Generally, plants fall into two life strategies, they can be perennial or annual. Perennial plants are plants will live for years, sometimes reproducing multiple times throughout their lives. In temperate regions, some herbaceous (not woody) perennial plants will die back during winter, but they will return from underground root structures when the weather gets warmer. Woody plants like trees, are also perennials. They may lose leaves, but their aboveground growth continues to live during cold months, and they can live for decades or even longer. Some examples of perennial invasive plants that we deal with here on the Pike NF are Canada thistle (Cirsium arvense) and yellow toadflax (Linaria vulgaris). An invasive tree species found on the forest is Russian olive (Elaeagnus angustifolia).
On the other end of the spectrum there are annual plants. Annual plants typically grow, reproduce, and die within the same season or year. These plants are usually prolific seed producers, as they do not reproduce through other means (creeping rhizomes, stolons) like perennials do. One common invasive annual plant on the Pike NF is downy brome (Bromus tectorum).
Now of course not all plants fall neatly into these two categories. We also have biennial plants, which complete their life cycle in two years. The first year they typically have vegetative growth and over-winter as a rosette, then flower and produce seeds the second year. Musk thistle (Carduus nutans) and common mullein (Verbascum thapsus) are great examples of biennial plants. Once they produce seeds these plants die. Depending on the climate of where some plants are grown, the same species can exhibit life strategies of annuals, perennials, or even biennials as well. Diffuse knapweed, which is usually a biennial, has also been observed as an annual or even a short-lived perennial plant depending on its growing conditions in a particular year.
Knowing whether an invasive plant falls into any of these life strategies is incredibly important when it comes to deciding how you are going to manage them. For instance, if a biennial musk thistle plant has already completed its second year of growth and it has flowers, killing the plant with herbicide application at this stage will still allow it to produce seeds. At this point it may be best to remove the flower heads instead of using herbicide. This can save money as well as the unnecessary use of chemicals. If you find a patch of musk thistle rosettes in the fall or spring, you can use herbicide at this point of the life cycle to stop the plants from producing seeds. The life strategy of certain invasive plants also makes some treatment methods unsuitable for those species. Mechanical treatments such as mowing of perennial plants may stimulate the plant to produce more flower heads and more seeds as a result. Hand pulling large infestations of perennial weeds quickly becomes unfeasible when you know that there is an extensive root network underground, from which the plant will just regrow next season. The more you know about the specific invasive plant you are trying to control, the better outcome you will have in managing it!