21 Characteristics of weedy species

Whether a plant is classified as a weed or not depends on its location and relationship to human activities. Plants in gardens, agricultural, and natural settings that are considered undesirable or out of place due to appearance, contamination, or competition with desirable plants are often classed as weeds. Aquatic and terrestrial weedy species transported or migrated beyond their natural range that become established  in a new area may pose significant impact or injury to economic, environmental, or human health. These are categorized as invasive, noxious, or nuisance species by governing authorities. Examples of species monitored for management in British Columbia are listed at this link to Invasive Terrestrial Plants [New Tab]^[1].

Common characteristics of weedy species include aggressive growth, competition with other plants for light, water, nutrients, and space, an ability to grow in a wide range of soils and adverse conditions, and resistance to control measures. Some cultivated plants such as Lythrum salicaria ( purple loosestrife), Vinca minor (periwinkle) and Lamiastrum galeobdolon (yellow archangel) can overwhelm and displace other plants and ecosystems. A number of unwanted horticultural plants are identified in this link to the Field Guide to Noxious Weeds and Other Selected Invasive Plants of British Columbia [PDF] [New Tab]^[2].

When environmental conditions in a site change there will always be a change in the plant make up.  For instance, where the ground is fully covered with vegetation there will be no bare soil available for weeds to inhabit. Disturbances in vegetation cover and changes in environmental conditions due to natural events or human activities and management practices create opportunities for species with adapted life cycles and growth characteristics to become established, reproduce, and colonize a site.

Knowledge of family characteristics and life cycles is important for proper landscape and garden plant selection.  Species characteristics such as generalist pollination requirements, diverse seed and vegetative dispersal methods, the ability to adapt quickly to new environmental conditions may indicate the potential for invasive growth. Combinations of these characteristics are commonly exhibited in the Asteraceae (aster), Brassicaceae (mustard), Polygonaceae (knotweed), Fabaceae (pea), and Euphorbiaceae (spurge) families, as well as others.

Weeds are typically classified according to their life cycle. Depending on the degree of disturbance to a site, herbaceous plant species with annual and biennial life cycles will be the first to colonize followed by perennial herbaceous and woody plants. Annual weeds such as Galium aparine (cleavers) that produce high numbers of seed occur most frequently in regularly cultivated and disturbed areas such as vegetable gardens or annual borders. Their rapid growth can smother slower-growing plants and compete for moisture and light. An advantage for winter annuals such as Capsella bursa-pastoris (shepherd’s purse) and Cardamine oligosperma (snapweed) is that they germinate in the fall, overwinter as a rosette of leaves, and flower and produce many seeds in late winter and early spring. Biennial weeds such as Echium vulgare (blueweed) usually produce only a rosette of leaves in the first growing season. Energy stored in the roots over a winter cold period enables the plant to bolt (flower), produce seeds, and then die in the next season.  Removal of the rosette before flowering stops the biennial life cycle.

Herbaceous perennial weeds such as Cirsium vulgare (Canada thistle), Heracleum mantegazzianum (giant hogweed), and Equisetum arvense (horsetail) and woody species such as Buddleja davidii (butterfly bush) and Rubus armeniacus (Himalayan blackberry) survive adverse conditions by storing food reserves in roots, rhizomes, and tubers or bulbs in some species. Important control measures include early identification and removal before establishment.

In situations where weed populations remain below established thresholds of impact or injury for a given site and use, there are ecosystem benefits. For instance weed cover can provide protection from soil erosion, produce pollen, nectar, and habitat for beneficial organisms and wildlife, serve as indicators of soil conditions, and contribute organic matter for soil enhancement, as well as provide food and medicinal products for human use.