Anise Hyssop (Agastache foeniculum) is a perennial herb valued for its fragrant, anise-scented foliage and striking spikes of lavender-blue flowers. Native to the prairies and Great Plains of North America, it is generally not listed as an officially regulated or ecologically invasive species. Its reputation for spread stems from its vigorous self-seeding habits.
Defining Invasive Versus Aggressive
An “invasive” species is defined in an ecological context as a non-native organism that causes or is likely to cause economic or environmental harm, or harm to human health. These species outcompete native plants, fundamentally changing ecosystems and are often subject to government regulation and control.
In contrast, a plant described as “aggressive” spreads rapidly within a cultivated garden setting but does not pose a major ecological threat to native ecosystems. Anise Hyssop, being native to North America, does not fit the definition of an invasive species in its native range. When gardeners describe it as aggressive, they are referring to its ability to colonize garden beds quickly through seed, rather than its potential to harm the wider natural environment.
Understanding Anise Hyssop’s Spread Mechanism
The aggressive nature of Anise Hyssop in a garden is due to its prolific seed production. Each plant develops numerous flower spikes which, once pollinated, produce thousands of tiny seeds known as nutlets. These seeds are readily dispersed around the parent plant when the flower heads dry and break down in the late fall and winter.
The plant itself is a clump-forming perennial that grows from a small taproot and is short-lived, often lasting only two to three years. While some sources mention it can spread via short rhizomes, its primary method of multiplication is through self-sowing. This mechanism leads to a dense cluster of new seedlings appearing near the original plant the following spring, giving the impression of a rapidly spreading root system.
Practical Steps for Controlling Garden Growth
Gardeners have several practical control methods available to manage Anise Hyssop’s spread. The most effective technique is called deadheading, which involves removing the spent flower spikes before the seeds have a chance to mature and drop. This action must be performed promptly after the bloom fades, typically in late summer or early fall, to prevent seed dispersal.
If new seedlings appear in the spring, they are generally shallow-rooted and easy to pull out or transplant to a desired location. Thinning these new plants allows the gardener to control the density and location of the next generation. Since Anise Hyssop is a clump-forming perennial, dividing the main clump every three to five years helps to rejuvenate the parent plant. Planting the herb in a contained area, such as a large container, can also restrict its ability to self-seed into surrounding beds.