Sequoiadendron giganteum (Lindl.) J.Buchholz

Like its close relative and near namesake Sequoia sempervirens, Sequoiadendron giganteum is one of the great wonders of the world. Both trees have been so extensively considered, investigated and written about, in the popular and scientific press alike, often with entire volumes dedicated to them, that there is a real risk of any new discussion simply reinventing the wheel. As such, for Trees and Shrubs Online we aim to provide an overview of their natural and horticultural histories, and at appropriate points throughout both articles direct readers to suggested further reading.

The principle reason for both trees’ celebrity is their sheer size, while great longevity adds to their allure. Sequoia, the Coast Redwood or to some simply the Redwood, currently holds the title of the world’s tallest tree and examples are known to be over 2200 years old, while Sequoiadendron, the Sierra or Giant Redwood or Giant Sequoia or Wellingtonia or Big or Mammoth Tree, is generally considered the world’s largest by girth or volume and stumps have been dated to over 3200 years (Debreczy & Rácz 2011; Earle 2024). Old growth stands of both species are an astonishing sight, and a visit to these can be a deeply moving experience whatever one’s interest – or disinterest – in the natural world.

Sequoia, growing near the California coast, must have been known to explorers and settlers for many hundreds of years, but Sequoiadendron, ensconced in the western foothills of the Sierra Nevada, would not come to prominence until the early 1850s. Ever since, countless travellers have attempted to put into words what it is to walk among these trees. Even allowing for how personal such an experience is, few have succeeded, and the early impressions of the famous naturalist John Muir have perhaps stood the test of time better than those of many who have followed in his footsteps. One senses from his writing that to step into these forests is to step into another world. As the 21st century advances new generations of visitors might qualify this sentiment; that to walk in these forests is to walk in a world that is disappearing, for the threat they are now facing is arguably far graver than any they have faced before.

In the late 19th century, when armies of loggers were putting the axe to entire forests of Sequoia and Sequoiadendron, this very tangible threat to their continued existence gave rise not only to America’s first National Parks but also the modern conservation movement (Earle 2024). Over a century later, the threats posed by climate change remain too remote or seemingly existential to far too many, especially in the most polluting nations, to generate the forcefulness of public opinion or political will that is needed to bring about sustained behavioural change, and so with each passing year we witness more extraordinary events, such as wrapping the bases of the largest Sequoiadendron in fire retardant blankets to protect them from encroaching forest fires, which are burning with a frequency, ferocity and on a scale that has no recorded precedent (Robbins 2023).

Fire has always been an important part of Sequoiadendron ecology. The interaction between the tree, its wider habitat and fire has been long studied, although much remains to be understood (Earle 2024). Certainly, fire has been shown to be an important factor in seedling recruitment (e.g. Mayer & Safford 2011) and most of the extant stands are thought to have developed in response to fire cycles which first create expansive clearings with large canopy gaps and almost no understorey; these fires leave behind them a bed of fertile ash for seed to germinate in (like most Cupressaceae Sequoiadendron is a light-demanding early successional species, although Sequoia is quite different; this will germinate under canopy, grow in the shade and even regenerate from old stumps – Barbour et al. 2001). Sequoiadendron colonised these clearings along with a host of associates including Incense-cedars (Calocedrus decurrens), pines and firs, which grew together until the next fire (a period of a few years to a few decades) would significantly reduce the proportion of thin-barked, fire-susceptible pines and firs, while Sequoiadendron, with their thick, fibrous, bark, were better able to withstand the fire and would survive and go on to dominate the grove, with the competition kept in check by occasional fires (Mayer & Safford 2011; Parsons & DeBenedetti 1979).

However, a well-intended but catastrophic policy of fire suppression throughout much of North America during the early 20th century meant that without prompt action small unplanned fires could quickly become infernos, laying waste to thousands of acres at a time as the accumulation of debris in the forest acted as tinder, significantly increasing a fire’s intensity. Recovery from such unnaturally large and intense fires takes much longer than recovery from smaller, faster-moving ones, besides which, in areas that remained unnaturally fire-free, the vegetation structure and age composition of the forest was affected in such a way as to increase vulnerability (Parsons & DeBenedetti 1979). In many areas, including reserves home to extant groves of Sequoiadendron, authorities now practice regular, carefully controlled burns to prevent the build-up of combustible debris on the forest floor and also to realise the wider ecosystem benefits that small fires can bring. An investigation following the Washburn fire, which affected parts of the Mariposa Grove in Yosemite National Park in July 2022, and which received extensive publicity due to the significant threat it posed to Mariposa’s Sequoiadendron, found that Yosemite’s fire management practices, including a policy of so-called prescribed burning dating back to the 1970s, was a significant factor in helping the authorities to bring the Washburn fire under control and minimise damage (Hankin et al. 2023). Such management regimes are set to become increasingly important, as there is evidence that the rapid increase in the frequency of severe fires is having a detrimental impact on Sequoiadendron’s resilience to fire, particularly in the largest and oldest trees (Shive et al. 2022).

Quite how many groves of Sequoiadendron exist in the western foothills of the Sierra Nevada depends on how one counts them; a list published by the Yosemite Association in 2000 gives 67 and this figure is generally accepted – it is used, for example, in the species’ IUCN conservation assessment (Thomas, Schmid & Farjon 2019). ‘Grove’ is an unusual means of describing wild populations of any tree, but it is widely used in the case of Sequoiadendron because the extant stands are very well defined and cover relatively small areas (Earle 2024). Most groves occur in the southern part of the distribution, in Sequoia & Kings Canyon National Parks and nearby in Fresno and Tulare counties (Debreczy & Rácz 2011). From the southernmost grove near California Hot Springs, north to the Converse and Indian Basin Groves near King’s Canyon, 59 groves are scattered over a north-south distance of c. 111 km, with barely 5 km separating one grove from another. These southern groves contain the very largest trees, including the famous ‘generals’ such as the General Sherman tree. Beyond King’s Canyon, another eight groves are widely scattered over another c. 273 km, in the vicinity of Yosemite valley, Calaveras, and one remarkable grove some 30 km west of Lake Tahoe; this, the northernmost grove, is more isolated than any other and contains only six mature trees (Earle 2024). This nearly-400 km long north-south distribution extends only 25 km east-west at its widest point, with the combined area of all the extant groves totalling just c. 14,200 ha (142 km²) (Thomas, Schmid & Farjon 2019). It is impossible to guess how many trees were lost to logging in the 19th century, but there is no evidence that the species ever occured beyond the restricted locations it is known to occur in even before logging began; whilst the number of mature individuals was severely depleted, this did not necessarily reduce the total area of occupancy (Thomas, Schmid & Farjon 2019; Earle 2024).

Discovery and introduction to horticulture

The native American peoples of the Sierra Nevada, known today as Miwok and Western Mono were largely unaffected by the Spanish and Mexican dominion of California. That changed after the arrival of Americans of (broader) European descent, however, and particularly during the gold rush when they were marginalised and persecuted, their lands taken and their cultures dismissed, so that little is known of their interactions with the Big Trees among which they passed their summers.

Former national park ranger-naturalist William C. Tweed laments this loss of knowledge that forces him to begin his otherwise authoritative King Sequoia – a particularly digestible account of Sequoiadendron’s modern history – in the mid 19th century rather than several hundred years earlier (Tweed 2016) and in which he notes that, despite the relative density of the groves south of the King’s River, these were not the first Giant Redwoods to be discovered by Euro-Americans. Rather, it was the somewhat isolated population at Calaveras, some 50 km south of the southern shore of Lake Tahoe.

Few interested in the botany or horticulture of Sequoiadendron will know the names Zenas Leonard or Augustus T. Dowd, but it was Leonard, a trapper-explorer, who was one of the first Euro-Americans to look upon the giant trees after the hunting party he was with – led by Joseph Walker – stumbled upon a grove in 1833. Although it is still debated exactly which grove the Walker party encountered the weight of evidence points to the north grove at Calaveras (Tweed 2016). Leonard published an account of his adventures in 1839, including mention of giant trees, but it received little attention and it was not until early 1852 or 53 when the hunter Dowd wandered into the Calaveras north grove (whilst hot on the heels of an injured grizzly bear), that things began to happen.

Dowd had a reputation as a storyteller; indeed, it took him some effort to convince his colleagues that he wasn’t spinning yet another yarn, but they would see the trees for themselves and the Big Trees of Calaveras soon began to attract wider attention, with stories about them appearing in the popular California press by the end of May 1853. Dowd dispatched specimens to the newly formed California Academy of Sciences in San Francisco where they were received by the botanist Albert Kellogg, himself recently arrived in California (Tweed 2016).

Kellogg was apparently a cautious man for he did not immediately name the new species, preferring to wait until he had consulted with fellow scientists. In the meantime he showed the specimens to other interested parties, including the English plant collector William Lobb who had been travelling in the Americas since 1840, collecting plants on behalf of the Veitch & Sons nursery firm in Britain. Sensing an opportunity, Lobb apparently dropped everything and travelled to Calaveras ‘via overnight steamboat and a thirty-six-hour stage ride’ (Tweed 2016) to collect seed and specimens for his employers. He returned promptly to San Francisco and thence embarked on a steamship for home, landing in Britain by mid-December 1853. On his return, Lobb’s employer sent some of his samples to botanist John Lindley who quickly described the material as a new genus. His new generic name Wellingtonia, commemorating the recently deceased British politician and military hero, was published in the Gardeners’ Chronicle on Christmas Eve (1853, p. 823).

Even by the standards of the time this was an arrogant move by Lindley and it infuriated Americans that the greatest tree in the world – their tree – should have been named after a Brit; Kellogg countered, eventually, by proposing the name Washingtonia after the great American hero and a ‘furious and highly nationalistic pathos’ ensued (Farjon 2017) but in the end both names would be rejected (Wellingtonia has however remained a common vernacular name in the United Kingdom). Indeed, the question of the generic name was only eventually settled in 1939 when the (thankfully) American botanist John T. Bucholz published Sequoiadendron. Lindley’s Wellingtonia had already been used for a member of the Sabiaceae, and by the time Kellogg got around to publishing anything there was already an unhelpful slew of alternatives in circulation, as botanists debated how best to classify the new discovery. Thankfully the rules of botanical nomenclature gradually began to bring some order and method to the chaos, and the name Sequoia gigantea became the accepted name for much of the period from the late 1850s to 1939, until the distinctiveness of Giant Redwood was finally proven to merit treatment in its own genus. At the time, many botanists balked at the publication of Sequoiadendron, but it gradually became widely accepted (Earle 2024); a small minority of botanists have continued to countenance its inclusion in Sequoia (e.g. Watson & Eckenwalder (1993)) but the legitimacy of Sequoiadendron has never been seriously challenged.

Meanwhile Lobb, together with his employer and probably Lindley too, could all have been forgiven for thinking that his was the first introduction of the new tree to Britain, but this was not the case. Among the many who travelled to join the California gold rush in the early 1850s was a Scottish surveyor, mining engineer and amateur botanist named John D. Matthew, son of Patrick Matthew, a wealthy Scottish merchant and landowner who lived in the Carse of Gowrie in Perthshire, between the cities of Perth and Dundee. Matthew set about procuring seed of this arboreal wonder for his father, who had a great interest in natural history, and made the journey to Calaveras at the beginning of July 1853, a month or so before Lobb, and immediately sent seed home which arrived in Perthshire a few weeks later, thus securing the honour of being the first person to introduce the new tree to Britain (Ravenscroft 1884, vol. 3). Patrick Matthew raised many young trees from his son’s consignment and planted them on his estate and gifted many more to friends and neighbours, while Lobb’s enormous haul would ensure Veitch & Sons a monopoly on its commercial supply through much of the remainder of the decade (Shepard 2003; Bean 1981).

Besides satisfying domestic demand, Veitch also dispatched material to more temperate parts of the British empire, notably Australia and New Zealand, as well as to many neighbouring European nations. European nurseries would soon secure their own, independent supplies, for example when M. Boursier de la Rivière sent seed to France from California in 1854 (Ravenscroft 1884, vol. 3) and Californian nurseries were soon in on the act, too; Sequoiadendron was being sold in the state certainly by 1858 and probably earlier (Jacobson 1996). Within a few years regular seed consignments ensured a supply to most horticulturally minded European nations and even in Britain Veitch’s monopoly would soon weaken. By 1884, Edward Ravenscroft was able to observe in his Pinetum Britannicum that the plant ‘is no longer rare or high priced’ (Ravenscroft 1884, vol. 3) and much as is the case with Monkey Puzzle (Araucaria araucana) in the UK and Ireland today, an extant tree’s status as a ‘Lobb original’ should not be taken on hearsay, although a far greater number of Lobb Sequoiadendron surely survive than Lobb Araucaria.

It has long been a source of frustration that the various trees raised and distributed by Patrick Matthew were not better documented. When Matthew received seed in August 1853 he divided it and sent shares to at least two friends, who raised their own seedlings and distributed the excess of their own efforts in turn. Ravenscroft (1884, vol. 3) was only able to track down eleven trees from this introduction with certainty, whereas Bean later considered that ‘From this sending most of the oldest trees in Scotland probably derive’ (Bean 1981). In reality, and in the absence of documentary evidence, it is impossible to say, at least of the oldest trees in Scotland, whether a tree might have come from Matthew or Lobb seed. Lobb’s much larger gathering was only four months behind, and by the following spring – when Veitch announced the availability of Lobb seedlings for summer dispatch – interest was at feverpitch, and the sheer size of Lobb’s introduction ensured supply was able to keep pace with ravenous demand (and caused this material to be credited persistently as ‘the first introduction’). The Scottish landowners lucky enough to have received a Matthew plant would have supplemented it with Lobb material at the earliest opportunity and after a few years such nuanced details as to ‘whence our trees’ would often be forgotten. Indeed, the contribution of the Matthew family to Britain’s Giant Redwood story went largely unacknowledged, and was only brought to the attention of the wider British horticultural community in an article in the Gardeners’ Chronicle of 15 December 1866. Detective work has occasionally turned up the odd original Matthew tree that Ravenscroft failed to identify, perhaps having been raised from one of the seed batches Matthew sent to trusted friends. A superb tree at Castle Leod, Strathpeffer, Scotland, was according to estate records received in 1853 and can therefore only have come from the Matthew introduction; it is now one of the finest examples in Britain and probably the largest tree at its latitude in the world (Rodger, Stokes & Ogilvie 2006).

It has been stated that Veitch continued to send out Lobb Sequoiadendron as late as 1860; this is perfectly plausible, but it brushes over the fact that subsequent seed introductions followed Lobb’s, albeit modestly at first, and later in greater quantity, from the 1850s right up to the present day. Particularly in the context of the early plantings of the 1850s and early 1860s this makes it virtually impossible, again in the absence of documentary evidence, to say whether or not a plant is from Lobb’s gathering. As other nurseries got in on the act Veitch’s market share reduced, but this was not necessarily a cause for concern, for Sequoiadendron had already produced golden cones for Veitch. In the middle of the 19th century newness was an irresistible lure for gardeners; coupled with Sequoiadendron’s status as probably the largest living thing on the planet and a wonder of the world, the tree became irresistible, the must-have item of 1854 and for several years after that: in Britain its association with the Duke of Wellington further boosted that popularity. When Veitch first offered young seedlings for sale in spring 1854 over 3000 are said to have been sold in a single day, with the largest fetching two guineas each (c. £200 in modern money) (Tweed 2016). Jacobson does not exaggerate when he says that Giant Redwood was ‘grown wherever possible’ (Jacobson 1996): the British upper middle class planted single specimens on the lawns of their suburban villas; aristocrats and wealthy industrialists peppered the grounds of their country estates with them; incurable show-offs planted avenues comprising many dozens of trees; while others were sequestered in all manner of locations, in vicarage gardens and churchyards, on hilltops and village greens, in quiet corners of woods and in the grounds of remote shooting lodges. One recent study identified just under 5000 ‘notable’ individual Sequoiadendron growing in the UK (which in this context can be taken to mean that the majority of these trees are mature) based on several data sources (Holland et al. 2024). It seems reasonable to assume that another several thousand will be unrecorded, but claims that up to half a million individuals grow in the UK should be interpreted cautiously. This figure seems to have arisen from a Forestry England estimate for the number of Sequoiadendron giganteum and Sequoia sempervirens growing in the UK, and would additionally appear to include planting figures from recent afforestation initiatives, not just mature individuals (forestryengland.uk).

Nevertheless, the clearly significant cultivated population of Sequoiadendron, especially in the UK and Ireland, has been cited an example of the potential value of garden plants to biodiversity conservation, but caution is advised. The thousands, perhaps tens of thousands of Sequoiadendron in cultivation clearly represent genotypes that do not exist in the wild, but it is important to remember that the first introductions all come from the same location, Calaveras. Recent research has indicated that the northern groves, which include this location, should be a conservation priority given their more fragmented distribution compared to groves south of the Kings Canyon (DeSilva & Dodd 2016) but it is important to note that within a few years of Calaveras’s discovery other groves would be found to the south, and later introductions would probably have sampled these, but there are remarkably few introductions other than the original Matthew and Lobb collections from Calaveras, and a startlingly small number of more recent collections resulting in a very small numbers of plants usually restricted to botanic gardens, that can claim known provenance. The near-total absence of provenance data from later collections means the conservation value of the global cultivated Sequoiadendron population must be viewed as severely limited, and would require considerable investigation to become useful. Clearly there is a case for a concerted effort to bolster ex-situ conservation of Sequoiadendron and to broaden the genetic base of the cultivated population of known-provenance material based on conservation priorities in the wild.

Cultivation

In the case of Giant Redwood at least, ubiquity did not breed contempt as it would for the much-maligned Monkey Puzzle (Araucaria araucana). While Veitch was merrily charging a small fortune for young Sequoiadendron, nobody knew how they were going to perform in cultivation. Among those many thousands of early plantings there were, relatively, very few failures indeed, and in most instances the trees took to foreign climates and soils with almost as much gusto as one would expect of them in the wild. Particularly in Britain and Ireland, ever the vanguard of new introductions to cultivation, they were found to grow excellently across the length and breadth of these islands in all but the very harshest of conditions; exposure rarely phased them, furthermore, they proved quite indifferent to soil type, faltering only on shallow chalk and proving slow on poor silty or sandy soils (Ravenscroft 1884, vol. 3; Mitchell 1996).

Once established Sequoiadendron grow rapidly into exceedingly elegant, perfectly symmetrical spires. Indeed, the performance of the earliest plantings probably helps to account for the sustained interest in them. Ten years after the species’ introduction it was still being planted in abundance: at Benmore in Argyll, Scotland, James Piers Patrick planted a new avenue of fifty Sequoiadendron to line the main drive to Benmore House, in 1863. It thrives to this day, containing some of the tallest examples in the UK and Ireland and forming one of the most impressive entrances to any botanic garden in the world – thanks largely to a recent conservation effort that saw over £100,000 spent on soil decompaction, reinvigoration and associated landscaping (Royal Botanic Garden Edinburgh 2022). (This extraordinary effort hints at two enemies of Sequoiadendron: compaction and poor drainage; in the wild they grow on moist but well drained, mostly alluvial soils which are very high in organic matter, containing millennia worth of organic debris). In 1864, King William I of Württemberg ordered seed from California. Legend has it he only wanted enough to plant a few specimens around the grounds of his Palaces, but due to a misunderstanding over 100,000 seeds duly arrived! Many thousands of plants were raised in the Wilhelma, Stuttgart, and later widely distributed throughout modern southwest Germany. Many of the surviving trees have become cultural icons in the region and are collectively known as the Wilhelma-Saat (Wilhelma seeds) (wilhelma-saat.de 2024).

In youth Sequoiadendron increases rapidly in height but especially in girth; after a while the vertical growth slows, while the increase in girth continues apace until that, too, slows. Finally, like most large Cupressaceae, trees reach an optimal height according to local conditions and then continue to fill out, slowly adding to their girth and occasionally developing additional, minor leaders in the crown. The famous British dendrologist Alan Mitchell notes that ‘[Sequoiadendron] took only about 80 years to be the biggest tree, in volume of timber, in every county of the British Isles’ (Mitchell 1996). Mitchell also quipped that across the south and east of England Sequoiadendron is ‘our national lightning conductor’. Indeed, the very tall, domed crowns present lightning with a large and convenient target, and lightning strikes have been common since the first generation of cultivated trees reached c. 30–40 m – which in the rolling hills of much of southern England puts them above mean canopy level. In areas with more dramatic topography, such as the valleys of the west country, Wales, northwest England and much of Scotland, this has always been less of an issue (Mitchell 1996). Sequoiadendron is also a remarkably wind-firm tree, with accounts of windblow virtually unheard of; it is more common for tops to be lost to lightning, or, in certain situations, for tops to be knocked out by falling sections of neighbouring trees, as happened to a tree growing near Dunkeld Cathedral, UK, following a particularly desctructive storm in 2011 (pers. obs.).

Sheltered conditions have consistently produced the tallest, if not necessarily the largest Sequoiadendron in cultivation. Examples in excess of 50 m are documented from the UK, Ireland, France, Austria, Germany, Italy, Spain and Slovakia; many of these tend to be rather slender, for example a tree near Dornbin, Austria, was 57.8 m tall with a girth of 7.21 m in 2022, while another near Bolzano in northern Italy was 52.1 m tall with a girth of 6.26 m in 2018 (monumentaltrees.com 2024). In Oregon, a tree that is probably the tallest Sequoiadendron outside its natural range was 64.6 m tall with a girth of 7.19 m in 2020 (Sillett et al. 2021). Growth rates such as these have helped to foster an interest in planting Sequoiadendron to offset personal carbon footprints, either in an ad-hoc domestic manner or else through organised, large scale planting, but one recent study has highlighted ‘the highly questionable nature’ of some commercialised approaches (Holland et al. 2024). Nevertheless, the same study has found that based on growth rates of trees in three different locations, ‘[UK] trees seem to have at least matched average growth rates, in terms of AGB [aboveground biomass] and carbon accumulation, of their counterparts of a similar age in the USA, at up to 150 kg AGB yr−1 (81 kg carbon yr−1)’ indicating that ‘particularly in open-grown well-managed conditions, they are likely to be a good choice purely from the perspective of carbon uptake’ (Holland et al. 2024).

The cultivated trees with the greatest girths are rarely very tall: a tree at Cluny House Gardens in Perthshire, UK, with a long history of measurement, was 11.4 m in girth with a height of 41 m in 2017 (Tree Register 2024). Several other UK trees have slightly larger girths, but most are either branched very low or else forked or planted on a slope. Nevertheless, there are larger-girthed trees in several countries: a tree in a private garden near Creuse, France, was >13 m in girth with a height of c. 25 m in 2013; a tree near Neuweiler in Germany (with an unusual, somewhat fluted bole) was 13.6 m in girth and 37 m tall in 2014 (monumentaltrees.com 2024).

Several cultivated trees combine girth and height: one in the grounds of the Palacio Real de la Granja in Segovia, Spain, was 46 m tall with a girth of 13.87 m in 2019; at Wanaka Station Park, New Zealand, a tree was 56.6 m tall with a girth of 11.9 m in 2023; at Frankton, Queenstown, New Zealand, a tree was 46 m tall with a girth of 14.99 m in 2018; and at Nahuel Huapi Lake in Argentina a tree was c. 50 m tall with a girth of >14 m in in 2020. All these statistics pale in comparison, of course, to the largest extant trees in the wild, where girths in excess of 25 m and heights in excess of 90 m have been recorded (monumentaltrees.com 2024).

Generally speaking, Sequoiadendron is largely untroubled by significant pest and disease issues as a wild plant, but it has shown significant susceptibility to fungal pathogens in cultivation which highlights the importance of implementing strict biosecurity protocols in the extant groves. In cultivation its susceptibility to honey fungus (Armillaria spp.) has been known for some time, but more recently a host of other organisms have been identfied as agents of concern. In 2021, the fungal pathogen Neofusicoccum parvum was identified as the most likely cause of cankers and sporadic and in extreme cases severe crown die back on established trees in Geneva, Switzerland (Haenzi et al. 2021). In the same year, a study examining the same symptoms in Croatia identified N. yunnanense and a related fungus Botryosphaeria dothidea, which had first been detected on Sequoiadendron in Bulgaria in 2016 (Kovač et al. 2021; Georgieva 2016). B. dothidea was also identified as the cause of dieback in mature Sequoiadendron at Arboretum Wespelaar, Belgium, in 2022–23 (J. Ossaer pers. comm. 2024) and it is likely established throughout Europe. Described as a weak pathogen, it tends only to affect its host through injuries and then only becomes a significant problem when trees are stressed, especially by drought. Symptoms include foliage branchlet discoloration followed by dieback – this can occur in large patches across the crown. These symptoms should not be confused with general drought stress which is relatively common in large, water-demanding members of Cupressaceae – e.g. Sequoia, Sequoiadendron, Thuja, Chamaecyparis, Cunninghamia etc. – especially following hot dry summers such as was experienced in northwestern Europe in 2022. As a consequence of this drought, many Cupressaceae exhibited browning, then death and shedding, of older foliage branchlets the following year (pers. obs.). Particularly in large, old trees this is most unsightly and always causes alarm and elicits many enquiries about tree health, but it is unlikely to suggest a fungal infection if the growth tips remain green and healthy, whereas these would also exhibit dieback if a fungal pathogen was at work. Furthermore, this response to drought stress tends to affect the entire tree, except perhaps the uppermost crown, whereas a fungal infection would usually be more patchy (pers. obs.).

Propagation

Propagation is generally by seed, which does not have a dormancy cycle and should therefore be sown as fresh as possible, on the surface of moist compost and lightly covered with grit or similar. It may be placed in a cold frame in areas prone to frost, and in warmer regions will benefit from a short period of cold scarification. Germination commences as soon as conditions are appropriate and should be relatively even, with young seedlings ready for pricking out after 3–4 weeks; seedlings are particularly vulnerable to grey mould (Botrytis cinerea) (Bean 1981) and maintaining good air movement and general hygiene is important. Growth can be very rapid and young plants in containers will soon become pot-bound unless the very greatest care is taken; cultivation in Air-Pot™ containers is recommended. Vegetative propagation is also possible; semi-ripe cuttings taken in mid-summer, treated with hormone and kept in very humid conditions will root readily, and many of the cultivars can be propagated this way. Grafting is also possible and is the preferred method for propagating cultivars by some growers. Historically, some cultivars, especially ‘Pendula’, have been grafted onto Sequoia sempervirens rootstocks (Bean 1981) and whilst this is clearly compatible, there is little contemporary advantage to this method when Giant Redwood seed is so readily available from merchants, and from cultivated trees which will produce an amount of viable seed in areas with reliably warm summers.

Cultivars

Within a few years of its introduction to European horticulture nurseries were selecting cultivars of Giant Redwood. A few of the oldest remain available, notably ‘Glaucum’ and ‘Pendulum’, and these are treated individually below. Many more selected from the mid-1850s through to the early 20th century are probably lost to cultivation, several of these are listed below for completeness but we acknowledge their unresolved and likely unresolvable status. The majority of cultivars treated here seem to have been selected in Europe, but American selections are gradually gaining ground.

When Giant Redwood was first described, as Wellingtonia gigantea, the Latin name was feminine. It remained so when it was reclassified as Sequoia gigantea, a name that was in use up until the end of the 1930s. As such, many of the oldest cultivar names were originally written in the feminine, and are often still written as such on labels, in catalogues and other literature. However, when the accepted name became Sequoiadendron giganteum in 1939, the gender became neuter, thus cultivars should now end in –um or –e, e.g. ‘Glauca’ becomes ‘Glaucum’ and ‘Pendula’ becomes ‘Pendulum’, and so forth. The cultivar notes have been drawn from several sources but most especially the indispensible RHS Encyclopaedia of Conifers (Auders & Spicer 2012).