Solidago canadensis L., Canadian Goldenrod

Account Summary

Growth form and history of introduction

This very tall, familiar rhizomatous perennial of N American origin was introduced to gardens in Britain in 1648 and probably arrived in Ireland shortly afterwards. It bears a large, terminal, pyramidal, 'paniculum' inflorescence on a central axis, with very many recurved-secund flowering side-branches bearing capitula or flowerheads. The shape of the S. canadensis inflorescence is distinctive amongst other Solidago species, the somewhat less familiar botanical descriptive term 'secund' being derived from the Latin 'secundus' for 'following', 'recurved-secund' therefore meaning, 'twisted back downwards and arranged on one side of the stem'. Sell & Murrell (2006) describe it rather differently as, "[capitula] in long, out-curving racemes at the ends of branches, the whole forming a broadly pyramidal inflorescence". (Note, there is no mention in the latter description of the secund nature of the inflorescence branches, nor of its central axis.) The inflorescences are striking and brightly yellow-coloured and over many years this, and their height, has made the species a useful, valued addition to wide herbaceous borders in larger gardens.

Preferred habitats

In its native territory, S. canadensis is a large weed characteristic of abandoned farmland, infrequently grazed pastures, waste ground and tall-grass prairies on a wide variety of preferably moist soils. It avoids both very wet and very dry soils and has a preference for substrates described as 'medium' with respect to fertility, texture and organic content (Werner et al. 1980). Canadian Goldenrod is not a serious weed of arable crops since it is readily controlled by tillage, but it definitely is a pest in forest nurseries and also in perennial gardens and crops (eg orchards and nurseries). In Canada, in ground where trees have been felled, soils otherwise disturbed, or farmland abandoned, sites subsequently undergoing secondary succession may often become dominated for considerable periods (up to several decades) by colonies of S. canadensis (Werner et al. 1980). None of this may appear very relevant in B & I at the moment, but the current author (RSF) suggests that it is important to know something of the invasive potential of the species. Canadian Goldenrod has been introduced and has already become labelled an invasive, noxious weed in a number of European countries prior to 1980.

In England, and to a much lesser extent in the remainder of B & I, S. canadensis is recorded as a naturalised garden escape or discard found on lowland waysides, uncultivated field margins, riverbanks, railway embankments, waste-ground and anywhere garden rubbish is discarded, including on refuse-tips, in abandoned quarries and on spoil heaps. It can cope with a wide variety of soils, but shows a preference for light, sandy or chalky conditions (G. Halliday & K.J. Walker, in: Stroh et al. 2023).

Variation and identification

The true identity of most N American Goldenrod garden escapes is always, or very often, in doubt, since the genus is extremely variable and there are over 100 species recognised within Solidago, with numerous species, hybrids and cultivars previously or presently grown in decorative gardens. For instance, the Royal Horticultural Society Index of garden plants lists around 30 named species of Solidago that grow at least 1.0 m tall and that are, or have been recently, in cultivation (Griffiths 1994). Despite a large amount of relevant scientific literature, the taxonomy of the genus Solidago remains tentative and a thorough systematic study is required to properly (acceptably) define circumscribed species (Werner et al. 1980).

S. canadensis was first recorded in Britain beyond the garden wall in Bristol in 1849, but it did not become frequent and widespread 'in the wild' until around the 1930s. As an example of the insecurity of identifications from the wild, D.H. Kent examined garden escapes in the Cumbria region of NW England and pronounced the majority as them as being S. gigantea Aiton (Early Goldenrod), rather than S. canadensis (Halliday 1997). To distinguish these two commonly met Goldenrod species, S. canadensis has leaves that are scabrid-pubescent (ie rough or bristly hairy) on both surfaces and the stems are hairy in at least their upper half, whilst S. gigantea has ± hairless surfaces on both the upper leaves and the stem and the latter has a waxy coating that may give it a bluish appearance (Werner et al. 1980; New Flora of the BI 1997).

Possible hybrids

The two Goldenrod species mentioned above occupy a very similar or near identical range of wayside habitats in B & I, both being very common and widespread, even to the extent that they regularly occur together and form large populations. As a result, a wide range of variation has developed in B & I, some of which does not match that found in N American populations where both species originate. It is therefore highly likely that hybridisation is taking place to some, as yet unknown, extent since this is a topic that has attracted little or no attention to date (Stace & Crawley 2015).

A sterile hybrid between S. canadensis and S. virgaurea is known to exist (S. × niederederi Khek), but it is very rare, only three records being known in Britain, where it was first found in 1979. It has not been recorded in Ireland, but has been found in Austria, Denmark and Sweden (Stace et al. 2015).

Flowering and vegetative reproduction

S. canadensis flowers late, from August to October in B & I. The flowers are self-incompatible and out-crossing is obligatory (Werner et al. 1980). The flowers have a strong odour and sufficient nectar to attract honeybees, bumblebees, beetles and the many other smaller insect species that carry out pollination. Plants readily set seed (achenes with an attached pappus), which are said to be transient in soil, surviving less than one year (Thompson et al. 1997). Seed production is high, however, varying from 1,100 to nearly 20,000 per inflorescence branch, depending upon geography and local ecology (Werner et al. 1980).

The species not only sets seed, it also vigorously reproduces by vegetative means using its short (usually only between 5 and 12 cm long), but persistent, rhizomes. In this way, plants form both tufted, many-stemmed individuals and also spread clonally, so that eventually they may produce dense stands. Old ring-like clones up to 10 m in diameter are not uncommon in Canada, being formed after about 25 years' growth when the central portion of the stand senesces and dies, the centre then becoming available to other colonising species.

After flowering, aerial stems die off in winter, the dormant plant surviving through its rhizomes, which send up vertical stems again in late spring (Werner et al. 1980).

Fermanagh occurrence

To date (and it has only recently been recorded at all), Canadian Goldenrod has been found among discarded garden material on just three occasions in Fermanagh, so is probably most accurately described as casual, rather than a fully established introduction. FNEI 3 similarly reported it as occasional, plants setting seed and persisting for a time in a few waste ground sites across the three NW Ulster VCs of Cos Down (H38), Antrim (H39) and Londonderry (H40).

The details of the first Fermanagh record are given above and the other two VC records are: roadside waste ground at Ballycassidy, 6 km N of Enniskillen, 1987-99, I. McNeill; and again at Knockninny Quarry, August 2004, RHN.

British and Irish occurrence

Oddly, S. canadensis is much more commonly reported in the north of Ireland than elsewhere on the island. Likewise, in Britain, it is also very much more commonly recorded in England than it is in either Wales or Scotland. In Scotland, northwards of the Glasgow-Edinburgh conurbation, it becomes increasingly confined to coastal areas (New Atlas; Stroh et al. 2023).

European and world occurrence

A great variety of S. canadensis forms are native and widespread throughout N America, from inside the Arctic Circle to California, Texas, New Mexico and Florida in the south and from E to W. Widely introduced to Europe as a decorative ornamental species, Canadian Goldenrod was first recorded beyond the garden in Austria in 1838 and in Germany in 1853, with similar dates for many other states in W, N & C Europe. Since these mid-19th century introductions, it has naturalised widely and is now designated a highly invasive weed in many parts of Europe including Switzerland, Belgium, Denmark and Estonia (https://www.nobanis.org/globalassets/speciesinfo/s/solidago-canadensis/solidago-canadensis.pdf.

accessed 17/02/2024 through https://www.nobanis.org/fact-sheets/)

This pattern of introduction and spread is also the case in Australia, New Zealand, Japan, China, Taiwan and Siberia (Weber 2000). S. canadensis has become most seriously invasive in China, where it is said to have replaced native species.

Threats

None at present, but the species is becoming more frequent and widespread and, containing as much genetic variation as it does, there is the possibility of hybridisation with related species and a real potential for invasive expansion.

References

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