Mown Kikuyu border - evolving a long-term strategy for current trial

Kikuyu within the trial site boundary:

The kikuyu within Gahnia Grove in June 2018 covered c. 30 sq m of mown dense mat, and c.50 sq m long loose unmown.

See observations showing Kikuyu eradication in the Arena:
"https://inaturalist.nz/observations?captive=any&order=asc&order_by=observed_on&place_id=any&project_id=kikuyu-eradication-in-the-arena&subview=grid&verifiable=any

Eradication is close to complete in the Apron:
https://inaturalist.nz/observations?captive=any&order=asc&order_by=observed_on&place_id=any&project_id=31006&subview=grid&verifiable=any

As observed in the above links, all Gahnia Grove's unmown kikuyu, and most of the previously mown kikuyu, have been pulled back and rotted. No live stolons remain, but a few scattered deep rhizomes produce weak stolons from time to time, uprooted or suppressed with mulch as they arise.

All these areas of kikuyu pullback in winter are now supporting dense benign exotic herbs, from mature in the first-released areas, to very new seedlings in the areas of most recent release.

Occasional native seedlings have germinated here and there throughout, a few having close exotic herb foliage removed around them to ensure unhindered growth, but most are still very small and vulnerable to drought.

The previously mown area has been reduced by pullback and rotting to c. 8 sq metres at the top of the Arena, visible from the mown margin through the 4m gap between two large harakeke. This 8 m sq block, with mowing and trimming suspended in June 2018 at our request, had grown to 30-40cm long by early December. Further growth was suppressed by a modified, less efficient (about .5hrs × 3) and less effective pullback to halt its growth through summer, avoiding the need for cutting which would have made it more difficult to control later.

We called this ad hoc technique "comb and curl". It resulted in surprisingly rapid decay of leafy stolons, through a wider area than previously treated at one time, but in hot dry weather it produced little rotted material and a lot of loose dry leafy stolons, wasting potential soil conditioning and potentially contributing to fire hazard near the roadside pedestrian access. To avoid the need for intervention in Firewatch season, the process was halted and the remaining area of "combed and curled " previously mown kikuyu was covered with a thinly scattered layer of wood chip mulch for visual integrity and to reduce Fire hazard. The edges of this block have been explored and found to be rotting full-depth in the "standard" manner, with the kikuyu area reducible a few centimetres a week by uprooting of weakened rhizomes, but the moisture loss and the production of dry loose stolons make the process inadvisable in this season.

The most cost- and labour-efficient summer solution for a smallish dense mat of kikuyu where manual eradication is planned may be to pull back all stolons from the perimeter, (the longer they are, the more they suppress the growth of the remnant), cover it entirely with dense light-proof mulch or sheet, and wait until first rains before continuing pullback. This may need to be repeated during summer where growth continues through the mulch.

Kikuyu at the margin of mown kikuyu:

The outer margin of the site requires a different strategy and additional consideration to meet traditional expectations of appearance and to facilitate kikuyu control in a wide area by mowing.

At the end of December 2018, 3 cu m of Wood chip mulch was spread along c. 30m kikuyu margin for visual definition and amenity and fire safety, covering some of the loose dry grass, pampas, twigs and fallen leaves.

Reserve users have asked if it is to "stop the kikuyu". It was explained that the mulch will not in itself stop the kikuyu, but will temporarily suppress and direct its growth, and by defining the mown area allow any kikuyu beyond to grow long enough to be pulled back in due course. (One area of mown kikuyu, a "bay" between two planted harakeke extended the mowing and kikuyu control margin by 6-8 metres with no obvious benefit, was covered with a tarp to hold some of the delivered mulchpile for a week or two. After the mulch was used the area was included in Gahnia Grove, ie behind the cordon, for a trial of letting-it-grow followed by pullback, starting in summer when growth is fast and rotting is difficult to achieve. If this proves difficult to control, the tarp and/or more mulch can be laid).

In the remainder of the 30m margin, a month after laying the mulch border strip a strategy is evolving to manage kikuyu pullback along a mown margin using a border of wood mulch initially c. 50cm wide and c. 20cm deep.

Whether the mulched kikuyu rots or dries depends on moisture surrounding it. In summer, growth is rapid and rotting is minimal and only at ground level, with any loose or lightly-mulched kikuyu tending to dry and potentially presenting a fire hazard. The following strategy and observations relate to the control of kikuyu in summer.

1. A few stolons rapidly grow and emerge within the mulch border itself. These have been, experimentally, suppressed only briefly by kicking adjacent mulch over the emergent growth.
2. Any kikuyu long enough to be pulled back towards the mown area BEFORE mulch is laid over it, grows vigorously BACKWARDS, ie towards the mown area. An attempt was made, pre-cordoning, to continually "hide" this growth, but this was (as expected) time-inefficient.

The erection of a cordon defining the border has helped clarify the ongoing and systematic nature of the kikuyu control, the appearance of which is well-accepted by those passersby who have commented.

[Interpretive signage on the cordon surrounding the site is being planned in collaboration with Kaipatiki Project (KP), to include Invitations to participate in restoration in Reserves through Council and the KP. We are to trial the effect of various texts with a variety of home-made signs. The first one reads simply "Native Seedlings Growing - Kaipatiki Project", conveying the most important message that the apparentl bare ground behind the cordon is a seed-bed.].

It is so far proving relatively efficient to let the backward growth develop without intervention for a week or two (depending on weather) and then to scoop off the mulch from about 50cm lenth of border, pull any unrooted stolons back towards the mown area, and re-cover them, along with any remaining suspected hidden live rhizomes, with a similar quantity of mulch.

This can be done by working from one end along the border, removing the mulch from one short length of border onto the adjacent area that has just been pulled back. Covering the backward growth on the outer edge, while uprooting the rotted stems on the inner edge, moves the mulch border outwards 10-20cm each time this is done. Provided no live rhizomes are left unmulched, and no mulched area is left unexamined too long, this process creates an outward-expanding controlled area, which will in time consume the supplied wood-chip mulch. However, since the process adds decaying kikuyu to the wood mulch, sufficient mulch material may be maintained to sustain the process until mowing ceases in winter.

Alternatively, when Firewatch season ends ongoing growth can be allowed into the manually controlled area, to be pulled back where threatening other vegetation. When the ground is wet enough, pullback will result in full-depth rotting of kikuyu rhizomes and the creation of humus. In winter, pullback is likely to provide enough rotted kikuyu, mixed with soil by the process, to continue the process outwards until mowing starts again.

The areas where kikuyu pullback and rotting have been completed are immediately seedable or plantable. Within Gahnia Grove benign exotics have already matured throughout much of the previous area of kikuyu, and native seedlings are beginning to emerge. A similar development of selective revegetation starting with benign exotics is now starting to occur in the outer kikuyu margin.

Kikuyu control will of course continue to be needed along the new border. If expansion of the kikuyu-free area is wanted, the process of pullback can be repeated along the new border. If not, a method of preventing outward spread of the permanently mown area will have to be decided.

Ongoing pullback and regrowth of the same area could be done, but the need for ongoing intervention could be reduced or eliminated by an effectively-competing ground cover that does not inhibit native forest or create a nuisance.

Where dense tall vegetation is wanted and achieved, shade will do much of the control. The current trial seeks to avoid line-trimming or spraying under the dripline primarily in order to achieving the necessary density of vegetation at and near ground level, by selecting the natives and benign exotics from those species that spontaneously arise.

Ongoing selective weeding of any spontaneously arising vegetation in the margin will support increasing diversity and habitat in what it is hoped will become a self-sustaining native plant community echoing natural forest buffer ecology. Any experiences or sharing of knowledge in that regard will be very welcome.

Planting of Scrambling pohuehue has been discussed, but we don't know how effective this is at excluding kikuyu. It would seem to be at least advisable to ensure the area is thoroughly free of kikuyu, including underground rhizomes, before planting Pohuehue, to avoid having to fight one's way through a tangled mass to control kikuyu later. But again, any experience is welcome.

The interaction of the mulch border of Gahnia Grove with mowing has not yet been observed. To define the mowing area, and to deter reserve users from walking through the revegetation site, a cordon has been provided by Council Parks, and was erected 15 January with the assistance of the Kaipatiki Project, soon after the mulch was laid.

It is not yet known how close to the cordon mowing is possible and convenient for the mowing team, or desirable for amenity or ease of manual control. All observations are welcome in the evolution of a sustainable solution to the problem of mown kikuyu along the borders of plantings and regenerating forest.

Posted on January 22, 2019 11:39 PM by kaipatiki_naturewatch