Calling all green thumbs!
Join us on a journey to cover key plant terminology from A-Z, and learn why these terms are important for understanding invasive species. Today, we will cover P-R.
Perennial
Perennial: definition
A plant that lives for more than two years. Unlike annuals (which complete their life cycle in one year) or biennials (which take two years), perennials grow and bloom over the spring and summer, die back in the fall and winter, and return again from their rootstock the following year.
Perennials can be:
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Herbaceous – dying back to the ground each winter (e.g., peonies, hostas).
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Woody – retaining above-ground structures year-round (e.g., trees, shrubs).
Some perennials are short-lived (only a few years), while others can last for decades.
How this term applies to invasives
Perennial invasives are particularly difficult to control because:
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Simply cutting or mowing them won’t kill the root system.
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They can outcompete native plants for years, forming dense monocultures.
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Their longevity means they can build a strong seed bank and spread aggressively over time.
Examples of perennial invasive species include:
Japanese knotweed
Canada thistle
Propagule
Propagule: definition
Any structure that can give rise to a new plant, including:
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Seeds
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Spores
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Bulbs
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Tubers
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Rhizomes
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Fragments of stems or roots
How this term applies to invasives
Invasive species often spread through propagules like seeds, roots, or plant fragments that can grow into new plants. The more propagules they produce and disperse, the higher their chances of establishing and spreading in new areas.
Examples of invasive species that produce high quantity, frequency, and quality of propagules are:
- Purple loosestrife (Lythrum salicaria)
- Kudzu (Pueraria montana)
Purple loosestrife
Kudzu. Photo credit: David J. Moorhead, University of Georgia, Bugwood.org
Quiescence
Quiescence: definition
A state of temporary inactivity or dormancy, where growth and development are paused even though the plant (or seed) is still alive. This occurs when environmental conditions, such as water availability, temperature, and light are inadequate, helping plants to wait out unfavorable conditions.
How this term applies to invasives
Quiescence allows seeds or other propagules of invasive species to remain dormant in the environment until conditions are ideal for growth. This means they can persist in the soil for months or even years, making management harder—because even after removal efforts, new individuals can keep emerging from the seed bank long afterward.
For example, garlic mustard (Alliaria petiolata) seeds can remain quiescent in the soil for several years, allowing the plant to reinvade areas where it was previously removed.
Garlic mustard (photo credit: J. Hallworth)
Garlic mustard foliage (photo credit: J. Hallworth)
Rhizome
Rhizome: definition
A type of underground stem that grows horizontally, producing roots and shoots from its nodes.
Unlike roots, rhizomes store nutrients and can give rise to new plants, making them a key way for many species to spread and reproduce vegetatively.
How this term applies to invasives
For invasive species, rhizomes are a powerful tool for rapid and persistent spread. Because rhizomes grow underground and can produce new shoots even from small fragments, invasive plants with rhizomes can:
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Regrow quickly after being cut or disturbed, making them hard to eradicate.
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Spread laterally over large areas, forming dense, tough-to-remove colonies.
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Survive harsh conditions underground, outlasting control efforts like herbicides or mowing.
For example, orange hawkweed (Hieracium aurantiacum L.) spreads via rhizomes, making it difficult to control this invasive species.
Orange Hawkweed
Ruderal
Ruderal: definition
A ruderal plant is one that is adapted to disturbed environments, such as roadsides, construction sites, or areas affected by fire, grazing, or human activity. These plants typically:
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Grow quickly
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Reproduce rapidly
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Tolerate poor or unstable soils
How this term applies to invasives
In the context of invasive species, ruderal traits make certain invasives especially successful, because they can quickly colonize open or damaged areas and outcompete native plants during early stages of succession.
For example, great mullein (Verbascum thapsus) invades overgrazed lands and roadsides, forming dense rosettes that outcompete native seedlings.
Great mullein
Great mullein
Want to learn more?
- Check out the rest of the Green Thumb’s Glossary!
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