Pennsylvania Native Species Day is observed annually May 20 to celebrate local biodiversity and combat invasive species, as established by the Governor’s Invasive Species Council.

One of the very common invasive plants is garlic mustard, a highly-destructive, non-native.

It aggressively dominates forest under-stories, creating dense stands that choke out native wildflowers and tree seedlings, disrupting entire woodland ecosystems.

Garlic mustard is a biennial, clumping weed.

It has natural predators in its native habitat, but none in region. Deer don’t touch it.

Garlic mustard will set seeds and quickly grow in disturbed sites.

Colonies of garlic mustard are found along the edges of forests in low-light areas, especially where trees have been removed and along streams and riverbeds.

It prefers moist sites in full or partial shade and will be more robust and fruitful under those conditions.

Home gardeners will typically find it in their landscape beds, along fences and in wooded areas.

It is ubiquitous as it tolerates poor growing conditions, except for highly acidic soils.

Garlic mustard is a cool-season crop.

Its seeds begin germinating in March.

First-year plants overwinter as green rosettes.

An entire clump-like plant can reach from three- to four-feet-tall.

White flowers develop on the stem tips.

The flowers grow in clusters; the inflorescence is a raceme.

Individual florets have a cross-like form of four petals, less than one-inch across.

The flowers are pollinated mainly by bees, butterflies and some moths.

As the flowers fade, seeds develop in pods called siliques.

The siliques begin green, narrow and are one-and-a-half to two-inches-long.

By June or July, the seed pods have matured and become brown and dry.

The siliques will burst and spread seeds up to several feet from the parent plant.

The seeds are tiny, less than one-quarter-inch long.

Each plant produces on average 22 siliques, each with as many as 28 seeds.

Healthy mature plants can release as many as 800 seeds.

The seeds are viable in the soil for up to five years, thus making dense colonies.

Wind, water, animals and humans also disperse seeds.

Seeds germinate all season long.

Garlic mustard has another characteristic that allows it to out-compete natives.

It makes glucosinolates, an allelopathic chemical compound that inhibits the germination of other plants.

There are many ways to control this plant.

In your garden, plant natives and minimize ground disturbance.

Never let invasive plants go to seed.

Remove garlic mustard colonies when noticed.

Hand removal by pulling is easy and effective.

Remnants of the root left in the ground do not regrow, as do root sections of dandelions or thistles.

Do not compost.

The seeds can develop and become viable on stems that have been pulled or cut but are left lying on the soil.

Reduce plant colonies by successively cutting the colony.

Bag and remove the cut remnants.

It may take more than one season to eliminate the colony.

Pre-emergent chemical applications are ineffective with garlic mustard as the seeds germinate over several months.

Some herbicides are effective in killing garlic mustard.

For specific suggestions and timing of herbicide treatments, always check the label.

By becoming knowledgeable about specific invasive plants, we all can help minimize their negative impact.

Our native plants, wildlife, and habitat will all benefit from our efforts.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613