Weed Orchids

Orchids can be weeds. Some people are surprised by that fact because orchids tend to be perceived as beautiful and rare plants, the opposite of weeds.

Disa bracteata, an orchid native to South Africa is invasive in Australia and considered to be a threat to other plants including other species of orchids. In Puerto Rico a native weevil associated with the spread of non-native orchid Spathoglottis plicata increases reproductive failure in native orchid Bletia patula. Yet in Australia Spathoglottis plicata is considered vulnerable and the subject of conservation. So the same orchid can be both an invasive weed and a subject for conservation.  A working definition of a weed is a plant’s ability to grow abundantly in places where it has not been deliberately planted. But orchids growing as weeds are particularly interesting.

Orchids are often considered to be indicators of an ecosystem, because they are so strongly connected to other components of the system such as soil fungi and pollinators. In cases where orchids become invasive they still useful as visible warnings of fundamental changes in the ecosystem they are occupying.

Cyrtopodium polyphyllum is considered an invasive orchid in Florida, where it is pollinated by an invasive bee Centris nitida. Other orchids that are now being recognised as invasive in countries outside of their natural distribution include: Oeceoclades maculate, Dendrobium crumenatum, Epidendrum radicans, Phaius tancarvilleae, Vanilla planifolia, Vanilla pompona and Zeuxine strateumatica. Hawai’is Pacific Weed Risk Assessment identifies Arundia graminifolia, Spathoglottis plicata and Vanda tricolor as orchid species that have naturalized in Hawai’i and are considered to have the potential to be invasive species in Hawai’i and other Pacific islands.

Orchids can be excellent at adapting to new ecosystems. Dependency on fungal partners can be a limiting factor if fungal partners are not present in soil, but when they are present orchids that use them have an advantage.  Another limiting factor can dependency on particular pollinating partners, but some orchids can self-pollinate, and one orchid seed capsule can contain thousands of seeds. Other features of orchids that become invasive are rapid development of mature flowering plants from seeds, vegetative reproduction, and high phenotypic plasticity.

Evidence is increasing that once an invasive species becomes established attempts to control it are expensive and ineffective.  An alternative to futile control attempts is making use of invasives. There is not yet a full list of the orchids consumed as chikanda, in southern Africa, but it would be interesting to know if Disa bracteata is one of the Disa species collected for chikanda, in which case perhaps Australian cuisine could adopt chikanda as another dish in its  culinary repertoire.

Knowing that orchids can be weeds makes it clearer that orchids should not automatically be the object of conservation efforts everywhere, and sometimes other plants could be more worthy of support. Perhaps the next perception about orchids to challenge is that they are beautiful. Nominations for ugliest orchids?

Reference                                                                                                                   Clifford, P. & Kobayashi, K. ‘Naturalizing Orchids and the Hawaii Pacific Weed Risk Assessment’

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