Bulb weeds - Montbretia and Bulbil Watsonia
We cover both these bulb weeds here as the young leaves are often indistinguishable, so they get treated together. However, while montbretia is usually easily uplifted entire when it is mature, Watsonia bulbs are almost always impossible to uplift, at least here in this hard clay among shallow tree roots.
So eradication is aimed for by very different methods. A later report will cover the method being used for Watsonia in July 2019 throughout a larger area of canopy and grass margin downhill of Gahnia Grove, after the year's observations and various methods of control in the manuka canopy
and its grassed margins of Gahnia Grove, where all Watsonia were suppressed by either cutting or tearing off leaves, or trampling and mulching. After almost no evident leaves through summer and autumn, a new crop of leaves started to appear in July 2019.
Here are observations of a typical section of Watsonia-dominated space manuka-margin before and after a season of ongoing suppression.
Watsonia is very dense throughout the partial shade of the manuka canopy margins from the roadside to the lower recreational grass margins at the entry to the top forest path. Deeper within the forest, Watsonia becomes scarce to absent - whether because the invasion has not yet progressed that far into the forest, or because an earlier invasion has been suppressed by increasing shade, we do not know.
In places there is not a single point of earth without a leafy or hidden bulb. With bulbs occupying the ground so densely in many places as to prevent seedling development, we do not know whether the young solely-manuka canopy margin will develop deep shade sufficiently quickly to suppress the existing dense ground cover by Watsonia without intervention.
Observations of Watsonia in Gahnia Grove from May 2018 to the present:
https://inaturalist.nz/observations?order=asc&order_by=observed_on&place_id=128172&subview=grid&taxon_id=72425&user_id=kaipatiki_naturewatch&verifiable=any
We are not familiar with their seasonal and reproductive habit yet, but after removal and/or suppression of all leaves in the Gahnia Grove canopy margins, we saw no evidence of regrowth until the drought broke recently with autumn-winter rains. Over the same period, thousands of Watsonia wilted and dried along the outer canopy margin without apparent intervention. These were presumably immature plants, since bulbils have been seen only on a few plants which attained over a metre in height.
Montbretia
In most soil conditions of our experience, mature Montbretia bulbs are very easily uplifted, even in handfuls, by pulling their leaf tips, and are effectively controlled by dropping them in piles anywhere there is not a native plant. When they reshoot they are easily uplifted again, this time en masse. The development of leaf combined with failure of piled bulbs to reach nutrition consumes their stored energy, and the mass of bulbs is gradually replaced by the accumulated rotting leaves and bulbs.
This method of gradual eradication and decomposition in situ suits our Methodology, which depends on ongoing successive monitoring and intervention. However, in the interests of developing a transferable method requiring the least monitoring, at Gahnia Grove in Spring 2018 we collected all uplifted bulbs, including the few Watsonia bulbs that were able to be lifted, and cable-tied them all together in a number of heavy-duty plastic bags.
Watsonia bulbs are said to be prone to rotting in damp storage. Over summer and autumn some of the Watsonia bulbs did decay in the bags, (we have not examined the rotting mass closely yet) but many Montbretia bulbs appear to have survived in a viable condition.
On another site, we recently observed Montbretia bulbs buried both under and in large Tradescantia weedbags and in black kleensaks. All the Tradescantia had long decomposed, but some Montbretia bulbs had new green shoots shortly after release from burial.
Sone Montbretia bulbs may have decayed along with the Tradescantia, but the observation suggests disposal in water, eg barrels, is necessary for unmonitored decomposition.
We tipped out the bags of rotted foliage and mixed bulbs and will, for now at least, revert to our earlier method, pleased to have easy access to shooting bulbs in passing, and the ease of throwing them onto the nearest pile rather than having to drag along a bag for collection. We also prefer the appearance of the site unspoiled by inorganic human artefacts.
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