Benign exotic herbs
Benign exotic herbs in ecological restoration are those which do not suppress native vegetation, or only suppress seedlings and can be easily controlled if and when needed, by cutting down, trampling or uprooting.
Many other exotic plants are also benign if sufficient quantities of mulch to suppress that species are readily at hand when needed, eg when native seedlings arise or grow larger and are ready to occupy that space or need access to light, or root space.
Until the natives reach that stage, the exotic herbs provide ground cover and shelter from sun and wind, reducing dessication and weed invasion. Their roots penetrate the soil, aerating it, allowing subsequent seedlings to reach hydration and nutrition more readily, and during their decomposition extending the depth of the topsoil and quantity of humus available to subsequent revegetation.
From observations of the surrounding area from June 2018, by September 2018 our restoration plan for Gahnia Grove had developed to include extending our previous experiences of benign exotics in revegetation within the dripline of forest margins, into a monitored Trial of the use of Benign exotics in wild revegetation over larger areas.
The situation in Gahnia Grove was one of dense tree/shrub/vine weed infestation throughout areas up to 10x10 metres without a single native plant. The nature of the Japanese honeysuckle and blackberry required total control to make ongoing control manageable in the time available.
During the control of these weeds it was discovered that Japanese honeysuckle and blackberry, and (later) ginger, Alocasia, and Arum lily, could be controlled more efficiently by only partial uprooting, leaving the root heavily mulched with its own stems and suckers, with the addition of cut stems, uprooted material, and in fact any other plant material available. The roots buried under such mulch became weakened over weeks or months depending on moisture in the mulch pile.
By a variety of these means, the uprooting of these plants was complete over large areas by September 2018, and the previously-sprayed kikuyu margins were also bare of vegetation to compete with the bordering kikuyu.
The plan was to allow dense revegetation of both the kikuyu margins and the wide areas cleared of shrub and vine weeds, restoring ground cover as soon as possible to prevent dessication and weed invasion until existing canopy spread, and new native vegetation was established, to fill those functions in both kikuyu margins and on the wide banks below.
We watched the bare clay anxiously in Spring to see what, if anything, would arise in the hard, bare areas of clay, in some places heavily diluted with roading mix, (which we subsequently learned are likely to have created these banks during levelling and widening of Glenfield Rd, with subsoil and roading materials perhaps having been pushed back onto these banks, explaining the gravel throughout the top 10cm of soil of the kikuyu margin, isolated areas of granular material, and cracks below sharp drops below the road in some places).
The first plants to appear were wild carrot, common vetch and cleavers. The first to survive were broad-leaved docks and creeping buttercup, which were immediately suppressed by mulching lightly with available weed material, including that of their own species; ie scooping the leaves of a few docks provided the material to suppress hundreds of tiny dock seedlings nearby; tearing a few handfuls of leaves off Creeping buttercup plants, and uprooting any plants in soil loose and damp enough to allow it, provided mulch for adjacent buttercup plants. A week or two later, the mulched plants were loose enough to be uprooted, combined with those mulching them, and used elsewhere.
We were pleasantly surprised at how successful this was in keeping areas clear of unwanted weeds until more congenial species arose - fresh crops of the wild carrot and vetch, with oxtongue, Scarlet pimpernel, and all the others seen here:
The first areas to establish cover by benign exotics were under the few surviving karamu or manuka, or in areas where a little humus had been created by the decomposition of honeysuckle, blackberry, kikuyu and other "first-generation" weeds.
Tall Verbena (intially thought to be bonariensis, but apparently V. incompta) and Verbena litoralis were a later, and much denser, arrival. Whether the seed existed in the topsoil, or was windblown from nearby plants since June 2018, remains an interesting question.
Calystegia silvatica (x sepium spp roseata, ie the hybrid native/exotic common along this forest margin) arose suddenly and, as expected, aggressively, and was actively uprooted where soil was loose, or broken off, as it threatened to suppress the other exotic herbs.
All the exotics known by experience to be helpful, or considered possibly benign were supported in developing dense communities of their own ""choosing", with very little assistance by active weeding, resulting in the surprising spontaneous suppression of Creeping buttercup, Broad-leaved dock, trefoils, Vetch and other weeds initially being monitored as possibly obstructive to native revegetation.
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