Willows going wild Down Under

WILLOWS are native to the Northern Hemisphere so why is temperate Australia so awash with Salix species causing damage to the physical environment? Three things mark out Salix from the rest: so many different species – around 900 at last count; unrivalled readiness to hybridise; and super-fast growth rates frequently enhanced by hybrid vigour.

Unlike other alien invasive plants in Australia, willows are generally not garden escapes but the consequence of purposeful planting. Willows were taken to Australia for a range of reasons including protection of riverbanks, shade and shelter.

With regard to riverbank protection, invasive willow has had the opposite effect through obstruction and diversion of the country’s creeks to cause flooding. There is no disputing shade levels furnished by willows, but these are sufficiently high to ensure virtual destruction of native flora and flora within and alongside the country’s creeks. Moreover, there is a marked and permanent reduction in water quality to the detriment of native, freshwater aquatic animals.

Stretches of Cox’s Creek dry up in summer, but the willow persists until rain returns in autumn.

Some willows causing environmental problems are native to the British Isles and in all probability were taken to Australia from Britain and Ireland, like Salix cinerea (grey sallow). Others are native to North America and Asia. For instance, Salix nigra (black willow), one of the more aggressive alien willows now in Australia, is naturally distributed across the eastern half of North America, from Ontario, Canada in the north to northern Florida in the south. With this ability to thrive across such a broad range of North American climate it’s no wonder black willow took Australia by storm.

Others may not be widely recognised outside Australia because they appear to be natural Australian hybrids. A classic example is the highly aggressive hybrid of two introduced British native willows – Salix alba (white willow) and Salix fragilis (crack willow) – and which is the curse of creeks in states like South Australia. Commonly called gold-crack willow, Salix x rubens is widely naturalised in south-eastern Australia as a damaging environmental weed and one of a group of willows known as Weeds of National Significance (WoNS).

However, some Australians like to see willow along their creeks, although, given the size of some trees, it’s clear that even the most senior in the population have probably not known it any other way. I remember one South Australian in the Adelaide Hills saying how he loved the willows along the creeks, especially in autumn (March/April), when the foliage goes a uniformly deep golden yellow, hence the gold-crack willow common name for Salix x rubens. However, he is in an increasing minority following a concerted effort to clear out willows and other alien invasive plants from along temperate Australia’s watercourses.

The way in which willow has taken to temperate Australia and taken over the landscape has to be seen to be believed. The example hitting visitors head-on is Salix x rubens.

Branches of Salix x rubens break off, root, and establish mid-stream in Cox’s Creek despite the fast flow seen here in early spring.

A HERCULEAN HYBRID WILLOW

The Salix x rubens hybrid you see in Australia regenerates by vegetative means but also by seed, something not always achieved by pure species of willow, let alone hybrids. Salix x rubens in Australia appears to be a separate, self-contained and stable plant. Indications are the hybridisation occurred on Australian soil. Its vigour suggests that if it did arrive in Australia as a hybrid from Europe then its genetics consolidated further in Australia to produce this contemporary, highly invasive plant. And, if not, why is Salix x rubens or something similar not rampaging across wetland areas of countries like the United Kingdom where the Salix alba and Salix fragilis parents grow side by side?

To muddy the waters even more, hybridisation of this hybrid with yet another introduced British native willow (Salix viminalis – common osier) is mooted as the reason for the seed-forming and -bearing capacities of Salix x rubens, which are unusually high for a hybrid. Sorting out pure Salix species from each other is difficult, but with hybrids it becomes a nightmare.

Be that as it may, Salix x rubens is widely naturalised in south-eastern Australia. This willow is invasive along waterways, in wetlands, around dams and in riparian areas. Such extraordinary colonising capabilities are seen in the Adelaide Hills some 20 km south-east of Adelaide rising to over 400 m.

One clear case is a stretch of Cox’s Creek that runs through Bridgewater in the Adelaide Hills, alongside the historic Bridgewater Inn (1842), built shortly after the first European colonists arrived, and a flour mill (Bridgewater Mill – 1860) powered by the creek. It is highly ironic that this particular stretch of water around which the original settlement was built is now devoid of native Australian plants, due in the first instance to willow invasion but also other shrub and ground-cover aliens encouraged thereafter.

Vegetative regeneration, largely inherited from the Salix fragilis parent, is mainly responsible for colonisation. Like British native crack willow, also called brittle willow, branches and twigs snap off easily to root and establish mid-stream even in a fast flow. Willow trees many decades old are packed in along the waterway, with alien shrubs like bramble (blackberry), common (British) broom and Montpelier broom (a Mediterranean native) coming in behind.

Willow establishes mid-stream to obstruct and divert water flow to cause flooding. Flooded areas are colonised by thick ground-cover plants, all alien and including bulbous buttercup and docks, to give a totally alien landscape and ecosystem to the Adelaide Hills in South Australia. By late summer (March) the creek has dried up, leaving the still green willow shrubs stranded in the creek bed, accessing water from beneath the dried-up silt and waiting for the rains to resume in April and May.

Only 10 per cent of susceptible land has been affected so far, so there is scope to remove the invader, and that’s exactly what conservation groups are doing. Cox’s Creek, both upstream and downstream from Bridgewater, has been cleared of willow and replanted with native trees and shrubs. Downstream the creek flows fast and free with native trees and shrubs on either side. Upstream, where the creek is deeper and slower, the removal of willow and replacement with original native plants has had startling results, with the rapid return of Hydromys chrysogaster, the native water rat or water babe (Rakali in the native Aboriginal tongue), and common yabby (Cherax destructor), a small crayfish.

Given the size and age of the willows along Cox’s Creek, even the most senior of the local residents will probably not have known anything else than these alien invasive trees which have ousted all the original native species.

GREY SALLOW ON THE WANTED LIST

UK readers might be surprised to learn that the top willow on Australia’s hit list is our own grey sallow (Salix cinerea). For us, it is a generally benign willow of the waterside and damp places. Not so in Australia, where orders are out to destroy each and every tree to stop further regeneration through the female tree’s massive seed load.

There was a huge grey sallow in my daughter’s garden at Bridgewater in the Adelaide Hills. The tree had clearly been there since the house was built decades earlier and was best described as a coppard or low-cut pollard. The net result was a multi-stemmed tree comprising a number of grossly over-stood poles (stems) variously trimmed over the years.

The massive grey sallow tree was leaning across into a neighbour’s property and given the size of the stems there was some doubt about stability. The tree’s presence was not altogether negative because it apparently provided an occasional perch for the resident nocturnal possum. However, for safety reasons it was decided the tree should come down – indeed according to South Australia State legislation it should never have been there.

To this end, a local arborist was called in who promptly measured the circumference of the largest stem – apparently anything over a minimum size requiring permission to fell with reams of accompanying paperwork. In a tactfully roundabout way I pointed out that grey sallow was an alien invasive species and on South Australia’s hit list so perhaps the minimum girth stipulation did not apply. It didn’t. The tree was duly removed and the wood burnt in the stove.

Salix x rubens has been completely removed from a fast-flowing stretch of Cox’s Creek downstream from Bridgewater Mill and Inn and the banks planted with native plant species.

BE CAREFUL WHAT YOU WISH FOR

The message from willows Down Under is be careful what you wish for, perhaps applicable to those drawing up lists of exotic trees to combat climate change in the UK. On my very first visit to the Adelaide Hills I arrived at my daughter’s house at daybreak in late August. I ventured outside to watch the sunrise and heard a familiar cry from a grove of trees – an English blackbird singing its heart out on an English elm.

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