Escaped plant

Wild olives have escaped gardens mainly by birds dispersing the fruit.

All garden refugees belong to the so-called hemerochoric plants. This term is used across the board for plants that have been introduced directly or indirectly by humans. The term also includes the unintentionally introduced plants that were introduced through seed pollution (speirochoric) or through unintentional transport (agochoric).[4]

Plants escape from gardens in many ways, but one main cause of spread from the ornamental garden is by green waste dumping in bushland and road reserves and as well as by birds or other animals eating the fruits or seeds and dispersing them.[5] Others are accidental hitchhikers that escape on ships, vehicles, and equipment.[6] Garden escapees can be adventive, which means they can be established in an inappropriate area of origin site by human influence.[7]

Occasionally, seed contamination also introduces new plants that could reproduce for a short period of time. The proportion of adventitious species in open ruderal corridors at such locations can exceed 30% of the flora of these locations. Further, ornamental alien plants can easily escape their confined areas (such as gardens and greenhouses) and naturalize if the climate outside changes to their benefit.[8] In the US, there are over 5,000 escaped plants, many of which are escaped ornamentals.[9]

Tradescantia fluminensis escapees infesting woodland area.

Many invasive neophytes in Australia and New Zealand were originally garden escapees. The Jerusalem thorn forms impenetrable thorny thickets in the Northern Territory which can be several kilometers in length and width. Two other plants introduced as ornamental garden plants, Asparagus asparagoides and Chrysanthemoides monilifera, now dominate the herbaceous layer in many eucalyptus forests and supplant perennials, grasses, orchids, and lilies.[10]

Neophytes that develop aggressively, which displace and repel large areas of native species in many humid and shady landscapes, permanently change the biotope pose in many parts of the world, and sometimes create an economic problem. For example, species of Opuntia (prickly pears) have been introduced from America to Australia, and have become wild, thus rendering territories unsuitable for breeding; the same goes for European gorse (Ulex europaeus) in New Zealand.[11]

Rhododendron species introduced as ornamental garden plants in the British Isles crowd out island vegetation.[12] The same can be seen in many acidic peatlands in the Atlantic and subatlantic climates. Robinia pseudoacacia was imported from America to Central Europe for its rapid growth, and it now threatens the scarce steppe and natural forest areas of the drylands. Examples in forests include Prunus serotina which was initially introduced to speed up the accumulation of humus.

In North America, Tamarisk trees, native to southern Europe and temperate parts of Asia, have proven to be problematic plants. In nutrient-poor heaths, but rich in grasses and bushes (fynbos) in the region Cape in South Africa, species of eucalyptus from Australia are growing strongly. As they are largely accustomed to poor soils, and in the Cape region they lack competitors for nutrients and parasites that could regulate their population, they are able to greatly modify the biotope. In Hawaii, the epiphytic fern Phlebodium aureum has spread widely, and is considered an invasive plant.[13]

Particularly unstable ecosystems, already unbalanced by attacks or possessing certain characteristics, can be massively damaged by neophytes because the final vegetation is already weakened. In the humid forests of Australia, neophytes first colonize along roads and paths and then enter the interior of the regions they surround.[14]

Thunbergia mysorensis, native to India, invaded the rainforests around the coastal city of Cairns in Queensland and even invades trees 40 m high. In Central Australia, the Eurasian species Tamarix aphylla grows along river banks, repelling native tree species, and wildlife that go together, lowers water levels and increases soil salinity. As in the United States, tamarisks have proven to be formidable bio-invaders. The fight against this species of trees, which has spread widely since, appears to be almost hopeless.[15][16]

Garden escapees can fall within the definition of, and may have a relation to, these botanical terminologies below:

Wild tulips survive, multiply and grow wild without human influence.

• Agriophyte: Refers to plant species that have invaded natural or near-natural vegetation and can survive there without human intervention. Established in their new natural habitats, they remain part of natural vegetation even after human influence has ceased, and are independent of humans in their continued existence.[17][18] Examples in Central Europe are waterweed, Douglas fir or Japanese knotweed, and the sweet chestnuts introduced by the Romans in Germany. The group of ornamental plants and even the poppy (both archaeophytes) that normally grow in fields colonized pioneering sites in floodplains and is therefore now part of natural vegetation.[19]
• Epecophyte: Species of recent appearance, usually numerous and constant in the country, but confined to artificial habitats, such as meadows and ruderal vegetation. They are dependent on humans for existence that their habitats require constant renewal.[20]
• Ephemerophyte: Species that are only introduced inconsistently, that die briefly from culture or that would disappear again without constant replenishment of seeds. In other words, they can establish themselves temporarily, but they are not in a position to meet all the conditions relating to the territory. A cold winter, or an unusual drought, can lead to the death of these plants; most of the time, they are not able to fight against the local flora in extreme conditions.[21]
• Hemerochory: Plants or their seeds may have been transported voluntarily (introduction) or involuntarily by humans in a territory which they could not have colonized by their own natural mechanisms of dissemination, or at least much more slowly. They are able to maintain themselves in this new vital space without voluntary help from man. Many Central European cultivated and ornamental plants are hemorochoric – insofar as they have escaped and subsist independently of cultivation.[22] These are the forms of hemerochory:
  • Agochoric: Plants that are spread through accidental transport with, among other things, ships, trains, and cars. On land, agochoric plants used to be common in harbors, at train stations, or along railway lines. Australia, like New Zealand, has taken stringent measures to prevent the spread by seed or human transport. Agricultural implements imported into Australia must be thoroughly cleaned. Air travelers from other continents are forced to thoroughly clean the soles of their shoes.[23]
  • Ethelochoric: Deliberate introduction by seedlings, seeds, or plants in a new habitat by humans. Many cultivated plants which currently play an important role in human nutrition have been deliberately disseminated by humans. Wheat, barley, lentil, broad bean and flax, for example.
  • Speirochoric: Unintentional introduction by seeds. As all seed samples also contain the seeds of the grasses of the field from which they were obtained, the trade-in seeds of useful plants has also allowed the spread of other species. Speirochoric plants are therefore sown on soil prepared by man and compete with useful plants. Wild chamomile, poppy, cornflower, corn buttercup are example of plants that were unintentionally scattered.

Examples of escaped plants and/or garden escapees include:

• 
  Iceplant refugees along the California Coast
• 
  Dyer's woad's escape to disturbed roadsides
• 
  Red valerian finding refuge atop old walls
• 
  Castor bean usually finds refuge on wastelands
• 
  Tiger lily occurs as a garden escapee in Eastern U.S..
• 
  Buddleja self-sown along a railroad
• 
  Oxalis latifolia has escaped gardens through seed dispersal.

• Volunteer plant
• Adventitious plant
• Archaeophyte
• Assisted colonization
• Hemerochory
• Neophyte

• Angelika Lüttig, Juliane Kasten (2003): Hagebutte & Co: Blüten, Früchte und Ausbreitung europäischer Pflanzen. Fauna, Nottuln. ISBN 3-93-598090-6.
• Christian Stolz (2013): Archäologische Zeigerpflanzen: Fallbeispiele aus dem Taunus und dem nördlichen Schleswig-Holstein. Plants as indicators for archaeological find sites: Case studies from the Taunus Mts. and from the northern part of Schleswig-Holstein (Germany). Schriften des Arbeitskreises Landes- und Volkskunde 11.
• Herrando-Moraira, S., Nualart, N., Herrando-Moraira, A. et al. Climatic niche characteristics of native and invasive Lilium lancifolium. Sci Rep 9, 14334 (2019). Climatic niche characteristics of native and invasive Lilium lancifolium

• ESCAPED GARDEN PLANTS AS A KEY THREATENING PROCESS