Wayne served as president of Massachusetts Nursery and Landscape Association (MNLA), New England Nursery Association (NENA), and American Nursery and Landscape Association (ANLA) which is based in Washington, DC.
Wayne is the horticultural consultant for GrowingWisdom.com with Dave Epstein.
While I generally prefer to write about positive garden experiences, this article discusses a pervasive pest that is a problem in most every garden in the northeastern USA. When we allow garlic mustard to spread, we significantly diminish the potential value of our landscape plantings. And the invasive characteristics of this plant seriously threaten natural areas, as well. Effective management of garlic mustard should be a priority for all gardeners so we can continue to enjoy the pleasures our outdoor living areas provide.
Garlic mustard (Alliaria petiolata) is an herbaceous (but evergreen) biennial introduced from Europe in the mid 1800's as a culinary herb, soon escaping and becoming naturalized. Seeds typically germinate in summer and the young plant overwinters as a low groundcover, flowering and producing seed the following spring, eventually dying-off by mid summer.
Reaching from a foot to as much as 4 feet high by late spring, garlic mustard is easy to spot – it's the only plant so tall topped with cross-shaped white flowers in May. It appears in wooded and garden locations in virtually every town in this region. When crushed its 2-3-inch kidney-shaped leaves give off a mild garlic-like odor. Each second-year plant rapidly increases in height with warmer weather. The white fleshy roots grow in any soil, even in deep shade, and can be difficult to remove. First-year seedlings are much smaller and grow in rosettes along the ground with similarly shaped leaves, not producing flowers until the next year. Both first and second year generations coexist.
Left unmanaged, garlic mustard soon dominates the ground, self-renewing year after year. It can cover entire forest floors, effectively choking-out natural and desirable plants. Additionally, it is reported to be allotropic -- its roots emit toxic substances which interfere with the growth of other plants and inhibit their growth. A single plant produces hundreds of seeds, readily spread by wind, animals and water flow. Its seeds remain viable for 5 years lying dormant in the ground. Once it shows up in your garden, it is critical to take action to prevent its further spread.
Several methods of control can be effective, but diligence is vital. Even one plant left to produce seed will infect a large area, producing numerous seedlings and continuing to spread. For small infestations, pulling up plants, digging them out, or cutting below the root crown can be effective. Cutting with a scythe, weed whip or mower can work for larger populations, but this must be done before flowering begins, and repeated on any re-growth. Once flowers appear, viable seeds are produced within days. It is important to dispose of the tops carefully – don't just leave them on the ground – the flowers even on cut plants continue to develop their seeds. I pack them in plastic bags and dispose of them in the trash, never on my compost pile.
Winter management can also be successful. By December, most other plants in the landscape have gone dormant, but the foliage of garlic mustard is clearly visible. If the ground is not frozen, pulling up plants will prevent them from completing their flowering-seeding cycle next spring. Spraying with a contact herbicide is also efficient now and will kill the plants by spring.
Because this is a non-native pest in this region, no natural deterrents yet control its spread. Even deer refuse to browse on it. Some biological controls are currently under evaluation for effectiveness, and preliminary results appear promising. But the best control for now is persistent removal of infestations as they appear.
It is important to understand that even a single plant left to produce seed has the potential to cause damage to areas far beyond our own gardens. Many regions nationwide have developed intensive programs to eliminate garlic mustard and prevent displacement of natural woodland wildflowers. Much more information is available in newspapers, magazines and books, as well as on the internet. I urge you to recognize and take action this month against this significant threat to the health and enjoyment of our landscapes.