Is Agastache invasive?

Agastache, also known as hummingbird mint, giant hyssop, or anise hyssop, refers to a genus of aromatic perennial plants in the Lamiaceae (mint) family. The genus contains approximately 22 species native to North America. Some of the most popular garden varieties include A. foeniculum (anise hyssop), A. rugosa (Korean mint), and A. mexicana (Mexican giant hyssop).

Is Agastache considered invasive?

Most species of Agastache are not considered invasive plants. In fact, many Agastache varieties are prized by gardeners for their showy flowers that attract hummingbirds, butterflies, and bees. However, a few species have demonstrated invasive tendencies in certain regions:

• Agastache urticifolia – This species has naturalized and become invasive in some areas of Australia. It spreads readily and crowds out native vegetation.
• Agastache nepetoides – Yellow giant hyssop exhibits aggressive spreading habits. It has naturalized in parts of New England and the mid-Atlantic states where it crowds out native plants.
• Agastache scrophulariifolia – Purple giant hyssop has naturalized and become invasive in Hawaii, New Zealand, and Australia.

So while most Agastache are not invasive, several species do have the potential to spread aggressively outside their native ranges. Gardeners should take care when planting yellow giant hyssop, purple giant hyssop, and nettleleaf giant hyssop to avoid uncontrolled spreading.

Growth habits of invasive Agastache species

The invasive species of Agastache are perennial plants that emerge in spring and flower through summer into fall. They spread readily both by seeds and creeping rhizomes (underground stems). Here are some key growth habits of invasive Agastache:

• Quickly colonize disturbed areas
• Spread rapidly to form dense clonal colonies that overwhelm native plants
• Produce abundant seeds that readily self-sow in ideal conditions
• Extensive rhizome system allows the plant to regrow vigorously when cut back
• Can adapt to a wide range of soils and light conditions
• Mature plants can reach over 5 feet tall

These aggressive growing habits allow invasive Agastache to naturalize and dominate sites where they are introduced outside of their native ranges. Care must be taken not to plant them in areas where they can spread unchecked.

Where are invasive Agastache species problematic?

Here are some regions where non-native Agastache species have naturalized and become invasive pests:

• Hawaiian Islands – Agastache scrophulariifolia is able to invade disturbed forests and conservation areas from sea level up to 6500 ft elevation.
• Australia – Agastache scrophulariifolia and A. urticifolia have both naturalized and become invasive problems. They aggressively invade roadsides, grasslands, open woodlands, and other disturbed areas.
• New Zealand – A. scrophulariifolia invades waste areas, roadsides, streambanks, open forests, and other disturbed sites. It is designated an unwanted organism.
• Northeastern/Mid-Atlantic United States – The native A. nepetoides has become weedy and invasive in areas outside its normal range such as New England, crowding out native plants.

In these regions, invasive Agastache can overwhelm native vegetation, reduce biodiversity, and alter ecosystem function. Protecting intact native plant communities and limiting disturbances that allow Agastache colonization are important management strategies.

Are hybrid Agastache varieties invasive?

Gardeners and breeders have created many hybrid Agastache varieties by crossing species like A. foeniculum, A. rugosa, and A. mexicana. Popular hybrids include the following:

• Agastache ‘Blue Fortune’
• Agastache ‘Kudos Coral’
• Agastache ‘Kudos Yellow’
• Agastache ‘Black Adder’
• Agastache ‘Summer Glow’

These hybrids are sterile so do not spread aggressively by self-sowing. However, some hybrids produce vigorous rhizomes and can spread via underground creeping stems if not managed properly. They should be monitored and controlled in areas outside of flower gardens to prevent unwanted spread. Overall, most hybrid Agastaches pose minimal invasive risk compared to potentially weedy species like A. scrophulariifolia and A. nepetoides.

How to prevent invasive Agastache spreading

Here are some tips to prevent problematic spread if you are growing potentially invasive Agastache species:

• Plant in containers above ground or surrounded by barriers to contain spreading rhizomes
• Cut off spent flower heads before seeds form to prevent self-sowing
• Pull or dig out unwanted colonies promptly when noticed
• Monitor regularly for new growth around original plantings
• Plant only sterile hybrid varieties
• Grow in areas bounded by pavement or lawn that deter spread

With proper management, the benefits of Agastache like the nectar for pollinators can be enjoyed without allowing uncontrolled spread. Know the growth habits of the species you are planting and take action to contain and control growth where needed.

Suitable alternatives to potentially invasive Agastache

Many gardeners are drawn to Agastache species like A. foeniculum (anise hyssop) for the showy flowers, fragrance, and wildlife attraction. If you want to avoid a potentially invasive Agastache, here are some suitable alternative plants:

These perennials provide the floral display, fragrance, and pollinator benefits of Agastache without the risk of invasiveness. They make excellent alternatives in gardens where containing Agastache spread may be difficult.

Should I remove invasive Agastache from my garden?

If you are currently growing an invasive form of Agastache, you do not necessarily need to remove it from your garden completely. As long as you take some key steps, you can often manage invasive Agastache without fully eradicating it:

• Deadhead flowers – Remove spent blooms so they don’t form seed heads that drop unwanted self-sown seeds.
• Cut back frequently – Cutting plants back regularly helps curtail seed production and keeps growth contained.
• Divide overgrown clumps – Divide dense clumps to revitalize growth and prevent uncontrolled spreading.
• Monitor for new growth – Watch for new sprouts around your plantings and promptly dig them out.
• Grow in containers – Plant in pots or other containers that prevent underground rhizome spread.

With diligent monitoring and management focused on preventing spread, you can often keep invasive Agastache under control in the garden. But if plants persistently escape confinement, fully removing them may be necessary.

How to remove invasive Agastache plants

If invasive Agastache have escaped control in your garden, full removal may be needed. Here are some effective steps for removing unwanted plants:

1. Cut plants down to ground level when in full bloom to prevent further seed production.
2. carefully dig or pull out as much of the root system as possible. Agastache have spreading rhizome networks that need to be removed.
3. Monitor the area vigilantly for regrowth. Invasive Agastache often resprout from any remnants left behind.
4. Repeat manual digging/pulling of regrowth. This may take several years to fully eradicate plants.
5. Replant with non-invasive plants to occupy the space so Agastache cannot recolonize easily.
6. Properly dispose of plant debris by burning, deep burial, or bagging for landfill disposal.

Completely removing invasive Agastache takes persistence, but is worthwhile to prevent further spread. Combining manual removal with competition from other non-invasive plants leads to the best long-term control.

Conclusion

While valued for their ornamental flowers, some Agastache species do present invasive risks. Understanding which types exhibit aggressive spreading tendencies is key for gardeners who want to grow Agastache responsibly. With appropriate siting choices and vigilant monitoring for unwanted spread, certain Agastache can often be managed. But if plants escape confinement, complete removal may be needed to prevent impacts on native plant communities. By making informed choices about Agastache plantings, gardeners can balance enjoyment of these aromatic perennials with responsible stewardship.