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Goldenrod - Weed KillerGoldenrod

Solidago ssp


Common names
Goldenrod, Jimmy Weed, Canadian Goldenrod, Giant Goldenrod

History and introduction
The name "goldenrod" has been given to a number of flowering plants in different regions of the english-speaking world at different times. It might, therefore, be helpful to write a little on plants bearing the name, but which are not the main subject of this text.

The native Goldenrod, Solidago virgaurea, though capable of being over vigorous in a garden situation, is not an invasive plant. It is of smaller size than the introduced
species, with a less dense and not one-sided flowerhead. It is found in hedge banks, rocky places and open woodlands in the north and west of the UK, but is extremely scarce elsewhere. It had, and retains, many uses in herbal medicine, including the traditional one of healing wounds. Such was its importance in Tudor London, that the dried herb cost 12 1/2p per 25g! It might be deduced that this truly enormous price reflected truly enormous need at the time, allowing us to take comfort from the fact that the "knife culture" is obviously not new!

The other Goldenrod to get out of the way, is a shrubby, yellow-flowered perennial known as Goldenrod, Jimmy weed or Rayless goldenrod, which grows in the cattle ranges of southern Colorado into Texas, New Mexico and Arizona. Two scientific synonyms are acknowledged; Halopappus heterophyllus and Isocoma wrightii. This plant is a cumulative poison of cattle, in a worse way than Ragwort. The toxin it contains, called tremetone, produces appalling symptoms, coma and death.

The invasive Goldenrods are the Canadian Goldenrod, Solidago canadensis; its subspecies, S. canadensis var. scabra, known as tall Goldenrod; and S. giganteum, unsurprisingly called Giant Goldenrod. Also of potential concern is Solidago nemoralis, the Grey Goldenrod, which is so similar in its natural history to the others that its behaviour in the wild needs to be carefully monitored.

Canadian Goldenrod was first introduced into the UK in 1648, initially as a medicinal herb, and became widely planted for its stature and abundant late-summer blooms by nineteenth century gardeners. When they realised that it was a merciless thug in the herbaceous border, it was relegated to the wild garden or the midden in much the same way as Japanese Knotweed. Having thus been sprung from gaol, it has colonised the sorts of disturbed and waste ground that are favoured by invasive plants, such as, indeed, the Knotweeds, Buddleia etc.; though it has not become a major problem or much invaded and degraded wild areas in the UK.

Where goldenrod does become invasive, as it has in many parts of Europe and southern China, its huge production of wind-dispersed seed (perhaps 20,000 annually for each flowering stem) and subsequent rhizomatous vigour can result in the development of mono-specific stands, devoid of all native plant competition.

Matters and questions at the heart of the study of invasiveness are well illustrated in the behaviour of Goldenrod, and are worth discussion. It is widely observed that plants in the invaded ranges are taller and more robust than populations in their native areas, and bear stronger and bigger rhizome systems.
The usual explanation given for this phenomenon for most invasive plants is that they take advantage of more favourable cultural conditions and the absence of co-evolved pests and diseases. Another explanation, is known as the *EICA (Evolution of Increased Competitive Ability) hypothesis. This further postulates that through succeeding generations, without pressure from herbivores or pathogens, natural selection favours genotypes with the highest competitive ability, rather than those whose vigour is curtailed by the energy expenditure in evolving strategies for defence. This hypothesis can also be employed to explain the lag phenomenon often observed, whereby an invasive species remains comparatively contained until rapidly beginning to invade wild areas. In European invasive populations of S. giganteum, the time lag between escape and invasion of natural vegetation areas has been observed at roughly 100 years. It has been proposed that this is the time taken to develop varieties of maximum competitive ability.

If this theory is true for Goldenrod, with its lavish seed production, genetic plasticity and variability, it cannot be true for Japanese Knotweed, which in the UK is largely a single clone. Without selection through sexual reproduction, both the increased vigour and the time lag between wild introduction and serious invasion are yet both true. Perhaps part of the answer, at least for time-lag, lies in the mathematics of population doubling and the extra opportunities, with each doubling, to speed up the rate of the doubling itself, allied to the vigilance, distribution and density of the human population, which account for observations of lag phenomena.

Goldenrod is also true to invasive type in secreting ** allelochemicals into the soil. Chinese scientists have shown, under strict experimental conditions, that these substantially inhibit germination and root growth of all wild plant species examined.

The presence of certain mycorrhizal fungi which assist goldenrod colonization, have also been observed.
In the USA, Goldenrod has gained an unjustified reputation for producing severe allergic reactions such as hay-fever and asthma. Goldenrod is, however, insect pollinated, and its pollen is heavy and sticky, and not liable to wind dispersal. Frequently growing in the same habitats and flowering at the same time as Goldenrod are the far less conspicuously flowered Ragweeds (Ambrosia spp.) which produce huge amounts of some of the most highly allergenic wind-blown pollen in the world. The possibility of noxious Ragweeds becoming widespread in this country, needs (in my opinion) to be taken seriously. They are alien, over here and increasing. Their vigour, competitive ability and pollen production will all be greatly enhanced by higher temperatures and rising atmospheric carbon dioxide.

One further fact about Goldenrod needs mentioning. Thomas Alva Edison, (1847-1931) he of the light bulb, the phonograph and the cine camera, as well as over 1000 other patents, was looking for a possible source of home produced rubber, should imports fail in times of war. Among plants tested was Goldenrod, which contains 5% of the polyterpene latex.

*Blossey and Notzold, 1995. ** Chemicals, produced by plants which suppress or disadvantage other plant commensals in the ecosystem.

He instituted a breeding and cultivation programme which resulted in the production of plants 4 metres tall and containing 12% latex. Good rubber indeed was made, some of it surviving unperished to this day. Henry Ford showed initial interest in the project, and the "Model T" he gave to Edison, had tyres of goldenrod rubber! The coming of artificial elastomers, which could be more cheaply produced than the goldenrod derived product, led to the project being abandoned shortly before Edison's death.

Botanical Description and Natural History

Species: The main invasive members of the genus are; Solidago canadensis; Solidago canadensis var. scabra (syn. Solidago altissima) and Solidago gigantea.

Family: Asteraceae.

Description: Three species of variable, late-flowering, rhizomatous perennial plants, differing in stature and degree of hairiness or otherwise, of stems, leaves or other plant organs. Moisture availability, soil fertility and other local conditions influence the size and appearance of the individual plants, whichever the species, and populations vary because of the plasticity and variability of their genetic makeup. The majority of individuals of S. canadensis do not exceed 1.5m in height, whilst var. scabrais often up to 2m, and S. gigantea may achieve as much as 2.5m. In general, the var. scabra is the most uniformly hairy of the three, whilst S. gigantea is predominantly smooth. Stems, l-2.5m in height, pubescent to scabrid throughout in S. var. scabra, in the upper half of S. canadensis; and glabrous, often glaucous in S. gigantea. Leaves, largest at mid-stem height and falling early below this, to 13cm x 2cm, sharply serrate, of lanceolate shape with prominent lateral veins, and with attenuate bases. Upper surfaces glabrous; lower, more or less pubescent or scabrid. Inflorescence, yellow, composed of capitula in a markedly secund pyramidal panicle; subtended by an involucre of generally obtuse phyllaries. Ray florets, female, 10-17, to 2mm, disc florets, hermaphrodite, 3mm. Fruit, a shortly pubescent cypsela.

The goldenrods are a genus of perennial herbaceous plants, largely originating from North America, and comprising about 100 species. They prefer open sunny habitats, but, like many plants with invasive potential, are otherwise tolerant of a wide range of soil, moisture and climatic regimes.

Propagation is by copious production of wind-distributed seeds, or extension of the rhizomatous rootstock. Maximum rhizome growth is during the autumn months, and both the rhizome biomass and size of plants is greater in the invaded than the home ranges. Vegetative spread, through rhizome growth, produces areas of apparently individual plants arising unbranched from the ground. Each stem is, in fact, a clonal part of a single plant. Stems of this kind occur in many plant species, and are known as ramets.

The yellow inflorescences are produced in late summer, and the flowers are pollinated by insects. Nectar flow is substantial, given adequate rainfall, and the flowers are highly attractive to bees.
The North American Goldenrods are host to a number of beetle and moth species, though none of these occur in the invaded ranges. It is likewise not thought that pests of our native Goldenrod, Solidago virgaurea are able to feed on the introduced species.

Control Methods
Mechanical control by digging or pulling is only feasible where plants are isolated or young. Mowing of established invasions has little effect on plant survival, though it does not provide opportunities for increased germination or seedling establishment in preference to leaving the sward unmown. Mowing conducted in late summer does help exhaust any seed-bank and curtail seed production, so seed colonization of new areas can be much reduced.

Biological control agents to date, show little promise. In the native ranges, a high proportion of plants show damage from stem-galling or herbivorous insects; mainly of the species Eurosta solidaginis, Rhopalomyia solidaginis, Philaenus spumariusand Uroleucon caligatum. Even where damage is seen to be severe, little reduction in the extent of the invasion is observable. It is possible, however that insect attack may slow the rate of population growth, and allow chances for one or two native species to hold on in or re-invade mono-specific areas. It is worth pointing out that if the EICA hypothesis is valid, non-native populations will be more susceptible to pests and pathogens than those found in native ranges, so there remains a possibility of biological control agents being developed in the future.

Chemical controls are regularly employed to control Goldenrod, and may be applied as a foliar spray when the plants are in active growth, or applied to individual ramets by glove, rope-wick or similar applicator. Trials have been conducted to assess whether pre-cutting of ramets increases the efficacy of herbicide application, but these tests showed no conclusive advantage to be gained from pre-cutting. Chemicals found to be effective against Goldenrod include Triclopyr, Glyphosate and 2.4D. back to top

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