Identify a Weed - Identifying Weeds

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Tree of Heaven - Weed KillerTree of Heaven

Ailanthus altissima; sometimes described as Ailanthus glandulosa*.

Closely related species
Ailanthus altissima var. Sutchuenensis, and Ailanthus giraldii.

* Mainly by practitioners of homeopathic medicine

Introductory History
With something of the romance of the introduction to England of Japanese Knotweed, seeds of Ailanthus Altissima were sent from their native China, along the caravan routes to St Petersburg and thence to London and Paris, by a Jesuit missionary. He was Pere Nicolas d'Incarville (1706-1757) who spent most of his time between 1740 and 1756 in Pekin. In 1751, a letter thence from d'Incarville was read out to the Royal Society. "We are much confined," he writes, "and have not even the liberty of going where we please by ourselves to see things." One can imagine him, at very much this time, virtually smuggling letters and seeds out. The Royal Society gave the seeds to Philip Miller, (1691-1777) who was curator of the Chelsea Physic Garden. The specimens grown at Chelsea were thought erroneously to be the Chinese Laquer tree, Rhus vemiciflua, though whether the original error was on the part of d'Incarville or Miller is not known. Seeds were also sent to and cultivated by Philip Webb, at his estate at Busbridge near Haslemere. Webb was a treasury solicitor, and though perhaps lacking an untarnished reputation, was a dedicated cultivator of trees.

Introduction to the USA was rather later. It was first brought to Philadelphia from England in 1784 by an American gardener called William Hamilton, and subsequently widely distributed by the nursery trade in the eastern states. By 1840 it was almost universally listed. A second wave of introductions came, with Chinese immigrant miners, to Californian in the goldrush of the 1890's. The descendants of these seeds, planted because of their Chinese cultural significance and medicinal value, now form great stands on abandoned mine-workings.

It is only at the beginning of its invasive career in the UK, though naturalisation records go back to the 1930's. Climates of warmer temperate type are more favourable to seedling survival and establishment. Colonisation here is, however, noticeably increasing. Allanthus has become an aggressive invasive weed in the USA, where it is listed as invasive in 44 states, plus Puerto Rico and Washington D.C. Its invasive area also spreads to southern Canada and way south to Argentina. Problems are also being experienced in areas of Australia, New Zealand and the African continent. In Europe the affected areas are from Bulgaria in the east to Portugal in the west, and north to parts of Germany.

In China, Ailanthus is still used for ornamental purposes, also the leaves and bark are a component of traditional Chinese medicine. It is used to treat a variety of conditions including bowel ailments, asthma, epilepsy and heart arrythmia. Large doses of bark-tincture are, however, highly toxic. Though the wood is difficult to season and prone to splitting, it has some uses in furniture manufacture. A silkworm, Samia cynthia, which produces inferior silk to the Mulberry silkworm (Bombyx mori) is fed on cultivated Ailanthus leaves.

In the USA the undesirable qualities of the tree are restricting its use. Its pollution tolerance and speed of growth encouraged its use in urban plantings on a massive scale. The same qualities made it ideal for reclamation of worked out strip-mines. In the UK and Europe it has been likewise used in urban situations as well as large gardens and arboreta.

*The investigation of Allelopathic phytotoxins is a most exciting area of study, likely to furnish novel herbicides of great importance to weed managers of the future.

The Tree of Heaven is a fast growing deciduous, dioecious tree with a rounded crown. It reaches heights of up to 30 metres, exceptionally to 35 metres. Trunks measure up to 1 metre in diameter, exceptionally 1.5 metres. Young branchlets are minutely pubescent, and young shoots are reddish-bronze. Leaves emerge in late spring, and appear initially bright red in colour, pinnate and growing to 45 to 100cm in length. Leaflets reach 15cm; from 11 to 41 in number, are lanceolate/ovate and truncate at the base. They are also finely ciliate, bearing from 2 to 4 coarse teeth near the base, and are often furnished with conspicuous glands below. Flowers measure 7 to 8 mm across, with 5 to 6 petals and the same number of carpels. Its stigmas appear elongated, with 10 stamens. The fruits are 4 x 1.3cm, brownish red and oblong. The seed measures 5mm across, and is encapsulated in a twisted samara.

The generic name for the "Tree of Heaven" derives from the Indonesian vernacular name "Aiianto", which means "reaching for the sky". It is a fast-growing, deciduous tree which can express great drama and beauty. In the most favourable conditions, 2.5 metres of growth can occur in the first season of life, and heights of 15 metres or so can be achieved in 15 years. Mature specimens can be 30 metres in height, and such individuals are very striking, with the deeply fissured pewter-grey bark on the trunk and mature branches and the massive compound leaves. On vigorous shoots, the individual leaves can be up to 1 metre long. Generally maturing early and living to perhaps 50 years only, lifespans can be considerably longer, and specimens 150 years of age have been documented in the USA. (Trees evincing the aura of venerability in the same manner as the 200 year old Plane trees in parts of London have been seen, but which were only from 40 to 60 years old.) The oldest recorded UK specimen is likely to be a massive individual at Chichester, which is about 180 years of age.

Individual trees are either male or female and in late July, bear terminal panicles of flowers up to 30cm long which, in the case of female trees, give rise in August and September to large bunches of pink flushed fruits of the type known as known as Samaras. These fruits persist on the tree throughout the winter. Flowering and fruiting times given are for the UK, but substantial variations occur throughout the world range.

All parts of the tree are strongly and somewhat unpleasantly scented, with the male flowers bearing a highly offensive odour, which serves to attract pollinating insects. There are reports of the pollen being markedly allergenic. A curious characteristic is that honey derived from Ailanthus flowers initially takes on the offensive taint of the flowers themselves, but later matures to assume a desirable muscat flavour.

Reproduction is by seed, which is prolific as already stated, and by suckers arising up to 15 mteres from the main stem.

Cutting of the main stem, or indeed its death through other causes gives rise to vigorous re-sprouting from the base, as well as from the wide-spread shallow roots. Coppice spring from a stool can be as much as 4 metres in a season, and shoots can flower and have potential for seed production in as little as two growing seasons. In common with some other invasive species, precocious flowering is fairly common in Ailanthus, with seedlings producing flowers in their first growing season, even in plants only six weeks old! It is thought that seed is shed dormant, though not possessing a prolonged or complex dormancy requirment. Seedlings develop a tap root within three months, though the root system subsequently develops laterally, with root bases near the trunk becoming swollen to function as water-storage organs.

Amongst the characteristics which predispose Ailanthus to become invasive are several which encouraged its popularity as an ornamental and urban tree. It is fast-growing, extremely frost hardy, except in early youth, and tolerant of heat. It can adapt to arid conditions or areas of high rainfall. Typically, in the American ranges where naturalisation has occured, rainfall averages vary from 36cm to 229cm per year, and temperatures from -9°C to 36°C are regularly experienced. Despite the fact that young growth is damaged by frost, 6 year old trees have been reported to have recovered from exposure down to -33°C.

More characteristics of Ailanthus have parallels in other invasive species. It produces large quantities of viable wind-distributed seed,* and it will regenerate if cut down, suckering with great freedom and vigour from its wide-spreading root system.

* Up to 325,000 per year for a mature tree.

It is a pioneer plant, rapidly colonising disturbed areas and greatly inhibiting other plant species by producing allelopathic chemicals. Experiments in the early 1960's showed that an aqeuous extract of Ailanthus leaves was toxic to 35 angiosperms and 10 gymnosperms. Subsequent experiments on woody species showed toxic effects to 35 hardwood species, and 34 coniferous species. Competitor plants show greatest susceptibility to these allelopathic effects during early exposure.

The worldwide trend for Ailanthus shows a steadily increasing population. Where it causes problems, it has much in common with other invasive species. In towns and cities it can colonise derelict areas in the same way as Japanese Knotweed. Seed can also lodge between concrete slabs or at the junction of walls and paving. The roots are aggressive and extensive enough to damage paved surfaces, foundations and sewers. In South Virginia it has spread along one third of the highways, obliterating much native flora and impeding road visibility. It also spreads along railways to the same effect. These two instances again being similar in manner to Knotweed. Unlike the Knotweeds, Ailanthus is also an agricultural pest, with seedlings germinating in thousands in recently planted fields, and suckers migrating into similar areas from woodland margins. Though far less commonly established in areas where there has been no human disturbance, it is still able to establish in such areas because of its prolific seeding, with consequent loss of native species.

In forestry plantation situations, these allelopathic effects plus reasonable shade tolerance allow invasiveness to establish.

Control Methods
Planting of Ailanthus should at most be restricted to large garden areas, where the muckering potential cannot cause damage to built structures, and where the brittle nature of the wood produces minimal risks through boughs breaking. Selecting female only trees and propagating these by root cuttings, together with elimination of males, should reduce invasion by seed spread. The offensive smell of the male flowers and the allergernic pollen would also be avoided. Where trees have to be felled, or the main trunk dies, measures should be taken to kill the root system, so that regeneration through energetic suckering or stump sprouting can be prevented.

Little research has been conducted on suitable biological controls for Ailanthus, mainly because the pathogens and insect pests associated with it rarely cause significant damage. In China, 13 fungal species are found on it, with 9 of these not occurring on other genera. 32 species of insect or arthropod have also been identified, with 3 offering the possibility of being developed as control agents.

These are, a bug; Orthopagus lunulifer, and 2 weevils; Eucryptorrhynchus brandti and Eucryptorrhynchus Chinensis.

In the USA, three insects, including the introduced silk moth, Samia cynthia, are known foliar pests. The others are a beetle, Maladera castanea, and a moth larva Atteva punctella. It is unlikely that these have potential for development as biological control agents.

Few fungal or bacterial pathogens of any consequence attack the species in the USA, though two organisms; Verticillium Atro-album and Fusarium oxysporum have been isolated from dead or dying trees in Virginia, and are undergoing evaluation for development as mycoherbicides. Both these last two are wilt fungi, and have widespread occurrence in the UK. The Verticillium causes often fatal wilts to a large range of plants, and the Fusarium is found as a number of host-specific strains. Both these organisms survive in the soil for long periods: the Verticillium as persistent mycelium and the Fusarium as specialised spores known as chlamydospores. From time to time the Verticillium is known to kill large numbers of Ailanthus trees.

In the UK, fungal rots, which are probably saprophytic in nature, are associated with the decline of old trees. Correct identification of these as they occur, and accurate interpretation of their modes of action might be of long term interest.

Other control methods, both mechanical and chemical have been developed in the USA, often with reference to natural sites and forestry situations where Ailanthus is becoming dominant. All of these have some relevance to the UK situation, and to what may develop in future.

Where many seedlings are present in the prescence of desirable native species, timely hand pulling is highly effective. The stage of growth is important. The seedling must have become sufficiently woody not to be broken off, and the tap root must not have grown to the extent that it cannot not all be removed at a pull.

Where a natural area is infested, clearance by pulling or digging should be done in a specific order. Weed first the areas of least infestation, allowing the native plant species the chance to recover where their population is highest. Weed then the areas where there is no more than one weed to two desirable species. This will allow the most bio-diverse areas to return to their optimum, followed by the next best. The last stage of the operation is to clear of weeds the most degraded areas, which will be recolonised from their now clean and healthy neighbours. This method of hand clearance is known as the Bradley method.

Many methods, employing axes, brush cutters, chain saws etc. can make initial clearance of infestations and may serve to prevent seeding. Brush must be sensitively treated, in case it sheds maturing seeds or roots at the base of a pile. Burning may be necessary. Girdling of standing trees has been employed, and it appears that leaving the standing tree has some consequence in suppressing regrowth from below the level of the girdle. All mechanical methods used alone, other than complete removal, will result in resprouting. For this reason, herbicide applications appropriate to conditions, are the options most likely to succeed.

The chemicals most effective are Triclopyr, Picloram and Glyphosate.

Foliar sprays of 2% Glyphosate or Triclopyr are of use in controlling populations of seedlings. Spraying will also defoliate root suckers or shoots from cut stumps, but total kill of the root system may not be achieved.

Where trees are being felled and removed, cut stump applications of Triclopyr or Glyphosate, should immediately follow felling. Herbicide concentrations employed are reported as being effective between 20 and 100%, according to experimental results or product formulation. The area treated should encompass the whole cambial area and at least 20% of the total surface. This technique usually prevents stump sprouting, though insufficient translocation may occur to kill the whole root system and prevent further suckering away from the main stem.

Frill-girdling, before applying concentrated herbicides, is not recommended, as the continuous cut encourages stump sprouting and root sucker production. A related technique, known as "hack and squirt" involves making a series of downward axe cuts around the trunk at about one cut per inch of trunk diameter. As the cut is made, concentrations of herbicide of from 50 to 100% concentration are introduced wth a lance. With this method, the girdling is not continuous, and shoot production is not stimulated.

Basal bark spraying with up to 25% concentrations of herbicide in an oil based or synthetic penetrant have been well researched in the USA. This method can be particularly effective in specimens of up to 15cm diameter. The solution is applied to the bottom 30-45cm of each stem, ensuring thorough wetting, but avoiding run-off. Of herbicides listed for use in the UK, Triclopyr formulations at 20% concentration in oil, were found to be satisfactory. Almost 100% efficacy was obtained using a mixture of the Triclopyr formulation at 20%, with Picloram at 5% added. In the States as over here, Picloram is not formulated for oil based mixes, so various companies were employed to pre-mix the formulation in a non oil penetrant, and supplying in a ready to use form. back to top

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