ovary superior. Fruits are aggregated follicles, 2(–3), ± ovoid to lanceoloid, 3–4 mm. Seeds 2–4+, ± ellipsoid to fusiform. Flowering May to July. (Jepson Flora Project 2006; Strother 2014).
Distribution United States California, Channel Islands only
Habitat Canyons, ravines, ridges, and rocky slopes in chaparral, oak woodlands, openings; 20–500 m.
USDA Hardiness Zone 8
RHS Hardiness Rating H4
Conservation status Vulnerable (VU)
In his account for Flora of North America Phipps (2015) does not recognise the traditional division of this species into two subspecies with different leaf forms found on separate islands, citing studies that suggest a continuum, and this view is followed by Plants of the World Online (2020). For several practical purposes, however, this division makes sense and we retain it here. For example, conservation concerns have been raised over the potential for genetic contamination of wild populations through the planting of the wrong subspecies on the wrong island (Duff 1994), and a horticultural distinction is useful. In the wild the trees of both subspecies grow in groups or groves: polymorphic DNA analysis has established that plants growing in groves are largely genetically distinct clones, with just over 1000 genetically distinct individuals out of 32,000 trunks on Santa Cruz Island (Cooper et al. 1999)
L. floribundus subsp. floribundus only occurs on Santa Catalina Island and perhaps because its unlobed foliage is perceived to be less attractive than var. aspleniifolius it is also less widespread in cultivation, with few specimens recorded: Johnson et al. 2003 mention an example planted in 1983 at Chelsea Physic Garden, London. It would be interesting to know whether there are any differences in hardiness or cultivation requirements between the two subspecies that might contribute to this rather remarkable dichotomy
Santa Cruz Island Ironwood
L. asplenifolius Greene
Leaves 9–12 cm, palmately or pinnately divided, usually with 3–5(–7), pinnately lobed leaflets, margins ± revolute, entire, crenulate, or serrate. (Strother 2014)
RHS Hardiness Rating: H4
Valued for its columnar habit, attractive foliage, which is reminiscent of fern fronds or Cannabis leaves, and cinnamon-red young bark and shoots this has been described as ‘the most remarkable contribution that California has made to the world’s list of ornamental trees’ (Hall 1910); it is indeed a lovely thing, but Hall evidently overlooked the likes of Sequoia and Sequoiadendron. ‘On Santa Cruz Island, perhaps the most satisfying of all sights are large stands of Lyonothamnus. It grows high up the canyons and in those protected places is able to attain its full beauty. The foliage is fern-like and very lovely, and the small white flowers borne in broad terminal panicles, overspread the top branches, so that when a colony of the tree, cupped in the hollow of a hill, is seen from a vantage point above, the aspect is of a sea of creamy white upon green’ (L. Rowntree cited by Bean (1981).
Despite being introduced to cultivation in the USA in 1894, ten years after its discovery (San Marcos Growers 2020) it is only now becoming more widely grown. Of the two subspecies this is the more widespread with examples in public and private gardens in California and further along the American West Coast as far north as Seattle, where in 2020 Jacobson records a tree 13.8 m tall and 1.8 m in circumference (Jacobson 2016). Hogan considers this to be mostly suitable for mild Mediterranean climates, succumbing to summer heat in the interior US (Hogan 2008)
In the UK, a tree was raised from seed at Kew as early as 1900. The plant grew to about 6 m tall next to the Temperate House but was killed in the 1928/29 winter. Bean (1981) also mentions a tree at Borde Hill in Sussex that was cut in the winter of 1962–3 but later recovered. The current largest specimen at Kew was 9 m with a remarkable 112 cm girth in 2016, not far behind the British champion at 11.5 m in Belgravia in the same year (Tree Register 2020). It has been planted in several parts of London as a street tree, but needs careful maintenance to stay suitably upright. According to Owen Johnson (2015) the largest specimens he has seen were at Ventnor, Isle of Wight, and at Casa di Sole near Salcombe, Devon, but neither now features among the 16 noteworthy individuals listed by the Tree Register in 2020. These are mostly in the south of England and in Wales, but also the West of Scotland at Colonsay House in Argyll and Bute (9 m in 2012). It is possible that they are the largest specimens merely by the chance of having been planted earlier than the many now growing inland where, despite its provenance and perhaps due to climate change it now seems to do well, having survived temperatures down to –15°C in a private garden in Somerset (S. Senior, pers.comm. 2020) but also in Sussex (White 2020). Further examples can be found at Westonbirt Arboretum, the American Bank at Birmingham Botanical Gardens, and the Wynkcoombe Arboretum in West Sussex where a 7 m tree planted in 2002 thrives (Smith 2018).
In Europe there is a notable flowering specimen at the Jardin exotique & botanique de Roscoff on the North Western French Atlantic Coast. This was planted in 1995 and in the mild climate of the garden flowers every June (Le Jardin Exotique & Botanique de Roscoff 2019).
It can be planted either as a single specimen in small to medium-sized gardens and as a street tree or, when space allows, in groups that mimic the clonal groves that form in nature. It prefers well drained soils and little irrigation. The inflorescences are persistent from one season to the next, but become unsightly and are thus best removed if possible. The same goes for the old grey, strips of bark which hide the vividly red new bark underneath. Propagation is by means of cuttings in late summer or seeds sown in autumn (Hogan 2008).