Aliens are defined as species that did not naturally occur in the area prior to European settlement but which have since become part of the flora. Most are exotics, introduced from other continents, many unintentionally. A few are adventives, meaning that they are native elsewhere in North America but have moved into the area in response to changes in land use or climate. Many are disturbance species that are abundant only in ruderal habitats such as the continually disturbed farm fields, lawns, and roadsides. The most ecologically disruptive species are those that aggressively invade natural or less-maintained areas. Unencumbered by pests, predators, or diseases, they reproduce and spread rapidly, out-competing the natives for sunlight and water, and frequently reducing the flora in many areas to a small group of contending aliens. Deer speed up this process (and prevent its reversal) by preferentially browsing the natives. Many aliens, including most of the worst woody invaders, were originally introduced with or as ornamental or landscape plantings. Introduced species do not always immediately take over – it often takes years after the initial introduction for a plant to reach a ‘critical mass’ or adjust genetically before it starts dramatically expanding its range and numbers.
During this survey, 293 non-native species of plants and 9 introductions (38% of the flora) were recorded for New Garden Township, 75 of which are considered invasive. Of the invasives, slightly over 40% were recorded in at least 17 of the 21 sections and are most likely present in all sections. Besides well-known pests such as multiflora rose, autumn olive, bittersweet, Japanese honeysuckle, Norway maple, garlic mustard, and Japanese stilt-grass, the widespread invasives include privet, barberry, winged euonymus, Amur honeysuckle, wineberry, long-bristled smartweed, porcelain berry, Japanese hops, mile-a-minute, Phragmites, Ailanthus, and crabapple. Lesser celandine and Callery pear are frequent, but not yet ubiquitous. Purple loosestrife is present in half the sections, but is usually not dense. Recent invaders that are not yet common but are definite threats include the grass Arthraxon hispidus, Asian Hercules-club, several ornamental viburnums, photinia, Butterfly-bush, and Higan or weeping cherry. Unfortunately, many of these are still common in the nursery trade.
A township survey can reveal interesting patterns of alien movement and distribution. Since New Garden Township lies on the state line and possesses a considerable amount of disturbed ground and commercial traffic, it was a little surprising that no aliens new to the state were discovered. Two aliens, Arctium tomentosum (Tomentose burdock) and Matricaria chamomilla (Wild chamomile) were new for the township surveys, and two salt-tolerant grasses – Leptochloa fusca (sprangletop) and Puccinellia distans (alkali-grass) – were seen several times along roadsides. Also Rubus triphyllus or creeping raspberry, which is locally abundant (smotheringly so) along the White Clay in Delaware, was seen once in the Township. Rumex patientia (patience dock) is uncommon in the region, but seems to thrive in the waste ground around mushroom houses.
For more information, see Appendix 2: Invasive Aliens in New Garden Township.