Merremia

Decalobanthus peltatus. Previously, it was called Merremia peltata. It is a member of the Convolvulaceae.

South and Southeast Asia, East Africa, North America (Canada), Oceania. It is recorded from Australia, Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, Guam, Marshall Islands, New Caledonia, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, Vanuatu, and Wallis & Futuna.

It is native to the Malaysia-Indonesia region. Suggestions that it is native to American Samoa, Samoa, Tonga, and Vanuatu are disputed, but it has become naturalised in the South Pacific.

A very important invasive weed of the humid tropics, from coastal areas up to 700 m where it can dominate bush fallows and forests (Photos 1-4). It becomes especially aggressive in disturbed areas, smothering crops, as well as reaching to the tops of trees in logged forests, forming dense canopies that stifle regrowth.

There are differences of opinion whether it is native or an alien invasive to the Pacific islands. It is considered by some that removal of Decalobanthus might even increase the invaseness of other weeds, and that it is part of a regeneration of lowland rainflorest.

Decalobanthus produces smooth, climbing vines, from underground tubers, up to 30 m long that twine at the tips. The vines are woody at the base up to 10 cm diameter. Leaves, alternate along the stems, almost round, up to 30 cm across, purple-veined below, with wavy margins and pointed tips; leaf stalk is joined to the blade at the lower surface near the centre (Photo 5). Flowers are funnel-shaped, white (but there are yellow types), 5-6 cm long (Photos 6&7). The seeds are furry, one to four inside a round capsule (Photo 7).

Spread can be rapid, with vines growing up to 30 cm a day, and new plants forming as root develop from the nodes. Over longer distances spread is by seed, although there are reports of low seed vitality from Solomon Islands. Seeds and cuttings can be transported by machinery. Deliberate introductions occur when the weed is planted as an ornamental, or as a ground cover to protect land from erosion.

In Solomon Islands, the removal of the tree canopy during logging promotes massive growth of Decalobanthus , which has to be removed before replanting can begin, as well as kept in check during the early life of the young trees. Coconut replanting schemes of the 1980s estimated Decalobanthus control to be 30% of recurrent annual costs.

Damage to forest by cyclones is also said to be a factor contributing to the increased aggressiveness of merremia in Pacific islands, with Cyclones Ofa and Val that struck Samoa in the early 90s given as examples. Here, it is suggested that disturbance (cyclones and shifting cultivation) rather than competition with merremia is the greater threat to native foirests.

Tubers said to be edible in Malaysia. It is promoted as a rapidly growing ground cover following land disturbances reducing erosion. Reported to be useful in healing skin burns.

BIOSECURITY
Countries not yet infested by merremia should consider all likely pathways for entry, and apply quarantine measures accordingly. Special consideration should be given to the part played by the domestic and international trade in ornamental plants in the spread of this invasive weed. Seeds can be bought on the internet.

In Solomon Islands and Vanuatu, merremia is among the top 10 worse weeds.

Decalobanthus peltata is on the Global Invasive Species Database (2020) of information about alien and invasive species that negatively impact biodiversity, managed by the Invasive Species Specialist Group of the IUCN Species Survival Commission.

BIOLOGICAL CONTROL
It has been ranked among the top 24 candidate weeds for classical biological control in Pacific island countries by a New Zealand study, which documented prospects for biological control.

CULTURAL CONTROL

• Physical & Mechanical:
  • Slashing and mowing. A practical means of control if sufficient labour available; the disadvantage is that it promotes vigorous new shoots, and stem pieces produced by the slashing or mowing can produce roots and new plants.
  • Allow cattle to feed and trample on the vines. They are said to relish merremia. 
  • Hand-pulling is effective, but it is very difficult to remove thick vines and tubers from the ground.
• Spacing:
  • Reduce spacing of plantation trees as Merremia requires full sunlight to grow aggressively.
• Hygiene:
  • Treat vehicles and farm machinery. If moving from areas where the weed occurs to those weed-free, wash to remove soil. This is equally important if the machinery is being imported into a country or moved within a country.

CHEMCIAL CONTROL
Waterhouse & Norris¹ state the limitations for using herbicides to control Decalobanthus , including, the difficulty of spraying in 'rough country', with logging debris, dense growth of vines, and where it is difficult to guarantee rain-free periods of 4-6 hours.

Use the following herbicides:

• In Australia, MCPA is registered.
• Elsewhere, foliar applications said to be effective are: 2,4-D; MCPA; dicamba; triclopyr; picloram; glyphosate (e.g., Fiji); triclopyr + picloram.
• For cut-vine applications use triclopyr or glyphosate (e.g., Fiji).

For foliar applications, it is recommended that the plants are slashed, and the new growth is then sprayed.

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Note, EU approval to use glyphosate ends in December 2022; its use after that date is under discussion.

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When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval. Recommendations will vary with the crop and system of cultivation. Expert advice on the most appropriate herbicides to use should always be sought from local agricultural authorities.